Patent Ductus Arteriosus Ligation in Extremely Preterm Infants and Death or Neurodevelopmental Impairment

by

Dany Weisz

A thesis submitted in conformity with the requirements for the degree of Masters of Science.

Institute of Health Policy, Management and Evaluation University of Toronto

© Copyright by Dany Weisz 2016

ii

Patent Ductus Arteriosus Ligation in Extremely Preterm Infants

and Death or Neurodevelopmental Impairment

Dany Weisz

Masters of Science

Institute of Health Policy, Management and Evaluation

University of Toronto

2016

Abstract

Objective: Evaluate the association between patent ductus arteriosus (PDA) ligation vs. medical

management and neonatal and neurodevelopmental outcomes.

Methods: Retrospective cohort study of extremely preterm infants with PDA born between

2006-2012. The primary outcome was death or neurodevelopmental impairment (NDI) at 18-24

months. Secondary outcomes included death, chronic lung disease (CLD), and NDI.

Multivariable logistic regression (MLR) analysis and marginal structural models (MSM) were

used to adjust for perinatal and postnatal confounders.

Results: Of 754 infants with PDA, 184(24%) underwent ligation. Compared with medically

treated infants, ligated infants had similar odds of death/NDI (aOR 0.83, 95%CI:0.52-1.32), NDI

(aOR 1.27, 95%CI:0.78–2.06), and CLD (aOR 1.36, 95%CI:0.78–2.39), but lower mortality

(aOR 0.09, 95%CI:0.04–0.21).

Conclusions: PDA ligation is not associated with adverse outcomes and may reduce mortality.

Previously reported associations of ligation with increased morbidity are likely due to bias from

confounding by indication, rather than a detrimental causal effect of ligation.

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Acknowledgements Thank you to Dr. Prakesh Shah, my supervisor, for outstanding guidance and mentorship during

all phases of this project.

I also sincerely thank my thesis committee members: Dr. Lucia Mirea, Dr. Patrick McNamara,

Dr. Joseph Kim and Dr. Linh Ly, for their constructive review, feedback and direction.

Thank you to Dr. William Benitz (Stanford University) and Dr. Catherine Birken (University of

Toronto) for their excellent reviews, to Dr. Paige Church and Dr. Ed Kelly for their

collaboration, and to my colleagues in the Department of Newborn and Developmental

Paediatrics, Sunnybrook Health Sciences Centre for their support.

Most importantly, a loving thank you and appreciation to my wife, Sharon, and our children for

their patience and enthusiasm. I am so grateful and this would not have been possible without

your strong support.

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Table of Contents

Preamble ......................................................................................................................................... 1

1 Background and Rationale ....................................................................................................... 2

1.1 Patent Ductus Arteriosus: Anatomy and Closure ............................................................ 2

1.2 Physiology and Continuum of the Ductal Shunt In Neonates .......................................... 2

1.3 Natural History of PDA in Preterm Infants ...................................................................... 4

1.4 Clinical Importance of the PDA in Preterm Infants ......................................................... 4

1.4.1 Cerebral Intraventricular Hemorrhage ...................................................................... 4

1.4.2 Necrotizing Enterocolitis .......................................................................................... 5

1.4.3 Chronic lung disease ................................................................................................. 6

1.4.4 Neurodevelopmental Impairment ............................................................................. 6

1.5 Diagnosis of Haemodynamically Significant PDA .......................................................... 8

1.6 Therapeutic Approaches to the PDA: Rationale and Options for Treatment .................. 9

1.6.1 Pharmacotherapeutic Agents for Ductal Closure ...................................................... 9

1.6.2 Universal Medical Prophylaxis for Patent Ductus Arteriosus ................................ 10

1.6.3 Early Medical Treatment of Asymptomatic PDA Diagnosed by Echocardiographic

Screening ............................................................................................................................... 11

1.6.4 Medical Treatment of Symptomatic PDA .............................................................. 11

1.6.5 Surgical PDA Ligation ............................................................................................ 12

1.7 Association of PDA Treatment and Adverse Outcomes ................................................ 12

1.8 Surgical Ligation and Health Outcomes: Methodological Issues in Previous Studies . 17

1.8.1 Confounding by Indication ..................................................................................... 17

1.8.2 Selection Bias.......................................................................................................... 19

1.9 The PDA Ligation Decision: Uncertainty, Practice Variability and the Urgent Need for

Clarity ........................................................................................................................................ 20

2 Methods ................................................................................................................................. 21

2.1 Design and Setting ......................................................................................................... 21

2.2 Management of PDA ...................................................................................................... 22

2.3 Outcomes and Assessment ............................................................................................. 23

2.4 Data Sources and Collection .......................................................................................... 26

2.5 Potential Confounders .................................................................................................... 28

2.5.1 Perinatal Covariates ................................................................................................ 28

2.5.2 Postnatal Morbidities Occurring Prior to Ductal Closure ....................................... 28

2.6 Statistical Analyses ........................................................................................................ 30

2.6.1 Multivariable Logistic Regression .......................................................................... 31

2.6.2 Marginal Structural Models .................................................................................... 32

2.6.2.1 Marginal Structural Models: Background ........................................................... 32

2.6.2.2 Assumptions of Marginal Structural Models ...................................................... 33

2.6.2.3 Marginal Structural Models, Stage 1 Analyses: Estimation of Stabilized Inverse

Probability of Treatment Weights ...................................................................................... 33

2.6.2.4 Marginal Structural Models, Stage 2 Analyses: Weighted Estimation of the

Effect of Ligation on Outcomes ......................................................................................... 36

2.6.3 Subcohort Analyses to Reduce Selection and Information Bias ............................ 36

2.6.4 Infants Lost To Follow-up ...................................................................................... 37

2.6.5 Sample Size ............................................................................................................. 38

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2.7 Research Ethics and Data Sharing ................................................................................. 38

3 Results ................................................................................................................................... 39

3.1 Multivariable Logistic Regression ................................................................................. 48

3.2 Marginal Structural Models ........................................................................................... 60

3.3 Infants Lost to Neurodevelopmental Follow-up ............................................................ 64

4 Discussion .............................................................................................................................. 67

4.1 Main Findings and Comparison to Previous Literature ................................................. 67

4.2 Strengths ......................................................................................................................... 72

4.3 Limitations ..................................................................................................................... 72

4.4 Implications for Practice ................................................................................................ 75

4.5 Implications for Future Research ................................................................................... 76

5 Conclusions ........................................................................................................................... 78

Appendix 1: Data Collection Form .............................................................................................. 87

Appendix 2: Covariate Definitions .............................................................................................. 90

Appendix 3: Multivariable Logistic Regression Model Results (full cohort, n=754) ................. 92

Appendix 4: Missing Data ......................................................................................................... 102

Appendix 5: Marginal Structural Model Stage 1 Results – Pooled Logistic Regression Analyses

(full cohort, n=754) ..................................................................................................................... 104

5.1 MSM Numerator Term: No covariates ....................................................................... 104

5.2 MSM Denominator Term: Antenatal and Perinatal Covariates Only ......................... 105

5.3 MSM Denominator Term: Antenatal, Perinatal and Postnatal Covariates Arising Prior

to Ductal Closure ..................................................................................................................... 110

5.4 MSM Weighted Logistic Regression Analysis: Ligation vs. Medical Treatment ...... 115

5.5 Distribution and Q-Q plot of the stabilized inverse probability of treatment weights

(sweight) for each outcome adjusted for antenatal, perinatal and postnatal covariates. ......... 116

vi

. List of Tables Table 1: Neonatal and neurodevelopmental outcomes from previous studies for infants with a

patent ductus arteriosus treated with surgical ligation compared with medical management only.

Table 2: Classification of the severity of neurodevelopmental impairment in infants.

Table 3: Echocardiographic classification of PDA haemodynamic significance.

Table 4: Antenatal and perinatal characteristics of cohort of extremely preterm infants with

clinically and echocardiographically significant patent ductus arteriosus.

Table 5: Morbidity arising during the NICU course, period of ductal patency (prior to surgical

ligation or medical closure), and during the at-risk period for surgical ligation in the full cohort

of infants (n=754).

Table 6: Neonatal and neurodevelopmental outcomes of ligated vs. medically treated infants

(full cohort, n=754) estimated using multivariable logistic regression.

Table 7: Antenatal and perinatal characteristics of ligated infants who had previously failed

cyclooxygenase inhibitor therapy and medically treated infants with persistent

haemodynamically significant PDA after cyclooxygenase inhibitor therapy (n=308).

Table 8: Morbidity arising during the NICU course, period of ductal patency (prior to surgical

ligation or medical closure), and during the at-risk period for surgical ligation for ligated infants

who had previously failed cyclooxygenase inhibitor therapy (COXI) and the subgroup of

medically treated infants with a haemodynamically significant PDA after cyclooxygenase

inhibitor therapy.

