ARDS on Day 2 With Hypotension and Oliguria
A 48-year-old man (175 cm, 82 kg) is on day 2 of volume-control ventilation for influenza ARDS. Settings: VT 550 mL, RR 22, PEEP 14 cmH2O, FiO2 0.70, inspiratory flow 60 L/min. He is deeply sedated (RASS -4) and receiving cisatracurium. ABG: pH 7.28, PaCO2 52 mmHg, PaO2 70 mmHg, HCO3 24 mEq/L. Plateau pressure is 33 cmH2O, peak pressure 41 cmH2O, and a brief expiratory-hold maneuver shows total PEEP of 16 cmH2O. SpO2 is 94%. HR 112, BP 92/54 mmHg, CVP 14 mmHg, and the nurse reports urine output has fallen to 15 mL/hour over the last 3 hours. Breath sounds are equal, and there is no pneumothorax on today's film.
1. Calculate predicted body weight and the lung-protective tidal volume range. Where does he sit now? Interpret the plateau pressure, driving pressure, and measured total PEEP.
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Predicted body weight
- 175 cm = 68.9 inches; 68.9 - 60 = 8.9
- Male PBW = 50 + (2.3 x 8.9) = 50 + 20.5 = 70.5 kg
- 6 mL/kg = 423 mL; 8 mL/kg = 564 mL
Where he sits 550 mL is 7.8 mL/kg of PBW. He is inside the 6-8 mL/kg envelope but well above the 6 mL/kg target, and the plateau tells you it is costing him: 33 cmH2O exceeds the usual target of 30 cmH2O or less.
Driving pressure Pplat minus total PEEP = 33 - 16 = 17 cmH2O. That is above the 15 cmH2O threshold associated with worse outcome, and it is the number to follow when you change settings.
Total PEEP Set PEEP is 14 but measured total PEEP is 16, so there is about 2 cmH2O of auto-PEEP. Small, but it means expiratory time is not generous and it must be rechecked after any rate change.
Action Reduce VT toward 420-440 mL, accept permissive hypercapnia down to about pH 7.20 rather than driving the rate up and creating air trapping, and recheck plateau, driving pressure, and total PEEP after each change.
2. He is hypotensive and oliguric. What is your differential, and what do you do about it while preserving his lung-protective settings?
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Differential
- High intrathoracic pressure limiting venous return, exaggerated by hypovolemia or by PEEP that is not recruiting anything.
- Air trapping - here only 2 cmH2O, so a minor contributor but still worth rechecking after rate changes.
- Sepsis, including a secondary bacterial pneumonia on top of influenza.
- Right ventricular failure from high PEEP plus hypercapnia and acidosis (pH 7.28, PaCO2 52), both of which raise pulmonary vascular resistance.
- Sedation and neuromuscular blockade contributing to vasodilation and loss of compensatory tone.
- Pneumothorax - the film is reassuring but not definitive in a ventilated ARDS patient.
What to do
- Assess fluid responsiveness rather than guessing from CVP 14: passive leg raise, bedside echo or IVC assessment, and a lactate and perfusion exam.
- If responsive, give a fluid bolus and reassess; if not, start vasopressor support rather than loading more volume.
- Check for auto-PEEP and consider whether PEEP is recruiting or merely distending, using compliance and driving pressure as the discriminator.
- Treat the acidosis and hypercapnia within permissive limits, since both worsen pulmonary vasoconstriction.
- Do not reflexively drop PEEP. In a recruitable lung, lowering PEEP worsens hypoxic vasoconstriction and can make the hemodynamics worse; but if he is fluid-unresponsive and pressures remain high, a small stepwise PEEP reduction trial with oxygenation and hemodynamics monitored is reasonable.
- Track urine output, creatinine, and lactate as trend markers, and evaluate for a secondary infection.
3. His PaO2/FiO2 is about 100 on FiO2 0.70 and PEEP 14. What is the next escalation, what criteria justify it, and how do you judge whether it worked?
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Next escalation: prone positioning
He meets the standard criteria: P/F under 150 on FiO2 of at least 0.60 and PEEP of at least 5 cmH2O. Target 16 consecutive hours per session, with a checklist before the turn.
Preparation Secure the airway and lines, protect eyes and pressure areas, pause enteral feeding, confirm hemodynamic stability for the turn, and assign roles including one person dedicated to the tube.
If proning is contraindicated or fails Consider a monitored recruitment maneuver, inhaled pulmonary vasodilators as a bridge, and continued neuromuscular blockade. Escalate to ECMO referral if P/F stays under 80, or if pH stays under 7.25 with PaCO2 of 60 mmHg or more, despite lung-protective settings.
Judging response
- The relevant endpoint is the P/F and driving pressure after returning to supine, not the saturation during proning.
- Track compliance and driving pressure; a responder shows better compliance and a lower driving pressure.
- Watch PaCO2 and hemodynamics; the same maneuver that improves oxygenation can worsen venous return.
- Wean FiO2 before PEEP when oxygenation improves, and do not chase the pH of 7.28 with a higher rate or a larger tidal volume - permissive hypercapnia is the intended strategy here.