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Hough, Judith, Shearman, Andrew, Liley, Helen, Grant, Caroline, & Schi-bler, Andreas(2014)Lung recruitment and endotracheal suction in ventilated preterm infantsmeasured with electrical impedance tomography.Journal of Paediatrics and Child Health, 50(11), pp. 884-889.

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https://doi.org/10.1111/jpc.12661

1

Title: Lung recruitment and endotracheal suction in ventilated preterm

infants measured with Electrical Impedance Tomography

Hough JL1,2,3, Shearman AD1,4, Liley H1,4, Grant CA5, Schibler A3

Affiliations 1Critical Care of the Newborn Program, Mater Research, South Brisbane, QLD, Australia 2School of Physiotherapy, Australian Catholic University, Banyo, QLD, Australia 3Paediatric Critical Care Research Group, Paediatric Intensive Care Unit, Mater Children’s

Hospital, South Brisbane QLD, Australia 4Department of Neonatology, Mater Health Services, South Brisbane, QLD, Australia 5Institute of Health and Biomedical Innovation, Queensland University of Technology,

Brisbane, QLD, Australia

Address for correspondence

Dr Judy Hough

Department of Physiotherapy

Mater Children’s Hospital

South Brisbane 4101 QLD Australia

Phone: +61 (7) 3163 1143 Fax: +61 (7) 3163 1642

e-mail: judith.hough@mater.org.au

Key Words: Electrical impedance tomography; Ventilation distribution; Suction; Infant,

newborn

Australia New Zealand Clinical Trials Registry: ACTRN12610000452099

2

What is already known on this topic:

1. Suctioning is a standard procedure in ventilated preterm infants and there are significant

associated risks.

2. Transient hypoxia and loss of lung volume are some of the documented associated

risks.

3. Electrical impedance tomography (EIT) is a non-invasive clinical tool which can be used

to measure end expiratory level (EEL) and ventilation distribution after suction in

neonates to enable assessment of lung volume loss with suction.

What this paper adds:

1. Despite significant derecruitment with ETT suction, there was rapid recovery resulting in

an increase in EEL immediately post suction. EEL remained increased for the next 2

hours, suggesting that the benefit of suction occurs much longer than many of the

previous studies have described.

2. There were regional differences in ventilation distribution suggesting heterogeneity in

the behaviour of the lung.

3. Retrospective analysis found that the mode of ventilation had little impact on changes in

EEL or ventilation distribution

3

ABSTRACT

Aims: Although suctioning is a standard airway maintenance procedure, there are significant

associated risks, such as loss of lung volume, due to high negative suction pressures. This

study aims to assess the extent and duration of change in end expiratory level (EEL) resulting

from endotracheal tube (ETT) suction and to examine the relationship between EEL and

regional lung ventilation in ventilated preterm infants with respiratory distress syndrome.

Methods: A prospective observational clinical study of the effect of ETT suction on twenty non-

muscle relaxed preterm infants with RDS on conventional mechanical ventilation in a Neonatal

Intensive Care Unit. Ventilation distribution was measured with regional impedance amplitudes

and EEL using electrical impedance tomography.

Results: ETT suction resulted in a significant increase in EEL post suction (p<0.01).

Regionally, anterior EEL decreased and posterior EEL increased post suction suggesting

heterogeneity. Tidal volume was significantly lower in volume guarantee compared with

pressure controlled ventilation (p=0.04).

Conclusions: ETT suction in non-muscle relaxed and ventilated preterm infants with RDS

results in significant lung volume increase which is maintained for at least 90 minutes.

Regional differences in distribution of ventilation with ETT suction suggest that the behaviour of

the lung is heterogeneous in nature.

Word count: 196

4

INTRODUCTION

Each year over 8000 infants born in Australia and New Zealand will require assistance with

their ventilation [1]. The presence of an endotracheal tube (ETT) stimulates increased mucus

production necessitating regular airway suction to remove secretions. Although suctioning is a

standard procedure used in airway maintenance, significant associated risks are well

documented. These risks include atelectasis and loss of lung volume due to high negative

suction pressures [2-5], hypoxaemia [6-8], cardiovascular instability and changes in

cerebrovascular volume [9, 10]

The physiological effect of suction is commonly monitored with pulse oximetry and blood gas

analysis. Both measurements are poor estimates of changes in ventilation distribution or lung

volume making it difficult to assess the degree of the suction induced lung collapse. Electrical

impedance tomography (EIT) has been used as a non-invasive clinical tool to measure end

expiratory level (EEL, as an equivalent to functional residual capacity) and ventilation

distribution after suction in neonates [11, 12].