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Table 9: Neonatal and neurodevelopmental outcomes of ligated infants who had previously

failed cyclooxygenase inhibitor treatment vs. medically treated infants with haemodynamically

significant patent ductus arteriosus after cyclooxygenase inhibitor therapy (n=308), estimated

using multivariable logistic regression.

Table 10: Associations of ligation vs. medical treatment and the primary composite outcome of

death or moderate-severe neurodevelopmental impairment, and the secondary outcome of death

before discharge from NICU among cohorts restricted to survivors by postnatal age.

Table 11: Association between surgical ligation and neonatal and neurodevelopmental outcomes

for the entire cohort of infants (n=754) with a clinical and echo diagnosis of PDA, estimated

using stabilized inverse probability of treatment weights and marginal structural models.

Table 12: Association between surgical ligation and neonatal and neurodevelopmental outcomes

for the subgroup of infants (n=308) comprising ligated infants who had previously failed

cyclooxygenase inhibitor (COXI) treatment and medically treated infants with an

echocardiography confirmed HSPDA after COXI treatment, estimated using stabilized inverse

probability of treatment weights and marginal structural models.*

Table 13: Mean, standard deviation and Kolmogorov-Smirnov test results for stabilized weights

for each study outcome.

Table 14: Antenatal and perinatal characteristics, postnatal morbidity, ductal characteristics and

neonatal outcomes of surviving infants based on completion of neurodevelopmental evaluation at

follow-up.

Table 15: Comparison of pooled adjusted odds ratios from previous meta-analysis where studies

adjusted for antenatal and perinatal covariates only vs. adjusted odds ratios for primary and

secondary outcomes from the current study.

viii

List of Figures

Figure 1: Directed acyclic graph depicting the relationship between physiological instability and

systemic inflammation, PDA ligation and the outcomes of death, severe neonatal morbidities and

neurodevelopmental impairment.

Figure 2: Flow diagram of infants included in the study.

Figure 3: Kaplan-Meier analysis of time to PDA closure of medically (red) versus surgically

(blue) treated infants over the first 10 weeks of life.

Figure 4: Average daily mean airway pressure (cmH2O) with 68% and 95% confidence intervals

(based on the standard error) over the first 40 days of life for medically treated (blue solid line)

vs. ligated infants (red dashed line) prior to ductal closure. Infants no longer contributed data

after the date of ductal closure, leading to the widening of confidence intervals over time as the

number of infants with persistent haemodynamically significant PDA diminished with time. The

median date of ligation was day of life (DOL) 29 (vertical interrupted line), with interquartile

range [DOL 22, DOL 38] (solid grey box). The earliest date of ligation was on DOL 7.

Figure 5: Average daily mean airway pressure (cmH2O) with 68% and 95% confidence intervals

(based on the standard error) over the first 40 days of life and prior to ductal closure for the

subcohort of medically treated infants with echo-proven significant PDA after cyclooxygenase

inhibitor treatment (blue solid line) vs. ligated infants who had previously failed cyclooxygenase

inhibitor treatment (red dashed line). Infants no longer contributed data after the date of ductal

closure, leading to the widening of confidence intervals over time as the number of infants with

persistent haemodynamically significant PDA diminished with time. The median date of ligation

was day of life (DOL) 29 (vertical interrupted line), with interquartile range [DOL 22, DOL 38]

(solid grey box). The earliest date of ligation was on DOL 7.

Figure 6: Kaplan-Meier analysis of time to PDA closure for ligated infants who had previously

failed cyclooxygenase inhibitor therapy vs. medically treated babies who had

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echocardiographically significant PDA after cyclooxygenase inhibitor treatment but who were

not treated with ligation.

Figure 7: Kaplan Meier analysis of survival in ligated vs. medically treated infants in the

subcohort with echocardiography proven HSPDA after COXI therapy that were alive at day 20

of life. Infants were censored if discharged home or transferred to a community NICU. Ligated

infants (top red line) had significantly greater survival than medically treated infants (bottom

blue line) (Log-Rank test, p

x

List of Appendices

Appendix 1: Data Collection Form

Appendix 2: Covariate Definitions

Appendix 3: Multivariable Logistic Regression Model Results

Appendix 4: Missing Data

Appendix 5: Marginal Structural Model Results

xi

List of Abbreviations

ASQ Ages and Stages Questionnaire

BSID Bayley Scales of Infant Development

BW birthweight

CGA corrected gestational age

COXI cyclooxygenase inhibitors

CLD chronic lung disease

CP cerebral palsy

GA gestational age

GMFCS Gross Motor Functional Classification System

HSPDA haemodynamically significant patent ductus arteriosus

HUS head ultrasound

IPTW inverse probability of treatment weight

IUGR intra-uterine growth restriction

IVH intraventricular hemorrhage

MLR multivariable logistic regression

MSM marginal structural model

NDI neurodevelopmental impairment

NEC necrotizing enterocolitis

NICU neonatal intensive care unit

OR odds ratio

PDA patent ductus arteriosus

RCT randomized control trial

ROP retinopathy of prematurity

SD standard deviation

SGA small for gestational age

SNAP II Score for Neonatal Acute Physiology II

VCP vocal cord paresis

VILI ventilator induced lung injury

1

Objective: Does patent ductus arteriosus ligation in preterm infants increase death or

neurodevelopmental impairment compared with medical management?

Preamble

At the end of 20th

century, the clinical management of patent ductus arteriosus (PDA) in

extremely preterm infants was commonly characterized by aggressive treatment aimed at

achieving rapid postnatal closure, often within the first two weeks of life. The strong association

between PDA and adverse outcomes had fostered the ideology that an infants' duration of

exposure to ductal shunting should be minimized. Pharmacotherapeutic prophylaxis was

frequently administered at birth, and early echocardiographic screening and pharmacological

treatment aimed at ductal closure was provided. Infants with respiratory failure who

demonstrated clinical and echocardiographic signs of PDA uniformly received medical

treatment, and surgical ligation was promptly performed if medical closure failed or was

contraindicated.

Over the past decade, several large retrospective cohort studies have associated PDA ligation

with increased neonatal and neurodevelopmental morbidity, including chronic lung disease,

retinopathy of prematurity, cerebral palsy, cognitive deficits and hearing and visual impairments.

The publication of these studies has been associated with a secular trend toward a reduction in

treatment with surgical ligation for persistent symptomatic PDA. While the collective impact of

these studies has been to prompt concern about causing harm by treating with surgical ligation,

significant methodological shortcomings limit the validity of these studies, especially residual

bias due to confounding by indication.

As a result, neonatologists and paediatric cardiac surgeons frequently face significant

controversy with the PDA ligation decision; clinicians must navigate high impact literature

associating PDA surgery with increased morbidity, yet fraught with biases that make the true

risks, and any benefits, of ligation uncertain.

2

1 Background and Rationale

1.1 Patent Ductus Arteriosus: Anatomy and Closure

The ductus arteriosus (DA) is the fetal vascular connection between the main pulmonary artery

and the descending aorta. The DA is one of several normal developmental mechanisms that

divert oxygen-replete blood away from the high resistance pulmonary circuit to the systemic

circulation. After birth, an abrupt decrease in circulating prostaglandins combined with increased

arterial oxygen tension leads to ductal vasoconstriction and DA closure in nearly all term infants

within the first week of life.

In contrast, delayed postnatal closure of the ductus arteriosus, termed patent ductus arteriosus

(PDA), occurs in up to 60% of infants born at less than 29 weeks gestational age.1, 2

In preterm

infants, the anatomical characteristics of the DA and physiological pathways that facilitate ductal

closure are immature. Relative to the muscular DA that is groomed for rapid vasoconstriction in

term infants; the preterm DA is comparatively less muscular and has reduced sensitivity of the

metabolic pathways of ductal vasoconstriction and closure.

1.2 Physiology and Continuum of the Ductal Shunt in

Neonates

During fetal life, low systemic vascular resistance (SVR) due to the low resistance placenta,

combined with elevated pulmonary vascular resistance (PVR) result in pulmonary artery – to –

aorta ('right to left') flow across the ductus arteriosus (DA). During normal neonatal transition,

increased SVR associated with umbilical cord clamping occurs alongside a decrease in PVR

precipitated by ventilation in air and increased arterial pressure of oxygen (PaO2). The previously

right-to-left ductal shunt becomes balanced (symmetrically bidirectional) at 5 minutes after birth,

mostly left to right by 10-20 minutes, and entirely left-to-right by 24 hours of age.3, 4

3

In preterm neonates, the ductal shunt is variable in direction, reflecting the effects of underlying

disease states on pulmonary and systemic haemodynamics. The shunt may be conceptualized as

residing on a continuum between life-saving conduit, neutral bystander and pathological entity.

