Severe ARDS With an Elevated Driving Pressure
A 62-year-old man is in his second day of mechanical ventilation for severe community-acquired pneumonia. He is sedated and ventilated in volume-assist/control with a tidal volume of 420 mL (predicted body weight 70 kg), respiratory rate 26/min, PEEP 12 cm H2O, and FiO2 0.70. Arterial blood gas on these settings: pH 7.28, PaCO2 52 mm Hg, PaO2 62 mm Hg, HCO3 24 mEq/L. Plateau pressure is 30 cm H2O and peak pressure 38 cm H2O. SpO2 is 89%. MAP is 68 mm Hg on norepinephrine 0.08 mcg/kg/min; CVP is 8 mm Hg; lactate 2.0 mmol/L. Temperature is 38.6°C. Chest radiograph shows bilateral infiltrates without pneumothorax or pleural effusion. He has no known neuromuscular disease and no history of COPD. The team asks whether to increase PEEP, change tidal volume, or proceed to prone positioning.
1. Calculate the PaO2/FiO2 ratio and driving pressure. How do these values classify the severity of his ARDS and what do they suggest about the current ventilator settings?
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PaO2/FiO2 = 62 ÷ 0.70 = approximately 89 mm Hg. Under the Berlin definition, a ratio below 100 mm Hg with bilateral infiltrates and a non-cardiac cause is severe ARDS. Driving pressure = plateau pressure minus PEEP = 30 − 12 = 18 cm H2O. A plateau of 30 cm H2O is at the accepted upper limit, but a driving pressure above 15 cm H2O is associated with worse outcomes. The peak-to-plateau difference of 8 cm H2O suggests some resistive loading, but the main problem is the high driving pressure. The pH of 7.28 with PaCO2 52 is acceptable permissive hypercapnia, so it does not by itself require a change. The key judgement is that this patient is under-ventilated from a lung-protective standpoint: simply raising FiO2 would not address the mechanical stress, and raising PEEP without checking plateau and hemodynamics could worsen the driving pressure or reduce venous return.
2. Recommend the next ventilator changes. Include specific targets, how you would assess response, and what you would avoid.
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Use a higher-PEEP strategy cautiously rather than increasing FiO2 first. Increase PEEP from 12 to 14 cm H2O in one step, then recheck plateau, driving pressure, blood pressure, and ABG within 30–60 minutes. Targets: plateau ≤30 cm H2O, driving pressure ≤15 cm H2O, SpO2 88–95%, PaO2 55–80 mm Hg, and pH >7.20. If the plateau rises above 32 cm H2O or driving pressure exceeds 15 cm H2O after the PEEP change, reduce tidal volume to 380–400 mL (about 5.4–5.7 mL/kg predicted body weight) and accept PaCO2 up to 60–65 mm Hg if pH remains above 7.20. Do not increase FiO2 above 0.70 unless SpO2 is below 88%; do not use tidal volumes above 6–8 mL/kg; and do not chase a normal PaCO2. Check for auto-PEEP and reassess MAP and CVP after each change. If MAP falls below 65 mm Hg or CVP rises sharply, return PEEP to 12 cm H2O and consider the patient for prone positioning or ECMO referral.
3. If hypoxemia persists after optimization, what is the indication for prone positioning? What must be confirmed before turning him, and what complications do you monitor?
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Prone positioning is indicated for moderate-to-severe ARDS with PaO2/FiO2 below 150 mm Hg despite FiO2 at least 0.60 and PEEP at least 5 cm H2O, ideally started within 12–24 hours of meeting criteria and continued for about 16 hours per day. Before turning, confirm that the endotracheal tube and all lines are securely sutured, that there is no pneumothorax requiring drainage, that the patient is hemodynamically stable enough to tolerate the move, and that enteral feeding has been paused. Exclude or carefully weigh contraindications such as unstable spinal injury, open abdomen, recent sternotomy, or uncontrolled intracranial pressure. During and after the turn, monitor for tube dislodgement, transient desaturation, arrhythmia, pressure injury to the face and chest, facial edema, and line migration. If there is no improvement after an adequate trial and PaO2/FiO2 remains below 80 mm Hg, escalate to ECMO evaluation.