It has been shown that after endotracheal suction there is a transient hypoxia and a loss of

lung volume. These parameters recover within the first few minutes after reconnection of the

ETT and it has been speculated that maintaining spontaneous breathing is an important factor

in restoring EEL. Limited data are available on the effect of suction on respiratory mechanics,

EEL and regional ventilation distribution for the two hours post suction.

The purpose of this study was to assess the extent and duration of change in EEL resulting

from ETT suction and to examine the relationship between EEL and regional lung ventilation in

ventilated preterm infants with respiratory distress syndrome (RDS).

5

METHODS

Study design

This prospective, observational study investigated ETT suction in preterm infants on

mechanical ventilation to determine its effect on derecruitment of the lungs.

Subjects

Infants were recruited from the Neonatal Intensive Care Unit (NICU) at the Mater Mothers’

Hospital (MMH), South Brisbane, Australia. Inclusion criteria were infants’ ≤ 32 weeks

gestational age with a body weight of > 750 g who were less than 7 days of age at onset of

endotracheal ventilation and had an arterial sampling line (umbilical or extremity) in situ.

Exclusion criteria were cardiopulmonary instability, ETT leak >50%, lung anomaly affecting

regional ventilation, air leak syndrome, or poor skin integrity. The study protocol was approved

by the Human Research Ethics Committees of the Mater Health Services, South Brisbane,

Queensland. Informed written consent was obtained from the parents. All infants required

ventilation and surfactant therapy at birth for RDS. Ventilatory support was delivered in volume

guaranteed (VG) or the pressure controlled (PC) modes of synchronised intermittent positive

pressure ventilation (SIPPV) or synchronised mandatory ventilation (SIMV) (Babylog 8000+

ventilator, Dräger Medical AG Co, Lübeck, Germany) with a tidal volume (VT) determined by

the clinical team caring for the baby (usually 3-5mL/kg).

Suction procedure

ETT suction was performed via the Bodai Neo2-SAFEtm manifold within the RT235 ventilator

circuit (Fisher & Paykel Healthcare, Auckland, New Zealand). In all instances a size 6 FG

catheter was used for suction. The suction procedure commenced with the removal of the flow

sensor (ISO 15, Dräger Medical AG Co, Lübeck, Germany) from the circuit (RO sensor).

Following instillation of 0.2 – 0.3 ml saline, the catheter was inserted to just past the tip of the

ETT and suction was applied continuously at a pressure of 100mm Hg whilst the catheter was

6

withdrawn (S1). Suction took less than 10 seconds and was repeated (S2). After the

completion of suction the flow sensor was re-inserted into the circuit (IO sensor). The indication

for ETT suction was either routine 6 hourly suctioning or clinical signs of secretions in the

upper airways and/or ETT.

Measurements

A Göttingen GoeMF II EIT tomograph (VIASYS Healthcare, Netherlands) was used to measure

EEL, amplitudes and regional ventilation distribution. Sixteen conventional ECG electrodes

(Kendall, Kittycat 1050NPSM, Tyco Healthcare group, Mansfield, Massachusetts) were placed

circumferentially around the infants’ chest at nipple level. Prior to the commencement of the

suction procedure a one minute EIT measurement was performed with a frame rate of 44 Hz.

This period was used for referencing of all following measurements. A ten minute EIT

measurement was taken during the suction procedure and one minute EIT measurements

were repeated at 15 minute intervals for 120 minutes after completion of the suction procedure.

The infant was positioned supine throughout the study period. Data are reported during the

suction procedure and at 30, 60, 90 and 120 minutes. Software provided with the equipment

was used for data acquisition and reconstruction of functional relative EIT images [13]. Data

were further analysed off-line using Matlab 7.7 (R2008b, The MathWorks Inc, Natick, MA,

USA).

Data processing and analysis EIT data were band pass filtered to include the first and

second harmonic of the respiratory rate [14]. A cut-off mask of 20% of the peak impedance

signal was applied [15] to eliminate cardiac interference.[14] As the preterm infant often

displays a periodic respiratory pattern, noise-free, regular sections of data were selected for

analysis using predetermined criteria.[16]

Measurement of EEL. EEL, as measured by EIT, is the relative impedance measured at end

expiration and is analogous with functional residual capacity. Changes in EEL during and after

7

suction were compared to EEL immediately prior to suction for the global, the dependent

(posterior), and nondependent (anterior) lung.