In infants with critical congenital heart disease or myocardial dysfunction, patency of the ductus

arteriosus (DA) may be necessary for adequate pulmonary blood flow (e.g., tricuspid atresia) or

systemic blood flow (e.g., critical aortic stenosis). In severe persistent pulmonary hypertension of

the newborn (PPHN), the postnatal failure of optimal vasorelaxation of pulmonary arterioles

(e.g., due to asphyxia, respiratory distress syndrome, etc) results in persistently high PVR and

persistence of the right-to-left ductal shunt. The right-to-left shunt may reduce right ventricular

afterload and support post-ductal systemic blood flow, albeit with deoxygenated blood. A

bidirectional shunt in milder cases of PPHN may play a neutral role, merely permitting the non-

invasive estimation of the systemic-pulmonary pressure gradient.

If the ductus arteriosus remains patent after birth, preterm infants who experience the expected

fall in PVR may be susceptible to the effects of a large systemic-to-pulmonary (left-to-right)

shunt. Blood flows across the PDA continuously in systole and diastole, resulting in volume

overload of the pulmonary artery, pulmonary veins, and left heart. Shunt volume (Q) is directly

proportional to 4th

power of the ductal radius (r), and the aorto-pulmonary pressure gradient

( , and is inversely proportional to the ductal length (L) and blood viscosity ( ).

Increased pulmonary blood flow may lead to alveolar edema, reduced pulmonary compliance,

increased need for respiratory support, and pulmonary hemorrhage. Increased blood flow to the

left heart results in dilatation and increased end-diastolic pressures in the left ventricle and

atrium. Ductal diastolic 'steal' from the descending aorta, shorter diastolic (and coronary

perfusion) times due to tachycardia, and increased myocardial oxygen demand may result in

subendocardial ischemia and mesenteric hypoperfusion.

4

1.3 Natural History of PDA in Preterm Infants

The natural history of PDA in preterm infants has been described in small cohorts of infants for

who echocardiographic assessment of PDA was performed but no pharmacological or surgical

treatment was administered. While the merits of exclusively conservative management of the

PDA cannot be elucidated without an adequate comparator group (of infants treated for PDA),

these studies provide insight into the short and long-term likelihood of spontaneous closure.

In a retrospective cohort study of 103 extremely preterm infants who did not receive PDA

treatment, 91 survived beyond 72 hours, of whom 70 had an echocardiographic diagnosis of

PDA on day 3 of life.5 Of these, 51 (73%) experienced spontaneous ductal closure prior to

discharge home. In very low birthweight infants discharged home from the NICU with PDA,

most (87 – 93%) infants undergo spontaneous closure in infancy6 or early childhood.

7

1.4 Clinical Importance of the PDA in Preterm Infants

Although the natural history of the PDA is toward spontaneous closure, the clinical importance

of the PDA in preterm infants is underscored by its incidence and consistent association with

adverse neonatal outcomes. PDA is common in preterm infants, occurring on the third day of

life in up to 65% of preterm infants born at gestational age < 29 weeks.8 Persistent patency on

day 3 of life identifies infants at higher risk of all major complications and morbidities of

prematurity, including death, intraventricular hemorrhage (IVH), chronic lung disease (CLD),

necrotizing enterocolitis (NEC) and retinopathy of prematurity (ROP).8 While the association of

PDA with adverse outcomes is strong, causation remains unestablished. Clinical trials aimed at

facilitating closure of the symptomatic PDA have failed to demonstrate a reduction in severe

morbidities of prematurity. However, inconsistency across studies in the definition of PDA

haemodynamic significance has led to concerns of reduced validity.9

1.4.1 Cerebral Intraventricular Hemorrhage

Early neonatal cardiorespiratory instability is associated with germinal matrix bleeding and

potential extension into the ventricular system (intraventricular hemorrhage (IVH)) and/or

5

periventricular hemorrhagic infarction (PVHI). Mild and severe IVH are associated with

progressively higher odds of moderate-severe NDI compared with no IVH.10

Cerebral ischemia-

reperfusion injury is a possible pathophysiology. Most (90%) IVH occurs in the first week of

life, corresponding with the emergence of the left-to-right PDA shunt and associated increased

left heart volume loading and cerebral (pre-ductal) perfusion. The coincidental timing of PDA

and IVH suggests that the PDA may contribute to reperfusion injury in preterm infants. A

potential causal role for PDA is supported by trials that have repeatedly and conclusively

demonstrated a reduction in IVH and symptomatic PDA with the administration of prophylactic

indomethacin, possibly by mitigating the emergence of a significant ductal shunt. However,

prophylactic ibuprofen has been demonstrated to facilitate early ductal closure without reducing

IVH. The disparity between indomethacin and ibuprofen in IVH prevention suggests that either

early PDA closure is not the causal mechanism of IVH prevention or that differences in the

administration of the COXI (eg. timing, dose) may result in similar early PDA closure but a

differential impact on IVH. Taken together, both the mechanism of action of indomethacin in

IVH prevention and a causal relationship between PDA and IVH remain speculative. 11, 12

1.4.2 Necrotizing Enterocolitis

Necrotizing enterocolitis (NEC) is an inflammatory intestinal condition of preterm infants with a

multifactorial pathophysiology influenced by genetic predisposition, intestinal immaturity and

ischemia, imbalance in microvascular tone, abnormal intestinal microbial colonization, and

mucosal immunoreactivity.13

Infants with severe NEC develop a profound systemic

inflammatory response. Mortality is high and survivors have a high incidence of

neurodevelopmental impairment.14

A causal role for PDA in the development of NEC is supported by results of a randomized

controlled trial of early prophylactic surgical PDA ligation vs. conservative management which

demonstrated reduced NEC in the interventional arm, though the incidence of NEC in the non-

interventional arm was atypically high.15

Subsequent observational studies have strongly

associated PDA with the development of NEC in extremely preterm infants.8, 16, 17

However,

6

placebo-controlled trials of PDA treatment have failed to demonstrate a reduction in NEC

despite achieving ductal closure.18

The pathophysiological role of PDA in the development of NEC is not well understood, but is

thought to be mediated by intestinal hypoperfusion. Patent ductus arteriosus is associated with

diastolic flow reversal in the abdominal aorta, celiac artery and superior mesenteric artery, and

with reduced mesenteric tissue oxygenation.19

1.4.3 Chronic lung disease

Preterm infants born before 32 weeks gestational age who require positive pressure ventilation or

supplemental oxygen at 36 weeks corrected gestational age are assigned a diagnosis of moderate-

to-severe chronic lung disease. A large ductal shunt leads to pulmonary overcirculation, alveolar

edema, decreased pulmonary compliance and an increased need for invasive mechanical

ventilation. Longer exposure to the ductal shunt and larger PDA shunt volumes, as assessed by

echocardiography, have been associated with increased mortality and CLD, supporting a

pathological role of the PDA.20, 21

However, placebo-controlled trials of PDA treatment have

failed to demonstrate a reduction in CLD despite achieving ductal closure. Although trials have

been criticized for suboptimal patient selection and open-label treatment, these results suggest

that ductal closure may not modify the increased risk of CLD associated with PDA.22

1.4.4 Neurodevelopmental Impairment

Among infants born extremely preterm, death or neurodevelopmental impairment (NDI) is

estimated to occur in up to 50%.23-25

NDI is commonly defined as a composite outcome which

includes neuromotor impairment (e.g., cerebral palsy), neurocognitive impairment (cognitive or

language delay) and neurosensory impairment (hearing, vision or both).25, 26

Cerebral

injury/dysmaturation and subsequent NDI is likely the final common pathway after

cardiorespiratory instability leads to hypoxia-ischemia-reperfusion injury, inflammation, or

arrested development of sensitive, immature white and grey matter (Figure 1).27-35

Isolated

7

cerebral specific injury (e.g., isolated arterial thrombo-embolic stroke, meningitis without

associated sepsis) is uncommon.

Predictive models have identified risk factors for death and NDI at different time points during

NICU care. At birth, GA, birthweight (BW), multiple gestation, antenatal corticosteroids, intra-

uterine growth restriction (IUGR) and gender are the most important prognostic perinatal risk

factors for death and NDI.23, 36

Postnatally, large PDA and sepsis are significantly associated

with NDI.37, 38

At the time of NICU discharge, neonatal morbidities of prematurity such as CLD,

retinopathy of prematurity (ROP), sepsis and major brain injury can reliably predict NDI at 18-

24 months corrected age.23, 39

Figure 1: Directed acyclic graph depicting the relationship between physiological instability

and systemic inflammation, PDA ligation and the outcomes of death, severe neonatal morbidities

and neurodevelopmental impairment (From Weisz and McNamara, J Clin Neo 201440

).

