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% 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%

Copyright restrictions may apply. Esteban, A. et al. JAMA 2002;287: 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

Mortality: –ICU mortality 30.7% –Hospital mortlaity 39.2%

Copyright restrictions may apply. Esteban, A. et al. JAMA 2002;287: 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/C R + Vt/T i * R + PEEPtotal Where CR = compliance of the respiratory system, Ti = inspiratory time and V T /Ti = Flow, R R = 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

Waveforms

Ventilator change Flow (lpm) Pressure (cm H2O) Volume (mL)

Waveforms Airway pressure Time Pause

Waveforms Pause

What changes on the ventilator should you make for hypoxemia? What changes for hypercapnia and respiratory acidosis? Hypotension on the ventilator?