A ventilated ICU patient with ARDS is transferred to theatre for a tracheostomy. Ventilator settings show tidal volume 320 ml, PEEP 12 cmH2O, peak inspiratory pressure 38 cmH2O. After a 0.5 second end-inspiratory hold, plateau pressure measures 27 cmH2O. What is the driving pressure, and what is the concern if it exceeds the accepted threshold?
- A 9 cmH2O, associated with barotrauma to healthy lung units
- B 15 cmH2O, values above this correlate with increased mortality in ARDS ✓
- C 26 cmH2O, indicating severe airflow obstruction requiring bronchodilators
- D 38 cmH2O, indicating excessive inspiratory flow rate
Correct answer: B. 15 cmH2O, values above this correlate with increased mortality in ARDS
Explanation
Driving pressure equals plateau pressure minus PEEP: 27 minus 12 equals 15 cmH2O. Values above approximately 15 cmH2O reflect high tidal strain relative to functional lung size and have been associated with increased mortality in ARDS. Peak inspiratory pressure includes resistive components such as tube resistance and secretions, so it overestimates alveolar stress; the plateau pressure measured during an end-inspiratory hold is the relevant alveolar pressure. The gap between peak and plateau here suggests a resistive rather than compliance problem.
Reference: Miller's Anesthesia, 9th ed.
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