BPD, bronchopulmonary dysplasia; ROP, retinopathy of prematurity

8

1.5 Diagnosis of Haemodynamically Significant PDA

Clinical examination is a common mechanism for the initial suspicion of PDA in preterm infants.

The clinical signs of PDA in preterm infants are related to the physiological effects of left heart

volume loading and diastolic 'steal' from the aorta to the pulmonary artery. The precordium is

active and a holosystolic murmur is present, often loudest at the upper left sternal border. This

may be accompanied by a wide pulse pressure, bounding peripheral pulses and diastolic

hypotension. Increased pulmonary blood flow reduces pulmonary compliance leading to a need

for more supplemental oxygen, increased work of breathing and ventilator support.

Echocardiography (echo) is the primary method for the definitive diagnosis of

haemodynamically significant PDA (HSPDA). The PDA can be easily and reliably imaged and

PDA severity may be classified as haemodynamically significant or non-significant. In general,

the haemodynamic significance of the PDA can be considered the interplay between the ductal

shunt volume and the compensatory capacity of the systemic and pulmonary circulations.

Studies have demonstrated that PDA size ≥ 1.5mm on the first day of life predicts a subsequent

symptomatic PDA41-43

and correlates well with Doppler flow pattern in assessments of

haemodynamic significance.44

The narrowest PDA diameter is typically recorded on all

echocardiograms and ductal diameter < 1.5mm is generally considered small and not

haemodynamically significant. Echo indices of left heart volume loading, such as left ventricle

and left atrium dilatation, and ratio of the left atrium to the aortic root (LA:Ao) correlate with the

need for treatment. LA:Ao ratio > 1.4 has a high sensitivity for HSPDA.

With each echocardiogram performed, a summary classification for the PDA haemodynamic

significance (large, moderate, or small) is provided that incorporates echo data such as ductal

size and flow pattern, indices of left heart volume loading and pulmonary overcirculation and

systemic arterial diastolic flow reversal. The echocardiographic significance of the PDA, and

longitudinal exposure to a HSPDA correlates with key neonatal outcomes such as CLD.20, 21, 45

9

1.6 Therapeutic Approaches to the PDA: Rationale and Options for

Treatment

The consistent association of PDA with adverse outcomes has been the impetus for treatment

aimed at ductal closure despite a lack of evidence from clinical trials to demonstrate that

pharmacological or surgical ductal closure mitigates these outcomes. Methods to close or

minimize the effects of a PDA include conservative management (e.g. fluid restriction, diuretics,

ventilation strategies), cyclooxygenase inhibitors (COI) such as indomethacin or ibuprofen, or

surgical ligation.46

1.6.1 Pharmacotherapeutic Agents for Ductal Closure

Failure or delay of ductal closure in preterm infants occurs, in part, due to elevated early

postnatal levels of circulating prostaglandin E2 (PGE2), which is produced from membrane

phospholipids by enzymes such as prostaglandin H synthase (PGHS) complex, and prevents

ductal vasoconstriction. Pharmacotherapeutic strategies aimed at ductal closure have targeted the

cyclooxygenase (COX) and peroxidase (POX) moieties of PGHS to effect a reduction in

circulating prostaglandins and PDA closure.

Indomethacin and ibuprofen are the most studied and commonly used cyclooxygenase inhibitors

(COXI) to facilitate ductal closure. Commonly used treatment regimens include indomethacin

0.2mg/kg intravenously (IV) every 12 hours for three doses and ibuprofen 10mg/kg followed by

two additional 5mg/kg doses 24 hours apart. Ibuprofen (administered via intravenous or oral

routes) has similar efficacy in ductal closure as indomethacin, but with fewer adverse events

such as renal insufficiency or necrotizing enterocolitis, though these results may have been

influenced by heterogeneity in GA, dosages and route of administration.47

Prolonged courses of

indomethacin, or the administration of a total indomethacin dose > 0.6mg/kg has been associated

with increased necrotizing enterocolitis, without improved rates of ductal closure.48

Oral and

intravenous ibuprofen have similar efficacy for ductal closure and adverse events.47, 49, 50

COXI administration for PDA closure has been associated with a transient reduction in cerebral

and splanchnic blood flow, oliguria, weight gain, hyperbilirubinemia and gastrointestinal injury.

10

Therefore, renal failure, severe jaundice, intestinal perforation and necrotizing enterocolitis are

contraindications to COXI administration.

Since 2012, two randomized controlled trials of acetaminophen have demonstrated similar

efficacy to ibuprofen in the early treatment of PDA in preterm infants, though trials have

enrolled relatively mature infants.51, 52

1.6.2 Universal Medical Prophylaxis for Patent Ductus Arteriosus

Patent ductus arteriosus prophylaxis is defined as the routine administration of a COXI within

the first 6-12 hours of life without the use of screening echocardiography. Prophylactic

indomethacin (0.1mg/kg daily for three days) reduces the incidence of severe intraventricular

hemorrhage, periventricular leukomalacia, pulmonary hemorrhage, symptomatic PDA and the

need for surgical PDA ligation.11, 53

However trials evaluating prophylactic indomethacin

employed 'back-up' treatments for subsequent symptomatic PDA, and therefore only provide a

comparison between prophylactic and symptomatic treatment vs. symptomatic treatment alone.

The broader use of prophylactic indomethacin decreased after publication of the Trial of

Indomethacin Prophylaxis in Preterm infants (TIPP) which reported no difference in the primary

outcome of death or neurodevelopmental impairment at 18 months corrected postnatal age in

extremely low birthweight infants.43

However, the clear short-term benefits, coupled with a lack

of demonstrable harm at long-term follow-up and the known suboptimal reliability of

neurodevelopmental testing at 18 months, has led to its continued use in some centres. Studies

that have followed infants to school age have shown an improvement in neurodevelopment in

boys who received indomethacin prophylaxis.54

A recent small randomized placebo-controlled trial reported that prophylactic acetaminophen

accelerated PDA closure in moderately preterm infants, but did not affect neonatal outcomes.55

11

1.6.3 Early Medical Treatment of Asymptomatic PDA Diagnosed by Echocardiographic Screening

Given the high rate of spontaneous ductal closure (30-40% in preterm infants born at less than 29

weeks gestational age), indiscriminate administration of indomethacin prophylaxis has been

criticized for subjecting a large minority (up to 46%) of infants to treatment for whom there is no

benefit.53

An alternative strategy is to employ early echocardiographic screening for PDA with

the targeted administration of treatment to infants at risk of failure of spontaneous early ductal

constriction. Recent trials have reported that early asymptomatic treatment reduces the risk of

pulmonary hemorrhage and subsequent symptomatic PDA, but without improvement in neonatal

outcomes. Van Overmeire et al. randomized 127 preterm infants (GA 26 – 31 weeks) with PDA

diagnosed by echocardiography to early (Day 3) or late (Day 7) treatment with indomethacin.

The infants treated early had greater PDA closure but incurred more short-term side effects

(lower urine output, higher serum creatinine, higher FiO2 and mean airway pressure) and adverse

events (composite of mortality, NEC, or cystic periventricular leukomalacia (PVL)).56

Another

placebo-controlled trial of targeted early PDA treatment with indomethacin after

echocardiographic screening in the first 12 hours of life demonstrated a reduction in early

pulmonary hemorrhage in the treatment group, but there was no difference in the rate of all

pulmonary hemorrhage and the study was underpowered to detect differences in neonatal

outcomes.57

1.6.4 Medical Treatment of Symptomatic PDA

For infants with clinical signs of PDA (murmur, wide pulse pressure, active precordium,

cardiomegaly, etc), contemporary trials have primarily compared the timing of therapy (early

[Day 2-5] vs. moderately late [day 7-14]). This comparison was performed by randomizing

infants to early COXI treatment vs. placebo, with later treatment as 'backup' for persistent PDA.

Unfortunately, this methodology does not permit the evaluation of PDA treatment vs. no

treatment, and thus the benefits/sequelae of treatment remain uncertain.

Recent trials have evaluated whether infants should be treated at the first clinical signs of PDA

vs. a delayed approach with later treatment. Overall, the early treatment of clinically

symptomatic PDA (at Day 3) increases PDA closure rates but may also increase adverse effects

12

without improving neonatal outcomes (e.g., mortality, CLD), compared with moderately late

treatment at Day 7 to 14.58

In addition, treatment in the 'late' group was more selective, with only

45% of control infants receiving open-label treatment. The efficacy of PDA treatment after the

first 2 weeks of life, and its impact on neonatal outcomes, has not been evaluated.