Measurement of Regional Ventilation Distribution. Regional impedance amplitudes as a

measure of ventilation distribution were used to describe the magnitude of the regional tidal

volume change within an individual over time. Impedance amplitudes for the entire lung (global

impedance amplitudes), and the regional impedance amplitudes of the dependent (posterior)

and non-dependent (anterior) lung were calculated by averaging the impedance differences in

each pixel of the measurement period between the end expiratory and end inspiratory periods.

To account for the unequal number of pixels analysed in each region of interest (ROI), the

average amplitude for each ROI was reported.

Respiratory mechanics. Continuous flow and pressure measurement of the delivered

ventilator breaths were downloaded from the Babylog 8000/8000+ ventilator with v5.01

software (Dräger Medical AG & Co. KG, Lübeck, Germany). Tidal volumes (Vt), minute

volumes (MV), resistance (R), peak pressures (PIP) and PEEP were documented for each EIT

recording period. Due to the removal of the flow sensor during suction, these measurements

were unable to be taken during the suction period.

Physiological parameters. Invasive blood pressure (BP), inspired oxygen (FiO2), respiratory

rate (RR) and heart rate (HR) were monitored throughout the study. Oxygen saturations

(SpO2) were manually recorded at the time of each EIT recording using the Masimo Radical

pulse oximeter (software V.6.0.2) (Masimo, Irvine, CA, USA). Oxygen saturations of between

88 and 94% were targeted in the NICU by titrating the FiO2. As arterial blood gas sampling was

not undertaken due to concerns about excessive withdrawal of blood, the SpO2/FiO2 ratio was

calculated from the collected data as a surrogate measure for PaO2/FiO2.[17]

Mode of Ventilation. The delivered mode of ventilation was dependent on consultant

preference. Infants were ventilated using either the pressure controlled modes (PC) of

8

synchronised intermittent positive pressure ventilation (SIPPV) or synchronised intermittent

mandatory ventilation (SIMV), or the volume guaranteed mode (VG) of SIPPV and volume

guarantee (SIPPV+VG). With pressure controlled modes of ventilation, the maximum PIP was

limited and resulted in variable tidal volumes whereas in volume guarantee the tidal volume

was pre-set and the airway pressure changed in response to changes in lung compliance and

resistance. As there are differences in the way the different modes of ventilation deliver

ventilator inflations we performed a sub-analysis of our data to determine if the mode of

ventilation contributed to the differences found.

Statistical analysis. Results are described using mean and confidence intervals (CI), or

standard deviations (SD) for the biometric data. Mixed linear models (MLM) were used to

analyse the impact of ventilation mode (volume guarantee or pressure controlled, VG and PC

respectively) and time of measurement on impedance amplitude, EEL, physiological and

ventilator parameters. ANOVA with Bonferroni correction was then used to compare repeated

measures. A p-value of < 0.05 was considered significant. All statistical analyses were

performed using SPSS (v15.0, Lead Technologies, Inc., Chicago, IL, USA).

9

RESULTS

Patient characteristics. Twenty infants ventilated for RDS were included in the study. All

infants received surfactant therapy. The infants had a mean (±SD) gestational age of 26.6

(±1.8) weeks, study weight of 991.4 grams (± 234.1), and postnatal age of 1.9 days (±1.2).

Seven infants were ventilated using VG. There were no differences in demographics between

the infants ventilated with PC or VG (Table 1). At baseline VT was significantly lower in VG

mode (p=0.04) as was SpO2 (p=0.05). All other baseline characteristics were similar (Table 2)

Changes in EEL. No interaction between mode of ventilation and change in EEL could be

demonstrated (p=0.86). During suction the EEL dropped below the pre-suction level (p<0.01).

There was a distinct loss of lung volume after RO sensor with the greatest volume loss

occurring at S1. (Figure 1)

Post suction global EEL increased significantly (p < 0.01). Post hoc correction showed that the

improved EEL was maintained for 120 minutes (p<0.03) (Figure 2).

Similarly the anterior (non-dependent) and the posterior (dependent) lung showed an increase

in EEL irrespective of ventilation mode (Figure 2). EEL was higher for the first 90 minutes in

the anterior lung (p<0.10) and was back to baseline after 120 minutes (p=0.09). The posterior

lung showed an increased EEL even after 120 minute (p = 0.04).

