Severe ARDS in a patient with class III obesity: tidal volume, driving pressure, and when to prone
62-year-old man, BMI 44 kg/m², day 3 of ARDS from influenza pneumonia. Sedated, RASS −4, on volume-control ventilation: tidal volume 480 mL, rate 24, PEEP 14 cm H₂O, FiO₂ 0.70. Plateau pressure 31 cm H₂O, total PEEP 15. PaO₂ 68 mmHg, SpO₂ 91%, PaCO₂ 52 mmHg, pH 7.28. He is on norepinephrine 0.12 µg/kg/min with MAP 68 mmHg, CVP 16 mmHg, lactate 2.4 mmol/L and urine output 25 mL/h. Chest radiograph shows dense consolidation in the right middle and lower lobes, no pneumothorax. A recruitment maneuver earlier in the day produced a transient drop in MAP to 48 mmHg. Height 175 cm.
1. Calculate his driving pressure and predicted-body-weight tidal volume. How do his obesity and hemodynamics change your ventilator settings right now, and what will you actually change?
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Numbers first
- Predicted body weight (male, 175 cm) = 50 + 0.91 × (175 − 152.4) ≈ 70.6 kg. A 6 mL/kg tidal volume is ≈ 425 mL. His 480 mL is about 6.8 mL/kg, above the lung-protective target.
- Driving pressure = plateau − total PEEP = 31 − 15 = 16 cmH₂O. The usual safety threshold is ≤15 cmH₂O, and his plateau also exceeds the 30 cmH₂O ceiling.
Why obesity changes interpretation but not the immediate action
A BMI of 44 means high chest-wall elastance, so part of that 31 cmH₂O plateau may be spent inflating the chest wall rather than the lung. Esophageal manometry would show whether transpulmonary pressure is genuinely low and whether higher PEEP is safe. Without it, the defensible move is to reduce tidal volume to ~420 mL, keep PEEP at 14, and re-measure plateau, driving pressure, MAP and urine output after 30 minutes.
What not to do
Do not chase oxygenation by pushing PEEP to 18–20. With CVP 16 mmHg, urine output 25 mL/h and a documented hypotensive response to recruitment, the likely result is hemodynamic deterioration and more fluid loading, not better recruitment. Permissive hypercapnia to pH ≥7.20 is acceptable; he is at 7.28.
Bottom line: reduce tidal volume to PBW-based 6 mL/kg, reassess driving pressure, and address oxygenation with prone positioning rather than more PEEP.
2. PaO₂ 68 mmHg on FiO₂ 0.70 - should he be proned tonight, and what are the practical considerations in a patient with BMI 44 on norepinephrine?
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Yes - he meets criteria
PaO₂/FiO₂ = 68/0.70 ≈ 97 mmHg, in the severe ARDS range, on PEEP 14 and FiO₂ 0.70. Prone positioning for ≥16 h/day reduces mortality in moderate-to-severe ARDS when started early, and day 3 is still within the treatment window.
Norepinephrine is not a contraindication
A requirement of 0.12 µg/kg/min is manageable during the turn. Relative contraindications are an unstable spine, an open abdomen or recent sternotomy, raised intracranial pressure, and hemodynamic instability that will not tolerate the maneuver - none apply. Have a small fluid bolus and pressor ready, but do not give a prophylactic litre to a patient with CVP 16 and oliguria.
Practical issues at BMI 44
- Two-person airway team, secure tube and lines, protect eyes and pressure areas; the abdomen is compressed, so use chest and pelvic supports and keep the bed in reverse Trendelenburg.
- Continue enteral feeding at a reduced rate rather than stopping it.
- Measure PaO₂/FiO₂ after 1 hour and at the end of the session; sustained improvement justifies repeating daily.
- Because the consolidation is dense and non-recruitable, oxygenation may improve only modestly - that does not make the first session a failure.
Bottom line: prone tonight for 16 hours with airway and hemodynamic precautions, and predefine that 3–4 sessions constitute an adequate trial.
3. After two prone sessions on day 5 his PaO₂/FiO₂ is 75, pH 7.22, PaCO₂ 68 mmHg. What are the options, and how should his BMI influence the decision?
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Recognise failure of conventional ventilation
A PaO₂/FiO₂ of 75 after two 16-hour prone sessions, with pH 7.22 and PaCO₂ 68, means lung-protective ventilation and prone positioning have not worked. Referral for venovenous ECMO is appropriate: refractory hypoxaemia (P/F <80 for >6 hours) and hypercapnic acidosis (pH <7.25 with PaCO₂ >60 for >6 hours) are standard referral triggers.
How BMI factors in
Obesity alone is not a contraindication to ECMO and should not be presented to the family as one. It raises practical difficulty - cannulation, nursing, imaging, drug dosing, anticoagulation risk - but outcome data in obesity are not clearly worse. What weighs more is duration of conventional ventilation: pre-ECMO ventilation beyond about 7 days is a recognised poor prognostic feature, and he is at day 5, so the decision cannot wait another week.
Parallel paths
- Confirm nothing reversible is missed: bronchoscopy for mucus plugging, evaluate for ventilator-associated pneumonia and fluid overload.
- If ECMO is unavailable or declined, continue lung-protective ventilation, consider tracheostomy after 7–10 days if he stabilises, and revisit goals of care with the family using honest prognostic information.
Bottom line: make the ECMO referral now rather than after another 48 hours; treat BMI as a complexity factor, not an exclusion.