1.6.5 Surgical PDA Ligation

Surgical PDA ligation provides definitive ductal closure, and is usually only considered when

medical treatments have either failed or were contraindicated.59

Factors associated with the

decision to treat a PDA with surgical ligation, include clinical signs of respiratory failure and/or

systemic hypoperfusion, and echocardiographic criteria.60, 61

Failure to wean an infant with PDA

from invasive mechanical ventilation is the sine qua non of considering referral for surgical

ligation.

Ligation is performed under general anaesthesia with the infant supported by invasive

mechanical ventilation. The surgical approach comprises a left lateral thoracotomy, retraction of

the left lung, and the application of a ligature or clip to the PDA. Surgical mortality is low.62

Immediate surgical complications include bleeding, pneumothorax, chylothorax, inadvertent

ligation of a primary bronchus or branch pulmonary artery and vocal cord paresis (VCP). Left

VCP occurs due to intra-operative injury to the left recurrent laryngeal nerve in 5-50% of

infants.186,187

Post-operatively, preterm infants are at risk of a low cardiac output state known as

'post ligation cardiac syndrome' (PLCS) which likely occurs due to increased LV afterload.

1.7 Association of PDA Treatment and Adverse Outcomes

Over the past decade, several large cohort studies have reported increased neonatal morbidity

and NDI in early childhood among infants treated with PDA ligation compared to medical

management alone.25, 26, 63-67

A recent systematic review and meta-analysis of randomized trials

and controlled observational studies demonstrated higher CLD, ROP, and NDI in ligated

compared to medically-treated infants.68

In contrast, mortality was lower in ligated compared to

medically-treated infants.68

13

In a large retrospective cohort study of preterm infants born < 32 weeks GA with a symptomatic

PDA, Mirea et al compared neonatal outcomes according to PDA treatment assignment.63

After

adjustment for antenatal and perinatal confounders, infants treated with surgical ligation had

lower mortality but higher odds of CLD and ROP, compared with infants treated with medical

management alone (Table 1). Similarly, in a retrospective review of 426 extremely low (

14

epoch were less likely to be treated with surgery (66% vs. 100%) and had less NDI (aOR 0.07,

95% CI 0.00-0.96).69

Aspects of PDA ligation that have been postulated to contribute to the risk of NDI include

surgical and anaesthesia effects, and post-operative haemodynamic compromise. Vocal cord

paresis is a common surgical complication and is associated with an increased risk of death,

extubation failure and chronic lung disease, need for gastrostomy tube, and gastroesophageal

reflux disease.62, 70-72

Recent studies have associated use of halothane gases for anaesthesia in

young children with NDI.73, 74

Preterm infants are at risk of post-operative hypotension and

cardiogenic shock due to PLCS, which may result in cerebral hypoperfusion and injury.60, 61, 75-78

In light of concerns regarding NDI and neonatal morbidities, the safety of PDA ligation has

recently been questioned.69, 79-83

Concerns regarding the association of ligation and increased

CLD, severe ROP and NDI have driven a secular trend toward a reduction in infants being

treated with surgical ligation in North American centres.84, 85

However, some studies have

estimated lower mortality among infants with a PDA treated with ligation compared to medical

management alone (Table 1).

15

Table 1: Neonatal and neurodevelopmental outcomes reported for infants with a patent ductus arteriosus treated with surgical ligation

compared with medical management only.

Study Characteristics Odds Ratios (95% Confidence Intervals)

Death or NDI Death NDI Severe ROP CLD

Kabra

2007*

ELBW infants with symptomatic PDA

PDA ligation (n=110)

Medical only (n=316)

1.55

(0.97–2.50)

0.56

(0.29–1.10)

1.98

(1.18–3.30)

2.20

(1.19–4.07)

1.81

( 1.09–3.03)

Madan

2009†

ELBW infants with PDA

Primary ligation (n=135), Indo only

(n=1525), Indo and ligation (n=775)

No treatment (n=403)

1.03

(0.80 – 1.33)

0.46

(0.35– 0.62)

1.53

(1.16-2.03) -

3.10

(2.26-4.26)

Clyman

2009‡

Post-hoc analysis of RCT comparing

early prophylactic ligation (n=40) vs.

delayed selective ligation (n=44) in

ELBW infants

- 1.15

(0.48–2.78) - -

3.79

(1.10-13.11)

Mirea

2012§

Infants with GA ≤ 32 weeks with a PDA.

Conservative (n=577), Indo only

(n=2026), Indo+ligation (n=626),

Primary ligation (n=327)

- 0.41

(0.31–0.54) -

1.91

(1.51–2.41)

2.30

(1.91–2.77)

Janz-

Robinson

2015ǁ

Infants with GA ≤ 28 weeks. No PDA or

clinically insignificant PDA (n=826),

Pharmacological treatment only (n=569),

Ligation (n=78)

- - 2.87

(1.21–6.86)

10.7

(0.66–173) -

Bourgoin

2016¶

Infants with GA ≤ 28 weeks with PDA.

Conservative (n=505), Ibuprofen only

(n=248), Ligation (n=104)

- - 1.9

(1.1-3.1) - -

CLD, chronic lung disease; GA, gestational age; Indo, indomethacin; NDI, neurodevelopmental impairment; RCT, randomized

controlled trial; ROP, retinopathy of prematurity;

* Adjusted for antenatal steroids, gestational age at birth, sex, multiple births, mother's education, and total dose of indomethacin

received per kg of bodyweight between birth and discharge from the study center

https://en.wikipedia.org/wiki/Pilcrow

16

† Data shown for Indomethacin and Ligation vs. Indomethacin only. Adjusted for center, gestational age at birth, birthweight, gender,

prophylactic indomethacin, Apgar score, severe RDS, growth restriction, antenatal steroids, antenatal/postnatal infection, maternal

marital status and maternal age.

‡ Unadjusted odds ratios computed from randomized controlled trial.

§ Data shown for any ligation vs. no ligation. Adjusted for gestational age, antenatal steroids, multiple births, gender, small for

gestational age, Score for Neonatal Acute Physiology II. This data was provided by the primary author and is unpublished.

ǁ Compared ligation vs. no treatment (either no PDA or clinically insignificant PDA) for the outcome of bilateral blindness and

cognitive impairment > 2 standard deviations below mean. Adjusted for SGA, antenatal corticosteroids, multiple gestation, gender and

other perinatal confounders (not specified).

¶ Compared ligation vs. conservative management. Adjusted for gender, GA, birthweight Z-score, antenatal corticosteroids,

gestational hypertension, clinical chorioamnionitis, Apgar score, place of hospitalization, place of birth, year of birth, delivery

characteristics

https://en.wikipedia.org/wiki/Pilcrow

17

1.8 Surgical Ligation and Health Outcomes: Methodological

Issues in Previous Studies

Observational studies to date have associated PDA ligation with lower mortality but increased

neurodevelopmental impairment compared with medical management alone. The divergence of

these competing outcomes (Table 1) may be explained by several possible situations: First,

surgical ligation may improve the survival of infants with PDA but may be simultaneously

neurologically detrimental. Second, ligation may improve the survival of infants with PDA, but

the infants referred for ligation are at higher pre-ligation risk of NDI (confounding by indication

and increased pre-ligation illness severity). Finally, the decreased mortality may be a spurious

finding influenced by survival bias (where moribund infants with a PDA die before becoming

eligible for ligation), and the increase in NDI may be either a true detrimental effect of ligation

or the effect of confounding by indication.

1.8.1 Confounding by Indication

A serious concern in observational studies is bias arising when treatment assignment is not

independent of baseline prognostic factors. The methodology used in the observational studies

described above suggests the authors did not adequately address sources of treatment selection

bias. Multivariable analyses were conducted controlling for GA and other antenatal or perinatal

covariates (Table 1). This set of covariates, if complete, would be sufficient to balance baseline

prognostic factors for interventions that occur shortly after birth. However, PDA ligation

typically occurs several weeks after birth, and the interval accumulation of PDA related and

unrelated postnatal comorbidities influences both treatment assignment and outcomes. No

studies to date have addressed this time-dependent confounding by indication – that infants

referred for ligation may be more 'ill' and have larger ductal shunts at the time of the decision to

treat with surgery, compared with infants who are treated with medical management alone.

Illness severity, characterized by postnatal morbidities such as IVH and sepsis, and parameters of

physiological instability such as hypotension predict both neonatal morbidities and NDI.27, 29, 39,

86, 87 Severe IVH is a true confounder as it is associated with both PDA ligation and NDI

88-92, and

18

is not on the causal pathway. Although IVH has been found to occur after treatment with ligation

and indomethacin93

, most (90%) IVH occurs in the first week of life94

, preceding the timing of

surgical ligation reported by most studies.93, 95-98

Other studies have demonstrated that IVH does

not worsen after PDA treatment with indomethacin or ligation.12, 99, 100

Additional potential

postnatal confounders include hypotension, postnatal sepsis and NEC, which increase illness

severity and are associated with both PDA ligation and death or NDI.64, 87, 101-103

These factors,

however, can occur both before, during, or after PDA treatment and thus may be confounders in

some infants, and intermediates in others. Therefore, it is necessary to obtain data on the timing

of hypotension, IVH, NEC and sepsis, relative to surgical ligation to correct for possible bias due

to these potential confounders in multivariable analyses examining the impact of surgical PDA

ligation. Taking these time-dependent covariates into account would enable a more reliable

estimation of the causal effects of ligation, compared with medical management.