Changes in regional ventilation distribution. There was no interaction between change in

global and regional amplitude and ventilation mode (p=0.68). There were no differences in

impedance amplitude during the suction procedure compared to pre-suction. The amplitude in

the anterior ROI was larger than during the pre-suction period (p<0.03), however no

differences remained post suction. The global amplitude increased 30 min post suction but the

change was not significant (Figure 3). The anterior amplitude decreased after suction and

remained lower, but the differences did not reach statistical significance. The posterior

amplitude increased after suction but not significantly.

10

Changes in Respiratory Mechanics and Physiological Parameters. There were no

significant differences pre and post suction for minute volume (p=0.60), PIP (p=0.91) or VT

(p=0.99). PEEP was maintained at 5 cm H2O for all patients. There was a significant difference

in resistance over time (p=0.03) with a reduction in resistance post suction compared to pre

suction.

There was a significant interaction between time point of measurement and SpO2 (p=0.02) with

SpO2 lower at 60 mins and 120 minutes post suction compared with pre-suction. There were

no differences pre, during and post suction for FiO2 (p=0.98), SpO2/FiO2 (p=0.46), RR

(p=0.39), HR (p=0.10) or mean BP (p=0.57).

11

DISCUSSION

Open ETT suction in non-muscle relaxed ventilated preterm infants with RDS results in a

significant increase in lung volume up to 120 minutes post suction.

Significant derecruitment and rapid recovery during open suction has previously been

described in the paediatric population during conventional ventilation [18, 19], during HFOV [5]

and in preterm infants on HFOV [11]. The consistency of findings over several studies

suggests that derecruitment caused by ETT suctioning is only transient and that a recruitment

manoeuver is not necessary. Whether closed or open suction was used did not impact on

outcome. Most studies only observed the first few minutes post suction and little is reported on

what happens over the longer time period post suction. Our study data shows that EEL

increased significantly after suction and remained increased for the next 2 hours, suggesting

that the benefit of suction occurs much longer than many of the previous studies have

described. As there was a concomitant reduction in resistance post suction, it would appear

that the clearance of secretions allowed gas exchange to occur in previously blocked airways.

This may be due to reopened alveolar spaces with little change in lung perfusion, but this

physiological phenomenon can only be speculated.

As some infants were in VG mode, it was hypothesised that the rapid reinstallation of the flow

sensor may have influenced the VG algorithm and delivered an increased PIP to counteract

suction induced derecruitment. On further retrospective analysis it was found that the mode of

ventilation had little impact on changes in EEL or ventilation distribution. This lack of difference

could be due in part to the small numbers in each ventilation mode. Additionally, there was no

difference in the delivered tidal volume or PIP between the two groups.

After ETT suction, there was a trend toward increased ventilation in the posterior (dependent)

lung compared to the non-dependent anterior lung illustrating that regional distribution of

ventilation in ventilated preterm infants with RDS is heterogeneous in nature. Heterogeneity in

12

regional ventilation distribution following ETT suction has been described previously [11, 12]

although no clear pattern has yet been established. Our findings are supported by those which

found that PEEP elevation induced a decrease in ventilation in the ventral lung regions with an

accompanying shift in ventilation towards the dependent dorsal regions [20]. However, in a

study performed on pigs, lung volume loss with disconnection and suctioning was more

pronounced in the dorsal lung regions and volume restitution was more rapid in the ventral

lung regions [12], and that during a decremental PEEP trial, derecruitment initially occurred in

dependent lung regions [21].

This finding of heterogeneity of regional ventilation distribution suggests that, despite

spontaneous recovery of global lung volume after suction, regional atelectasis and

overdistension might still be present. As it has been previously noted that regional ventilation

distribution is dependent on applied positive end expiratory pressure (PEEP) in ventilated

subjects [22], the use of a recruitment manoeuvre involving increasing PEEP may provide

some benefits in improving homogeneity of ventilation.

Previous studies have suggested that the magnitude of ETT occlusion and degree of applied

negative pressure are the most important factors in determining EEL and tidal volume change

during and after suction [23-25]. Although our study did not control for these factors, there was

no significant difference in ETT diameter between the PC and VG groups, in all instances a

size 6 Fg catheter was used for ETT suction, and a pressure of 100mm Hg was the standard

set suction pressure used.

