MECHANICAL VENTILATION

R. Duncan Hite, M.D.Associate Professor of Internal Medicine

Section on Pulmonary and Critical Care Medicine

OBJECTIVES

Mechanics of Ventilators

Weaning (“Liberation”) from Mechanical Ventilation

Non Invasive Mechanical Ventilation (NIPPV)

Respiratory Failure

Hypoxic Respiratory Failure

Hypercarbic Respiratory Failure

VENTILATION

Physical Parameters of Ventilation

PressureVolumeFlow

VENTILATION

Negative Pressure• Spontaneous• Mechanical

Positive Pressure • Mechanical

Invasive Non Invasive (CPAP, BiPAP)

MECHANICAL VENTILATIONVentilation Modes

Volume-Cycled (Vt fixed, P variable)Assist Control (AC, CMV)Intermittent Mandatory Ventilation (IMV, SIMV)

Pressure-Cycled (P fixed, Vt variable)Pressure Support (PSV)Pressure Control (PCV)Airway Pressure Release (APRV)

MECHANICAL VENTILATIONVentilation Modes

MacIntyre, Chest, 1993, 104, 560.

COMPLIANCE

Compliance = V/P

Key physiologic variables:Lung ComplianceChest Wall ComplianceAirway Resistance

Static = Vt/Pplat-PEEP

Dynamic = Vt/PIP-PEEP

MECHANICAL VENTILATIONNomenclature

MECHANICAL VENTILATIONTriggering

MECHANICAL VENTILATIONRespiratory Rate

Assist Control Set rate = sets time duration for ventilator to give breath if not

triggered by patient. Possible for no breath to be given without patient triggering

All breaths are preset tidal volume

SIMVSet rate = sets time interval during which assisted breath must

be given with or without patient triggeringPreset tidal volume only provided during assisted breaths. Vt

for other breaths are spontaneous or PSV-assisted.

PSVNO set rate. Preset pressure provided during all breaths.

PCVAble to set rate, same as AC. Preset pressure provided for all.

MECHANICAL VENTILATIONSIMV

Terminology of PEEP

Extrinsic (i.e. Machine generated)

Terminology of PEEP

Extrinsic (i.e. Machine generated)

PEEP = CPAP

Intrinsic = autoPEEPObstructive lung disease - “air trapping”Mechanical (Inverse Ratio Ventilation)

Prolonged Inspiration/Intrinsic PEEP

Terminology of PEEP

Extrinsic (i.e. Machine generated)

PEEP = CPAP

Intrinsic = autoPEEPObstructive lung disease - “air trapping”Mechanical (Inverse Ratio Ventilation)

“Physiologic” (?)

Amato, etal. Am J Respir CritCare Med, 1995, 152, 1835.

Low Volume Ventilation StrategyARDS

ARDS Network, NEJM, 2000,342,1301.

Low Volume Ventilation StrategyARDS

ARDS Network, NEJM,2000,342,1301.

100

110

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190

Day 0 Day 1 Day 3 Day 7

Pa

O2

/FiO

2 R

ati

o

Lower

Traditional

20

25

30

35

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45

50

Day 1 Day 3 Day 7P

aC

O2

Lower

Traditional

Permissive Hypercapnea

Hickling, etal. Crit Care Med, 1994,22,1568.

• Management Issues– Cardiovascular

• Reduced Myocardial Contractility

• Pulmonary Vasoconstriction

• Systemic Vasoconstriction (increased afterload)

– Central Nervous System• Anesthetic

• Increased Intracranial Pressure

• Cerebral Vasodilation

– Musculoskeletal• Reduced Contractility

– Endocrine

Weaning Guidelines

“Weaning” (Extubation) Parameters

Parameters:• Strength

• MIP (< -20 cmH2O)• VC (> 10 cc/kg)• Vt (> 5 cc/kg)

• Demand• Ve (< 10 Lpm)• RR (> 30 bpm)

• Combined• f/Vt (< 105 bpm/L)

“Weaning” (Extubation) Parameters

Hilberman, etal. J Thor Cardiovasc Surg, 1976, 71, 711.

Yang, Tobin. NEJM, 1991, 324, 1445.

“Weaning” (Extubation) Parameters

Modes of Ventilation for Weaning

Volume-Cycled (Vt fixed, P variable)Assist Control (AC, CMV)Synchronized Intermittent Mandatory

Ventilation (SIMV)

Pressure-Cycled (P fixed, Vt variable)Pressure Support (PSV)Pressure Control (PCV)Airway Pressure Release (APRV)

Comparison of Weaning Modes

SIMV v. PSV??PSV - more patient

comfort, requires more observation

SIMV - less apnea alarms!

Esteban,etal. NEJM,

1995, 332, 345.

Impact of Daily Screen at NCBH

Ely, etal. N Engl J Med, 1996, 335, 1864.

MECHANICAL VENTILATIONSpontaneous Breathing Trials

Esteban,etal. Am J Resp Crit Care Med, 1999, 159, 512.

Modes: AutoFlow or Flowby PSV = 5 T-Piece Trial

Duration: 30 - 120 min

Non Invasive Ventilation

• PEEP = CPAP

• BiPAP = CPAP + PSV modes

• ePAP = CPAP

• iPAP = CPAP + PSV

Non Invasive Ventilation

• Clinical Indications:

• Hypercarbic Respiratory Failure• COPD Exacerbation

• BiPAP• OSA / Obesity Hypoventilation• Neuromuscular Disease

• Hypoxic Respiratory Failure• Cardiogenic Pulmonary Edema

• CPAP (?BiPAP)• Other ??

Mehta, Hill. Am J Respir Crit Care Med, 2001, 163, 540.

Non Invasive VentilationPatient Selection

Predictors of Success• Younger Age• Lower acuity of illness• Better Neurologic score• Cooperative and able to coordinate breathing with ventilator• Less air leaking; intact dentition• Hypercarbia; not too severe (PaCO2 > 45 and < 92 mm

Hg)• Acidemia, but not too severe (pH < 7.35, > 7.10)• Improvements in gas exchange, HR and RR within first 2 h

Mehta, Hill. AJRCCM, 2001, 163, 540.

Non Invasive VentilationDelivery Issues

Mehta, Hill. AJRCCM, 2001, 163, 540.

Post Extubation Resp Failure• Likely Improves Reintubation rate in COPD patients• No change in other etiologies• May increase mortality (?)

Staff Requirements• No increase in nursing time• Significant increase in RT time in first 8 – 48 hours• Requires MD re-evaluation after first 2 hours• Results not as favorable when performed outside of ICU