The intensity and duration of invasive mechanical ventilation is a particularly important

confounder in the association between ligation and adverse outcomes. Prolonged ventilator

dependence is a commonly used clinical criterion in the decision to treat with ligation (vs.

medical management) and also strongly predicts neonatal morbidities such as CLD, ROP and

NDI.104-107

The need for invasive mechanical ventilation reflects a combination of the effects of pulmonary

overcirculation and alveolar edema from the PDA and underlying respiratory insufficiency due

to severe respiratory distress syndrome, extreme prematurity and ventilator induced lung injury

(VILI). The relative independent contribution of the PDA to ongoing respiratory insufficiency is,

at present, difficult to quantify. Infants with similar echocardiographic indices of PDA

haemodynamic significance may have varying degrees of respiratory failure (severe to none)

owing to differences in nascent lung disease and tolerance of the increased pulmonary blood

flow from the ductal shunt. Infants with greater respiratory failure are more likely to be referred

for ligation, and the association of pulmonary insufficiency with adverse outcomes renders this a

key confounder. However, no studies to date have adequately accounted for the intensity and

duration of invasive mechanical ventilation support required prior to the administration of PDA

treatment.

19

1.8.2 Selection Bias

In previous studies26, 63, 96, 108

, survival bias may have influenced the reported lower mortality

among ligated infants. Ligation was generally undertaken later in life after failure of medical

therapy, meaning that ligated infants were more likely to have already survived the critical

period of high early neonatal mortality. This PDA treatment paradigm implies that some of the

sickest infants, treated initially with conservative management and/or indomethacin, may have

died prior to becoming 'eligible' for ligation, resulting in selection bias in assembling the cohort

of ligated infants. The possibility of survival bias is supported by studies which found no

difference in mortality between medically and surgically treated groups of infants who both

received treatments at a similar postnatal age.26, 63, 64, 109-112

If ligation truly improved survival (and survival bias was not present), then a delay or reduction

in rates of surgical treatment would be expected to increase mortality. Some studies have

demonstrated consistent mortality rates after moving to a delayed selective ligation strategy from

an early routine ligation strategy following indomethacin failure.69, 81

Several studies have

reported higher mortality rates in infants with persistent PDA refractory to medical therapy,

though the effect of ligation in potentially mitigating this risk was not assessed.113, 114

Taken

together, survival bias may be present and any beneficial effect of ligation on mortality remains

untested.

An additional form of selection bias may exist due to the common treatment paradigm where

medically treated infants must have a demonstrated persistent significant PDA after failure of

medical treatment in order to be considered eligible for ligation. Previous studies have uniformly

categorized infants by treatment assignment irrespective of ductal patency following medical

therapy. If infants whose PDA closed with medical therapy (and who were therefore never

considered for ligation) were healthier (e.g., lower illness severity and thus at lower risk of

adverse neonatal outcomes) than medically treated infants whose PDAs remained patent, then

the inclusion of the former may introduce bias in favour of the medically treated group.

20

1.9 The PDA Ligation Decision: Uncertainty, Practice

Variability and the Urgent Need for Clarity

Over the preceding decade, several large observational studies (Table 1) published in high-

impact peer-reviewed scientific journals have strongly associated PDA ligation with adverse

outcomes, with a resultant secular trend toward avoidance of PDA surgery. Wide practice

variability also exists, with rates of surgical ligation in Canadian centres ranging from 0% to

25% among infants with gestational age < 33 weeks.115

Altogether, the decision to refer a

preterm infant with PDA for ligation has evolved into a patient management conundrum, as

clinicians must navigate literature suggesting that surgery for PDA may be a choice between

death and neurodevelopmental impairment, but fraught with critical methodological biases, of

which confounding by indication is the most prominent. Neonatologists and paediatric cardiac

surgeons remain unable to answer the following key question: is PDA surgery associated with

neurodevelopmental impairment and other adverse outcomes?

21

2 Methods

The objective of this study was to evaluate the association of PDA ligation vs. medical

management on neonatal outcomes using a study design and analytical methods that addressed

the major sources of possible bias including confounding by indication and selection bias.

2.1 Design and Setting

This was a retrospective cohort study of extremely preterm infants born at GA 23+0

to 27+6

weeks treated in the three tertiary neonatal units in Toronto, Canada from January 1, 2006 to

December 31, 2012. All infants born at this gestation in Toronto were admitted to one of these

centres. The NICUs at Sunnybrook Health Sciences Centre (SB) and Mt. Sinai Hospital (MSH)

are tertiary perinatal centres that respectively admit over 150 and 250 extremely preterm infants

(both inborn and outborn) annually. The NICU at the Hospital for Sick Children is a quaternary

centre providing care to approximately 50 outborn extremely preterm infants annually and is the

referral centre for paediatric cardiac and general surgery (including all PDA ligations) and other

subspecialty care. Only extremely preterm infants were included in this study as PDA ligation is

uncommon in infants born after 28 weeks gestation.

Infants were included in the study if they had a clinically significant PDA and had undergone at

least one echocardiogram demonstrating a significant PDA, defined as a ductal diameter ≥

1.5mm. This requirement for an echocardiographic diagnosis of significant PDA was

incorporated to reduce selection bias by excluding medically treated infants with trivial or small

PDAs (diameter < 1.5mm) who would never have been candidates for ligation, as all ligated

infants would have had at least one echocardiogram demonstrating a haemodynamically

significant PDA (HSPDA). Infants with major congenital anomalies or other congenital heart

defects were excluded (except for small ventricular septal defects or small- moderate atrial septal

defects).

22

2.2 Management of PDA

Treatment for PDA was at the discretion of the attending neonatologist and typically occurred for

a clinically and echocardiographically significant PDA. Medical treatment aimed at facilitating

ductal closure (with COXI (intravenous indomethacin 0.2mg/kg q12h for 3 doses, or intravenous

ibuprofen 10mg/kg for the first dose, followed by 2 doses of 5mg/kg q24h) was used as first-line

therapy. Infants with a clinically and echocardiographically significant PDA refractory to

medical therapy, or in whom cyclooxygenase inhibitors were contraindicated (due to renal

failure, oliguria, active bleeding, severe thrombocytopenia, necrotizing enterocolitis, or

spontaneous intestinal perforation) were considered for surgical ligation. Oral acetaminophen

(15mg/kg/dose q6h for 3-7 days) was used for a very small number of infants (n=10) beginning

in late 2012 as rescue therapy for refractory PDA in infants being considered for surgical

ligation.

Infants in whom COXI had failed or were contraindicated may have been managed using

intensive care strategies to either mitigate or improve an infant's tolerance of the ductal shunt,

termed conservative management. These included the use of diuretics, fluid restriction, increased

positive end-expiratory pressure, or permissive mild acidosis or red-cell transfusion to increase

pulmonary vascular resistance. Conservative management may have been instituted as primary

therapy (i.e. without prior or subsequent COXI treatment).

The decision to refer an infant for PDA ligation was made by the attending neonatologist at each

site in conjunction with a dedicated team of neonatologists with expertise in targeted neonatal

echocardiography. Infants referred for ligation were reviewed and triaged according to clinical

and echocardiographic indices of PDA severity. At the time of ligation, infants at MSH or SB

NICUs were transported to The Hospital for Sick Children, which is the only site for PDA

ligation surgery. All infants were transported to the operating theatre in a heated isolette. Infants

were endotracheally intubated and supported with conventional mechanical ventilation. The

surgery was performed via a left lateral thoracotomy, an intra- or extrapleural approach, and the

PDA was attenuated using either a clip or ligature. Surgical approach and choice of clip or

ligature was at the discretion of the attending surgeon. Previous studies have reported no

23

significant differences in neonatal outcomes between an intra or extrapleural approach.116, 117

The

relative merits of the use of clip or ligature in preterm infants have had mixed reports, with one

study reporting reduced operation and anaesthesia times and decreased bleeding118

with clip

application and other studies suggesting increased risk of VCP with clip use.118-120

Post-

operative intensive care was supported by targeted neonatal echocardiography and the

administration of targeted milrinone treatment in infants with early post-operative critically low

cardiac output.76

Generally within a few days after the operation, infants from SB and MSH were

transported back to their original NICU for the remainder of their hospital stay.

