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%
• 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%
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Esteban, A. et al. JAMA 2002;287:345-355.
Ventilator Modes Used Each Day During the Course of Mechanical Ventilation
• 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
Copyright restrictions may apply.
Esteban, A. et al. JAMA 2002;287:345-355.
Kaplan-Meier Curves of the Probability of Survival Over Time of Mechanical Ventilation
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:– 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
• 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– 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