Mechanical Ventilation
Epidemiology
• 28 day international study
– 361 ICUs in 20 countries
– All consecutive adult patients who received
MV for > 12 hours
– 33% Patient admitted to those ICUs received
mechanical ventilation
• Mean age 59
• M > F (61 v. 39%)
Esteban et al. JAMA 2002
• Indication for mechanical ventilation
– Acute respiratory failure 68%
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Post-op (21%)
Pneumonia 14%
CHF 10%
Sepsis 9%
Trauma 8%
ARDS 4.5%
Aspiration 2.5%
Cardiac arrest 1.9%
– Acute on chronuic respiratory failure
• COPD 10%
• Asthma 1.5%
• Chronic respiratory disease (non_COPD) 1.8%
– Coma 16.7%
– Neuromuscular disease 1.8%
Ventilator Modes Used Each Day During the Course of Mechanical Ventilation
Esteban, A. et al. JAMA 2002;287:345-355.
Copyright restrictions may apply.
• Duration of mechanical ventilation
– Overall 5.9days
– COPD pts 5.1 days
– ARDS pts 8.8 days
• ICU LOS: 11.2 days
• Hospital LOS: 22.5 days
• Mortality:
– ICU mortality 30.7%
– Hospital mortlaity 39.2%
Kaplan-Meier Curves of the Probability of Survival Over Time of Mechanical Ventilation
Esteban, A. et al. JAMA 2002;287:345-355.
Copyright restrictions may apply.
Mechanical ventilation
• Physiology:
– Positive pressure ventilation versus naturanl negative
pressure ventilation
• Effects:
– Heterogeneous ventilation
• Preferential ventilation of the non-dependent regions
– Increased physiologic dead space
– Improvement of physiologic shunt causes by
atelectasis and/or alveolar filling
– Rapid disuse atrophy of the diaphragm
– Impairment of mucociliary clearance
• Cardiovascular effects:
– Decreased venous return
• Exacerbated by:
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Auto-PEEP
Applied PEEP
Intravascular volume depletion
Cardiac tamponnade
– Increased right ventricular afterload:
• Compression of the pulmonary vascular bed  Increased
PVR
– May decrease left ventricular afterload
• Lung exansion decreased extramural pressure
Mechanical ventilation
• Benefits
– Improves gas exchange by improved V/Q
matching predominantly be decreasing shunt
– Decreased work of breathing
Mechanical ventilation:
Complications
• Barotrauma
– Incidence ~3%
– To Avoid: Keep plateau pressure < 35 cm
• VILI
– Over stretch
– Atelectotrauma
• Auto-PEEP
• Asyncrhony
Mechanical ventilation: Modes
• Choices:
– Mandatory v. non-mandatory
• Mandatory
– Volume v. pressure limited ventilation
– Mandatory rate
– Modes:
» SIMV
» Assist Control
» PCV
» Hybrid Modes: PRVC, SIMV/PRVC
• Non-mandatory or assisted breaths
– PSV
Variables: some default values
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Trigger sensitivity: -1 to -3 cm
Tidal volume: 6-8mg/kg/IBW
Rate: 10 to 14
PEEP: 5 cm H2O
Flow rate: 60 L/min
I to E ratio
Volume limited v. pressure limited
• Volume limited
– Physician sets:
• Tidal volume
• Rate
– Guaranteed constant tidal
volume
– Guaranteed minute
ventilation
– High peak pressures
• Pressure limited
– Physician sets:
• Peak airway pressure
• Inspiratory time
– Tidal volume and minute
ventilation depends entirely
on patient factors:
compliance and airway
resistance
– Associated with lower peak
airway pressure
– Associated with more
homogenous gas
distribution
No difference in mortality, oxygenation, or work of breathing
P = Vt/CR + Vt/Ti * R + PEEPtotal
Where CR = compliance of the respiratory system, Ti = inspiratory time and VT/Ti = Flow, RR = resistance of the
respiratory system and PEEP total = the alveolar pressure at the end of expiration = external PEEP + auto (or
intrinsic) PEEP, if any. Auto PEEP = PEEP total – P extrinsic (PEEP dialed in the ventilator) adds to the inspiratory
pressure one needs to generate a tidal breath.
• Peak pressure
• Plateau pressure
– Surrogate for peak alveolar distending
pressure
• Peak – Plateau
– Resistive pressure
• Mean airway pressure
– Pressure applied acorss the lung and chest
wall averaged throughout the ventilary cycle
• Patient factors:
– Airway resistance
– Compliance of the respiratory system
• Chest wall recoil
• Lung recoil
Assist-Control
• Set variables
– Tidal volume
– Flow rate or Ti
– PEEP FiO2
– Mandatory rate
• Spontaneous breaths
– Additional cycles can be triggered; they are
identical to the mandatory breath
SIMV
• Set variables
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Targeted volume
Flow rate
Manatory frequency
PEEP
FiO2
PS augmentation for spontaneous breaths
• Spontaneous breaths
– Unrestricted and aided by the selected level of
pressure support
PCV
• Set variables:
– Peak pressure
– Inspiratory time
– Frequency of mandatory breaths
• Spontaneous breaths
– PCV (AC): same as mandatory breaths
– PCV/SIMV: unsupported or pressure
supported
Tidal volume during PCV
• Changes in mechanics
– Increased airway resistance
– Decreased respiratory system compliance
• Increased auto-PEEP
• Decreased inspiratory time
Waveforms
Waveforms
Waveforms
Waveforms
Ventilator change
Flow (lpm)
Pressure (cm H2O)
Volume (mL)
Waveforms
Airway pressure
40
30
Pause
20
10
0
Time
Waveforms
Pause
• What changes on the ventilator should you
make for hypoxemia?
• What changes for hypercapnia and
respiratory acidosis?
• Hypotension on the ventilator?