2.3 Outcomes and Assessment

Infants who survived to hospital discharge underwent standardized developmental evaluations at

established intervals after discharge for the purposes of identifying NDI. All sites conducted an

18-24 month neurodevelopmental assessment using a combination of clinical examination, visual

and hearing assessment, and cognitive evaluation using the Ages and Stages Questionnaire

(ASQ) and/or Bayley Scales of Infant Development, Third Edition (BSID III). Clinical

examination and standardized motor assessments identified the presence or absence of cerebral

palsy, which was classified according to the Gross Motor Functional Classification System

(GMFCS).121

Cognitive and language impairments were assessed using the BSID and/or ASQ.

Across North America, the BSID III is considered the standard developmental assessment tool

for preterm infants at 18-24 months corrected gestational age and has been used in the majority

of large clinical trials where neurodevelopmental impairment was a defined clinical outcome.

Prior to 2008, the neurodevelopmental follow-up inclusion criteria and assessments differed

between the three sites. High-risk preterm infants at SickKids Hospital and Mt. Sinai Hospital

were assessed in the Neonatal Follow-Up Clinic at SickKids. All infants with birthweight <

1000g or GA < 26 weeks were assessed in follow-up. Infants with birthweight 1000-1500g with

at least one risk factor for NDI (chronic lung disease, IVH > Grade 1, or a high risk social

environment) were also routinely assessed in follow-up. The 18-24 month neurodevelopmental

assessment (prior to 2008) at SickKids comprised an ASQ and a Receptive-Expressive Language

Test. In comparison, at Sunnybrook Health Sciences Centre, prior to 2008 all infants < 30 weeks

24

at Sunnybrook Health Sciences Centre gestation had complete neurodevelopmental assessments

at 18-24 months, including a BSID III. After 2008, neurodevelopmental assessments were

standardized across all three sites and included all infants < 30 weeks GA and a BSID evaluation

at 18-24 months.

An anticipated potential source of measurement bias involves differences in the scoring results of

the BSID III and ASQ in the assessment of neurodevelopmental outcome. Correlation between

ASQ and BSID has been reported as moderate (r = 0.52 to 0.65) in preterm infants.122, 123

The

ASQ has moderate sensitivity (73-78%) and specificity (65-75%) for detecting NDI in the

population of extremely preterm infants assessed at 24 months CGA with the BSID.122-125

Several studies have revealed a systematic, significant elevation in the scores of infants tested

with the BSID III compared with the second edition (BSID II).126-128

Recently, a conversion scale

has been developed127

and a recent large RCT129

used a similar conversion method to compare

assessments performed using the different BSID editions. Similarly, for this study, a BSID III

score of 115 was taken as the mean, with a standard deviation of 15. Thus moderate-severe

neurocognitive impairment was defined by a BSID III cognitive and/or language score < 85.

The primary outcome was a composite of death or moderate-severe neurodevelopmental

impairment, evaluated at 18-24 months corrected gestational age. This primary outcome is

common to major neonatal trials.130

Moderate-severe NDI was defined as a composite of

neuromotor, neurocognitive and/or neurosensory impairment (Table 2) Secondary outcomes

included death before discharge from NICU, moderate-severe NDI, moderate-severe chronic

lung disease (defined as the need for oxygen or positive pressure ventilation at 36 weeks CGA)

and severe ROP (defined as the need for laser surgery or intravitreal vascular endothelial growth

factor inhibitor).

25

Table 2: Classification of the severity of neurodevelopmental impairment in infants.

Domain / Test Normal Mild Impairment Moderate Impairment Severe Impairment

Neuro-

motor Clinical Examination No impairment GMFCS 1-2

121 GMFCS 3-5

Neuro-

cognitive

ASQ Subtests:

(1) Problem

solving

(2) Personal-social

(3) Communication

At most one subtest

score 1-2 SD below

mean, and

remaining subtests

within 1 SD of the

mean

Two or more subtests

score 1-2 SD below

mean, and no subtest

with more than 2 SD

below the mean

1 or 2 subtests more than

2 SD below mean

All subtests more than 2

SD below mean

BSID III Subtests:

(1) Cognitive

(2) Language

Both subtests with

score ≥ 100

Any subtest with score

85-99 but not < 85

Any subtest with score

26

2.4 Data Sources and Collection

Each site maintains a comprehensive database of admitted infants through the Canadian Neonatal

Network that includes all demographic and clinical information, including gestational age,

birthweight and all neonatal morbidities such as death, PDA, CLD, NEC, and ROP. All infants ≤

27+6

weeks GA with a clinical diagnosis of PDA were identified from the CNN database. Each

infant's echocardiography reports, available on the patient chart or electronically available on the

hospital echocardiography database, were reviewed to identify the subset of infants with an

echocardiographically significant PDA, defined as at least one echocardiogram with PDA

diameter ≥ 1.5mm. The hospital records of all eligible infants were reviewed to abstract

antenatal, neonatal, and outcome data (Appendix 1).

The date and PDA haemodynamic significance of all echocardiograms were recorded from

echocardiography reports to define the onset and longitudinal course (duration and severity) of

each infant’s exposure to a ductal shunt. When surrogate markers of ductal shunt volume were

available in the echocardiogram report, these were used to determine the PDA haemodynamic

significance (Table 3). In cases where this data was unavailable on the echo report, the summary

interpretation of haemodynamic significance (large, moderate-large, moderate, small-moderate,

or small) provided by the interpreting Cardiologist or Neonatologist was recorded.

The dates of echocardiographic and/or clinical PDA closure were recorded. For ligated infants,

the date of PDA closure was recorded as the date of surgical ligation. For medically treated

infants, echocardiographic closure was recorded as the earliest date of the echocardiogram

demonstrating ductal closure, without subsequent re-opening. Clinical closure was determined as

the earliest date of disappearance of established clinical signs of PDA (murmur, active

precordium, and bounding pulses) in an infant in whom these signs were definitively present. An

infant's PDA was deemed to be no longer haemodynamically significant if it underwent clinical

or echo closure or was 'small' or 'small-moderate' on echocardiography. For this study, a small-

moderate PDA was deemed to be 'non-significant' based on the assumption that infants with a

PDA shunt of this size would not be eligible for surgical ligation (whereas a 'moderate' PDA

would be eligible).

27

Table 3: Echocardiographic classification of PDA haemodynamic significance*

Haemodynamic

Significance

PDA

Diameter

(mm)

LV output (ml/kg/min) /

LV dilatation† LA:Ao ratio

Large ≥ 3.0 mm > 400

Severe dilatation (2.5 < Z-score) > 2.0

Moderate-large 2.1 - 2.9 > 300 but ≤ 400

Moderate dilatation (2.0 < Z-score ≤ 2.5) 1.5 - 2.0

Moderate‡ 1.5 - 2.0 ≥ 250 but ≤ 300

Mild dilatation (1.5 < Z-score ≤ 2.0)

Small‡ < 1.5 < 250

No dilatation (Z-score < 1.5) < 1.5

* Classification based on most severe parameter recorded

† Severity of LV dilatation determined by LV internal diameter in diastole z-score, or by

description in echocardiography report if z-score unavailable

‡ PDAs classified as 'small-moderate' in the echocardiography report retained this summary

classification if other echo parameters were unavailable to distinguish as either 'small' or

'moderate'

LA:Ao, left atrium to aortic root ratio; LV, left ventricle

28

2.5 Potential Confounders

In this observational study, the main threat to validity was confounding by indication, where the

'sickest' infants (characterized by higher intermediate neonatal morbidity and larger PDA), who

are at higher risk of adverse outcomes, were more likely to be assigned to PDA ligation.

Potential confounders in this study can be classified as antenatal/perinatal (occurring prior to or

during birth), or postnatal (occurring after the birthing period), as detailed below.

2.5.1 Perinatal Covariates

Data was abstracted on perinatal factors including gestational age (in completed weeks and

days), birthweight, antenatal corticosteroids ('full' course defined as two doses of intramuscular

betamethasone administered prior to delivery), multiple gestation, small for gestational age

(defined as birthweight < 10th

percentile for GA) and gender, mode of delivery, intensive

delivery room resuscitation, Apgar scores, indomethacin prophylaxis and Score of Neonatal

Acute Physiology II (SNAP-II).36, 131

2.5.2 Postnatal Morbidities Occurring Prior to Ductal Closure

This study focused significantly on the collection of data on the timing of onset, frequency and

severity of postnatal confounders. Factors associated with both PDA ligation and death/NDI

include NEC 103

, sepsis87, 101, 132

and IVH or cystic PVL30, 88-91, 94, 133, 134

. In addition, the need for

treatment with systemic corticosteroids135

, hypotension requiring inotropes136

and duration and

intensity of mechanical ventilation39, 105

are quantitative indicators of physiological instability

that are associated with subsequent PDA ligation as well as death/NDI, and may confound

analyses.

