Lung-Protective Ventilation and Prone Positioning in ARDS
A 52-year-old man with influenza A pneumonia has been intubated for 48 hours. He is on volume-control ventilation with a set tidal volume of 500 mL, respiratory rate 24, PEEP 12 cmH2O, FiO2 0.70, and inspiratory flow 60 L/min. His predicted body weight is 60 kg. Recent measurements are peak pressure 38 cmH2O, plateau pressure 30 cmH2O, SpO2 90%, and arterial blood gas pH 7.27, PaCO2 53 mmHg, PaO2 62 mmHg. Chest radiograph shows bilateral infiltrates, bedside echocardiography shows normal left ventricular function, and there is no evidence of fluid overload. The resident writes orders to increase PEEP to 18 cmH2O and increase tidal volume to 600 mL to clear carbon dioxide. The nurse reports blood pressure 118/64 mmHg on norepinephrine 0.05 mcg/kg/min.
1. Calculate tidal volume per predicted body weight, driving pressure, and the PaO2/FiO2 ratio. Classify the ARDS severity.
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Tidal volume per predicted body weight is 500 divided by 60, which equals 8.3 mL/kg. The lung-protective target is 6 mL/kg, so this patient is being overdistended.
Driving pressure is plateau pressure minus PEEP: 30 minus 12 equals 18 cmH2O. The target is below 15 cmH2O; 18 is associated with higher mortality in ARDS.
PaO2/FiO2 is 62 divided by 0.70, which equals approximately 89 mmHg. Under the Berlin definition, with PEEP of at least 5 cmH2O and bilateral infiltrates not explained by cardiac failure, a ratio at or below 100 mmHg classifies as severe ARDS.
The pH of 7.27 with PaCO2 of 53 mmHg is acceptable permissive hypercapnia. It is not a reason to increase tidal volume, and doing so would raise plateau and driving pressure further.
2. Which of the resident's orders do you question, and what settings do you recommend instead?
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Question both orders. Raising tidal volume to 600 mL, which is 10 mL/kg predicted body weight, would increase plateau pressure and driving pressure and worsen ventilator-induced lung injury. The ARDS Network trial showed that 6 mL/kg predicted body weight reduces mortality compared with 12 mL/kg; clearing carbon dioxide is not worth that cost.
Increasing PEEP to 18 may be reasonable if recruitment is likely, but it should follow volume reduction, since a lower tidal volume will itself lower plateau pressure. Reduce tidal volume in steps to 360 mL (6 mL/kg), keep plateau pressure at or below 30 cmH2O, and keep driving pressure below 15 cmH2O. Use the ARDS Network PEEP/FiO2 table: with FiO2 of 0.70, lower-table PEEP is 10 to 14 cmH2O and higher-table PEEP is 16 cmH2O. Titrate FiO2 to SpO2 of 88% to 95%.
If pH falls below 7.20 from permissive hypercapnia, increase respiratory rate toward 35 rather than tidal volume, and consider bicarbonate. Monitor hemodynamics with each PEEP change.
3. After these changes the PaO2/FiO2 ratio is 110 on FiO2 0.60, PEEP 14 cmH2O, plateau 27 cmH2O, and driving pressure 13 cmH2O. What is the next intervention, and what criteria support it?
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Prone positioning. The PROSEVA trial enrolled patients with a PaO2/FiO2 ratio below 150 mmHg on FiO2 of at least 0.60 and PEEP of at least 5 cmH2O after 12 to 24 hours of optimization, and showed a mortality benefit with sessions of at least 16 hours per day. This patient now meets those criteria, with a ratio of 110 on FiO2 0.60 and PEEP 14 cmH2O, and the plateau and driving pressures are within target.
Before proning, confirm hemodynamic stability, secure the airway and lines, protect pressure points and eyes, and plan for enteral feeding pauses. Measure the response by repeat gas exchange and compliance after 1 to 2 hours; a responder usually shows improved oxygenation.
Also consider a 48-hour course of cisatracurium in early severe ARDS, continue lung-protective ventilation, and move to a conservative fluid strategy once shock has resolved. Reassess the ratio daily to decide when proning can stop.