Worsening oxygenation in influenza-associated ARDS
Ms L is a 44-year-old woman admitted with influenza A pneumonia and secondary bacterial infection. She is intubated and ventilated on volume-controlled mode: tidal volume 420 mL, respiratory rate 28/min, PEEP 12 cmH2O, FiO2 0.70. Her weight is 110 kg, height 160 cm (predicted body weight 52 kg); plateau pressure is 31 cmH2O, driving pressure 19 cmH2O. Arterial blood gas: pH 7.28, PaCO2 52 mmHg, PaO2 68 mmHg, HCO3 24 mmol/L. Chest radiograph shows bilateral infiltrates; echocardiography shows normal LV function, no significant valve disease. She is sedated with propofol and fentanyl, has been proned once for 16 hours with improvement, and is now supine again. Noradrenaline 0.1 µg/kg/min maintains MAP 68 mmHg. Urine output is 0.5 mL/kg/h.
1. Calculate the P/F ratio, tidal volume per kg predicted body weight and driving pressure. What immediate ventilator changes would you make, and why?
Read the worked answer
Predicted body weight (PBW): 45.5 + 0.91 × (160 − 152.4) = 52.4 kg. Tidal volume is 420 mL = 8.0 mL/kg PBW, above the 6 mL/kg target. Driving pressure = plateau − PEEP = 31 − 12 = 19 cmH2O, above the 15 cmH2O threshold associated with harm. P/F = 68/0.70 = 97 mmHg, i.e. moderate–severe ARDS.
Immediate changes:
- Reduce tidal volume to 6 mL/kg PBW (about 315 mL), then check pH and plateau. Accept permissive hypercapnia down to pH ~7.20 if haemodynamics allow; her pH 7.28 with PaCO2 52 gives room.
- Recheck plateau after the change; target ≤30 cmH2O and driving pressure ≤15.
- Consider increasing PEEP only if it improves oxygenation and compliance without raising plateau; use a PEEP table or oesophageal pressure if available.
- Keep sedation deep enough to tolerate the strategy; consider neuromuscular blockade for 48 h if there is dyssynchrony or very high driving pressure.
- Use a fluid-restrictive strategy once shock has resolved.
The reasoning: the mortality benefit in ARDS comes from minimising volutrauma and atelectrauma, so tidal volume and driving pressure matter more than the FiO2 number itself.
2. She improved during proning. The team wants to stop because of pressure sores and staff workload. Argue whether to continue, including how many sessions and what endpoints you would use.
Read the worked answer
Continue proning. In moderate–severe ARDS (P/F <150), prone positioning for at least 16 h/day reduces 28-day mortality (PROSEVA, number needed to treat about 6). One 16-hour session with improvement is not a treatment course; the benefit comes from repeated, long sessions during the period of severe hypoxaemia.
Practical approach:
- Target at least 16 h per session, for at least 3–4 consecutive days or until P/F >150 with PEEP ≤10 and FiO2 ≤0.6 in the supine position.
- Use a dedicated team and checklist; pressure areas are manageable with careful positioning, silicone dressings and eye care. Pressure sores are a real harm but are outweighed by the mortality benefit.
- Stop criteria: lack of oxygenation improvement after 2–3 sessions, haemodynamic instability, or an adverse event such as unplanned extubation.
- Reassess after each supine period; do not use proning as a substitute for lung-protective ventilation and fluid balance.
The team's concern about workload is legitimate but should be solved with staffing and training, not by withdrawing an intervention with a mortality benefit.
3. On day 5 she remains P/F 110 on FiO2 0.8, PEEP 14, with PaCO2 65 mmHg and pH 7.22. Discuss ECMO referral criteria and what you would do while awaiting transfer.
Read the worked answer
ECMO referral should be considered when, despite optimised ventilation and prone positioning, P/F remains <80 mmHg for >6 h, or <50 mmHg for >3 h, or pH <7.25 with PaCO2 ≥60 mmHg for >6 h (EOLIA criteria). She has P/F 110 on FiO2 0.8/PEEP 14, so she is not yet at the hypoxaemic threshold, but her PaCO2 65 with pH 7.22 meets a hypercapnic criterion; the overall trajectory matters.
While awaiting transfer:
- Optimise: confirm PBW tidal volume, measure plateau and driving pressure, and continue prone sessions. Consider a recruitment manoeuvre only if safe.
- Review fluid balance and diurese if she is net positive; treat infection and review sedation.
- Avoid injurious escalation: do not raise PEEP or tidal volume simply to chase numbers.
- Call the ECMO centre early for advice; transfer is easier before cardiovascular collapse.
- Discuss with the family the uncertainty and the possibility of transfer.
The reasoning: ECMO is a rescue therapy for refractory hypoxaemia or hypercapnia, not a treatment for a single abnormal number. Early referral improves outcomes because it allows time for assessment, but late initiation after multi-organ failure is less effective.