No previous study has systematically adjusted for postnatal confounders, and some studies have

inappropriately used NEC and IVH as outcomes of ligation, despite the fact that these

morbidities typically occur prior to ligation.63

IVH uniformly occurs in the first week of life, well

before the time when most infants are referred for ligation, and thus can be considered a true

29

confounder. However, sepsis, NEC, extended duration of mechanical ventilation and use of

systemic corticosteroids may occur before or after PDA ligation. It is important to account for

the timing of postnatal factors relative to the exposure of PDA ligation; therefore, in this study,

postnatal factors were used for adjustment only when they had occurred prior to ligation in

surgically treated infants, or prior to ductal closure in medically treated infants.

Accordingly, data on postnatal morbidities (defined in Appendix 2) was collected for each DOL

for all infants from birth until discharge from the NICU or the date of death, whichever occurred

first. Specifically, the date of onset of all morbidities was abstracted in order to include them as

time-dependent covariates and characterize the timing of these morbidities in relation to surgical

ligation and medical ductal closure. Morbidities collected included: NEC ≥ stage 2, systemic

dexamethasone administered for chronic respiratory failure to prevent chronic lung disease

(defined as a minimum 5 day course), seizures requiring anticonvulsant therapy, systemic

hypotension treated with inotropes, culture positive sepsis (defined as clinical sepsis

accompanied by the pure growth of a pathogenic organism from a sterile site), culture negative

sepsis (defined as clinical sepsis without a positive culture that was treated with at least 5 days of

systemic antimicrobials), administration of inhaled nitric oxide (indicative of severe oxygenation

failure and suspected or proven pulmonary hypertension), and spontaneous intestinal perforation.

In addition, the dates and results of all cranial ultrasounds were recorded from radiology reports,

including the determination of the severity and laterality of intraventricular hemorrhage

(according to the classification of Papile et al.) and presence of periventricular echogenicity or

cystic periventricular leukomalacia.

Daily ventilation information, including mode of ventilation (invasive positive pressure, non-

invasive positive pressure [non-invasive high frequency oscillatory ventilation (NiHFO), non-

invasive positive pressure ventilation (NIPPV), nasal continuous positive airway

pressure(nCPAP) or heated and humidified high flow nasal cannula (HHHFNC)], low flow

oxygen, or none) was abstracted. The most intensive mode of ventilation used on each calendar

day was recorded.

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Daily average mean airway pressure (MAP) was also collected. Estimation of the daily average

MAP was derived from the detailed respiratory therapist daily logs (providing hourly readings of

ventilator settings) and was performed as follows: if the MAP was unchanged for at least 80%

of the calendar day, then this MAP was assigned as the daily average MAP. When the MAP

fluctuated for more than 20% of the calendar day, the average daily MAP was estimated as the

average of the highest and lowest recorded MAPs for the calendar day.

For analyses, postnatal factors were considered time-dependent, and were used to represent

cumulative illness severity that was estimated for each infant on each DOL when infants were

considered “at risk” for ligation. For ligated infants, the "at risk" period comprised the days of

life prior to surgical ligation. Medically treated infants were considered "at risk" for ligation

during the period that the PDA haemodynamic significance was at least 'moderate'. The "at risk"

period was deemed to conclude at the time of death, PDA closure or no longer

haemodynamically significant, or transfer to a Level 2 NICU, whichever occurred first. The at-

risk-of-ligation period for medically treated infants with a persistent significant PDA was

truncated at DOL 60 as this was the latest postnatal day of surgery in the ligated group.

2.6 Statistical Analyses

Perinatal and postnatal characteristics were descriptively analyzed using the count and percent

for categorical variables, and the mean and SD or the median and inter-quartile range for

continuous measures. The distribution of perinatal and postnatal characteristics between ligated

and medically-treated groups was compared using the Chi-square test or Fisher's exact test for

categorical variables and the T-test or Wilcoxon Rank-Sum test for continuous variables, as

appropriate. Time to PDA closure for ligated vs. medically treated infants was compared using

the Kaplan-Meier analysis.

The association between PDA ligation and adverse outcomes was estimated using logistic

regression analyses. Initially, unadjusted analyses estimated crude odds ratios (ORs) and 95%

confidence intervals (CI) for the association between ligation and outcomes (Model 1). To adjust

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for possible confounding, two statistical approaches were applied: 1) multivariable logistic

regression and 2) marginal structural models.

2.6.1 Multivariable Logistic Regression

Multivariable logistic regression (MLR) analysis was used to estimate the adjusted odds ratios

(AORs) and 95% CI for the association of ligation and the primary and secondary outcomes,

while adjusting for potential confounders:

log (p/(1-p)) = α + β1x1 +... + βjxj

where p is the probability of the outcome of interest, α is the intercept, and βj is the

coefficient for covariate xj (representing the log OR)

A multivariable model (Model 2) was constructed using only antenatal and perinatal covariates,

to provide a comparison with the results of previous studies which only adjusted for these

confounders.

The third and final model (Model 3) included covariates representing postnatal morbidities that

occurred during the period an infant was at risk of ligation (defined as having a

haemodynamically significant PDA), in addition to the antenatal/perinatal confounders, in order

to account for postnatal preligation/preclosure illness severity.

Variable selection for the final model was determined by backward elimination. All covariates

were initially included in the model. Covariates with nonsignificant p-values (p>0.50) were

iteratively withheld from the model and removed if their absence did not result in a change in the

point estimate of more than 10%.137

Backward elimination procedures are considered superior to

forward approaches with regard to bias and root mean square error of regression coefficients,

though typically use more stringent significance level criterion (eg. α = 0.2).138

The use of a

broader significance criterion in this study (α = 0.5) is comparatively less likely to introduce

bias.137, 139

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Key assumptions for MLR were verified. Model fit was evaluated using the Hosmer-Lemeshow

statistic and influential outliers were identified using Pearson and Deviance residuals.

2.6.2 Marginal Structural Models

The association between PDA ligation and outcomes was also examined within the framework of

marginal structural models (MSMs). MSMs are used to estimate the causal effect of a time-

dependent exposure in the presence of time-dependent covariates that may be simultaneously

confounders and intermediate variables.140

In this observational study, where confounding by

indication was a likely major source of bias, MSMs can adjust for baseline and time-dependent

covariates influencing the assignment of an infant to PDA ligation treatment. As described

below, MSMs include 2 stages of analyses and provide unbiased estimates of causal parameters

provided that several assumptions are met.

2.6.2.1 Marginal Structural Models: Background

In the first stage of MSM analyses, a model is derived to estimate the probability of being

assigned to the treatment group (ligated or not ligated) that was observed for each infant,

conditional on available covariates. For each subject, the predicted probability of their observed

exposure is then used to compute a weight. In general, at a single time-point, for each subject i,

the weight wi is defined as:

wi = 1 / pr [Ai = ai | Zi = zi]

where Ai is the treatment assignment and Zi is the vector of covariates.

The weights obtained in stage 1 are referred to as inverse probability of treatment weights

(IPTW), and are used in stage 2 to perform weighted analyses.140

The effect of weighting is to

create a pseudo-population consisting of wi copies of each subject, in which the treatment

assignment is unconfounded by the measured covariates. Furthermore, the probability of the

event of interest given the treatment is the same in the pseudopopulation as in the original study

population.

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When covariates are strongly associated with treatment assignment, the conditional probability

of treatment assignment may vary greatly, and the weighted analysis may be dominated by a few

subjects with extremely large weights who contribute a very large number of copies of

themselves to the pseudopopulation. In this case, the IPTW estimator will have a large variance

and will fail to be approximately normally distributed. This variability can be mitigated by

replacing the weight wi with a 'stabilized weight' swi141, 142

computed as the product of wi and the

unconditional probability of treatment assignment:

swi = pr [Ai = ai] / pr [Ai = ai | Zi = zi]

2.6.2.2 Assumptions of Marginal Structural Models

Weighted analyses in stage 2 of MSMs will estimate unbiased causal effect parameters when the

conditions of exchangeability and positivity are satisfied, and the model is correctly specified.143

Exchangeability implies that there are no unmeasured confounders. While this is not testable in

observational studies, it relies on the identification of important confounders by expert

investigators. Positivity, also known as the experimental treatment assumption, is the condition

that there are both exposed and unexposed individuals at every level of each confounder, and this

can be assessed using the data. The assumption of an accurate exposure model specification is

required to ensure validity and precision of the final weighted parameter estimates. A necessary

condition for correct model specification is that the stabilized weights have an approximate mean

of one.144

Thus the mean and standard deviation of the estimated stabilized weights can be

examined for alternative model specifications (replacing linear terms with categories,143