This study has several limitations that need to be addressed. First of all, EIT measures relative

changes in ventilation so it does not provide absolute lung volume changes in response to ETT

suction. In this study, where intra-subject variation with suction was considered, this does not

represent a problem. EIT measurements on small preterm infants are technically challenging

with electrode placement difficulties increasing recorded noise. Additionally, the back

13

projection algorithm which is used to produce the image is based on the mathematical premise

that the chest shape is constant, whereas it is in fact dependent on the population studied.

CONCLUSIONS

ETT suction in non-muscle relaxed and ventilated preterm infants with RDS results in

significant lung volume increase which is maintained for at least 90 minutes. Regional

differences in distribution of ventilation with ETT suction suggest that the behaviour of the lung

is heterogeneous in nature.

Word count: 2490

14

Acknowledgments

We are deeply thankful to all parents of preterm babies who volunteered to participate in the

study and Kristen Gibbons, Statistician, Mater Research for statistical advice.

We wish to acknowledge the financial support of the ANZ Trustees Children’s Medical

Research Establishment Grant and the provision of equipment by Masimo Australia Pty Ltd.

15

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19

Table 1 Demographic data of the study infants: Mean ± SD

Infants on PC

(n = 13) Infants on VG

(n = 7) p-value

Mean difference

Confidence intervals

Birth weight (g) 991.2 ± 273.1 1011.7 ± 206.6 p=0.86 -20.50 -228.6 – 269.6

Study weight (g) 974.2 ± 265.3 1018.6 ± 200.6 p=0.70 -44.40 -197.5 – 286.3

Gestational age (weeks) 26.7 ± 1.8 26.4 ± 1.9 p=0.77 0.26 -2.1 – 1.6

Postnatal age (days) 2.0 ± 1.4 1.7 ± 0.8 p=0.63 0.29 -1.5 – 0.9

Crib score 10.6 ± 3.0 10.1 ± 4.0 p=0.77 0.47 -3.8 – 2.6

Predicted Death Rate (%) 21.9 ± 20.6 21.6 ± 1.6 p=0.97 0.36 -20.3 – 19.6

Male: Female

ETT Size (mm)

Surfactant (doses)

6:7

2.6 ± 0.2

1.9 ± 0.3

5:2

2.7 ± 0.3

2.0 ± 0

p=0.30

p=0.61

p=0.49

0.06

0.08

-0.2 – 0.3

-0.2 – 0.3

PC pressure controlled ventilation

VG volume guarantee ventilation

20

Table 2 Baseline differences in respiratory mechanics and physiological characteristics of the study

infants: (Mean ± SE)

Infants on PC

(n = 13) Infants on VG

(n = 7) p-value

Confidence intervals

HR (beats per minute) 139.08 ± 2.93 147.53 ± 4.12 p=0.10 -1.6 – 18.5

RR (breaths per minute) 72.54 ± 3.82 82.92 ± 5.18 p=0.11 -2.6 – 23.3

DBP (mm Hg) 27.58 ± 0.58 27.27 ± 0.80 p=0.75 -2.3 – 1.6

Mean BP (mm Hg) 35.67 ± 1.57 37.60 ± 2.78 p=0.57 -4.5 – 8.4

SpO2 93.69 ± 0.49 92.00 ± 0.67 p=0.05* -3.0 – -0.03

PIP 15.58 ± 0.63 14.36 ± 0.89 p=0.27 -3.4 – 0.9

FiO2 0.23 ± 0.005 0.23 ± 0.006 p=0.80 -0.01 – 0.02

SpO2/FiO2 407.92 ± 7.97 397.60 ± 11.32 p=0.46 -37.8 – 17.2

Vt (ml/kg) 4.10 ± 0.18 3.43 ± 0.26 p=0.04* -1.3 – -0.04

MV 0.27 ± 0.02 0.27 ± 0.02 p=0.89 -0.05 – 0.06

R 109.56 ± 13.81 123.47 ± 26.43 p=0.14 -43.02 – 70.84

PC pressure controlled ventilation

VG volume guarantee ventilation

21

Figure Legends

Figure 1 The change in EEL expressed in amplitude units of the global lung (A. upper panel)

and posterior and anterior lung fields (B. lower panel) in the ventilated preterm infants during

ETT suction. Mean and CI displayed.

Figure 2 Change in EEL pre and post suction (mean and CI). # EEL change after 30, 60, 90

minutes were significantly different compared to pre suction for both global (upper) and

regional (lower) EEL but not at 120 minutes for anterior lung

Figure 3 Global (upper) and regional (lower) tidal impedance amplitudes pre and post suction.

Mean and CI are shown.