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Refractory Hypoxaemia on Day Six of Influenza ARDS

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Refractory Hypoxaemia on Day Six of Influenza ARDS

A 38-year-old woman (160 cm, 78 kg actual, predicted body weight 57 kg) is on day 6 of ventilatory support for influenza A ARDS. She is sedated and ventilated in volume-controlled mode with a tidal volume of 350 mL, PEEP 16 cmH2O, FiO2 0.9, respiratory rate 30, and a plateau pressure of 34 cmH2O. Arterial blood gas on these settings: PaO2 55 mmHg, PaCO2 58 mmHg, pH 7.22, bicarbonate 23 mmol/L, lactate 2.6 mmol/L. She is on noradrenaline 0.35 micrograms/kg/min, has urine output of 0.2 mL/kg/h, temperature 38.2 degrees Celsius, and has been nursed supine since admission because she was considered too unstable for turning. Chest radiograph shows bilateral infiltrates with no effusion; echocardiography shows normal left ventricular function and no patent foramen ovale. The nearest ECMO centre is 90 minutes away by road and has a retrieval team. Her ICU has the equipment and staff to prone but has no written proning protocol.

1. Rank the next interventions you would make over the next six hours and justify each with the physiological reasoning and the evidence base.

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Priority order:

  1. Prone position, 16 hours or more per day is the single intervention with the best mortality evidence in moderate-to-severe ARDS (PROSEVA: P/F ratio below 150, 28-day mortality reduced from 32.8% to 16%). Haemodynamic instability requiring noradrenaline at 0.35 micrograms/kg/min is not a contraindication; only an open abdomen, unstable spine or raised intracranial pressure are. The team must be trained, with a checklist and a designated airway holder during the turn.
  2. Ensure deep sedation and add neuromuscular blockade for 24–48 hours to enable proning and abolish patient–ventilator dyssynchrony, which is a major driver of tidal hyperinflation and worsening lung injury. Cisatracurium is preferred.
  3. Optimise the ventilator to a driving pressure below 15 cmH2O. Current driving pressure is 34 minus 16 = 18. Consider reducing tidal volume towards 5 mL/kg predicted body weight (accepting permissive hypercapnia down to about pH 7.20), and titrate PEEP to the ARDSNet table or to the best compliance rather than to oxygenation alone; a cautious recruitment manoeuvre only if there is clear evidence of derecruitment, given the risk of barotrauma.
  4. Treat the cause: continue oseltamivir, screen for secondary bacterial or fungal infection with tracheal aspirate and blood cultures, and review antibiotics.
  5. Conservative fluid strategy: she is oliguric (0.2 mL/kg/h) with no clear hypovolaemia; a FACTT-type conservative approach shortens ventilation, but do not allow the patient to become prerenal.
  6. Call the ECMO centre now - see the next question - and prepare the patient for retrieval while continuing the above.
  7. Avoid routine high-frequency oscillation, routine recruitment manoeuvres, and inhaled nitric oxide outside a rescue setting; none improves mortality.

2. Which criteria would make you call the ECMO centre now, and how would you ventilate and manage her while awaiting retrieval?

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She already meets referral criteria. Common thresholds include a PaO2/FiO2 ratio below 80 mmHg for more than 6 hours, or below 50 mmHg for more than 3 hours, despite optimised conventional therapy; or uncompensated hypercapnia with pH below 7.25 and PaCO2 above 60 mmHg despite a respiratory rate up to 35 and a plateau pressure of 32 cmH2O or less.

Here the P/F ratio is 55 divided by 0.9, about 61 mmHg - well under 80 - and the pH is 7.22 with a PaCO2 of 58, close to the hypercapnic criterion. She also has a reversible cause (influenza), is young, and has no major comorbidity, so she is a good candidate. Call, and let the retrieval team triage; do not wait for further deterioration.

While waiting:

  • Keep tidal volume at 6 mL/kg predicted body weight or less, plateau pressure below 30 cmH2O where achievable, and driving pressure below 15 cmH2O; tolerate SpO2 of 88–95% and permissive hypercapnia.
  • Prone as soon as possible and keep her prone if she is stable, since retrieval teams often transport prone patients.
  • Avoid hyperoxia and further volutrauma; do not chase a normal PaCO2.
  • Secure large-bore access, arterial line, and prepare for anticoagulation discussion with the retrieval team; avoid femoral central lines if cannulation may be needed at that site.
  • Check and document contraindications to ECMO (severe irreversible neurological injury, advanced malignancy, uncontrolled bleeding), and discuss goals of care with the family early.
  • Continue sedation, paralysis and enteral feeding; treat fever and nosocomial infection.

3. During the first proning turn, SpO2 falls to 68% and peak airway pressures rise. How do you troubleshoot in a structured way?

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Desaturation during proning is usually mechanical, not a failure of proning itself. Work through it in order while turning the patient back if there is no rapid improvement.

  1. FiO2 to 1.0 immediately and increase the ventilator rate modestly.
  2. Tube and circuit: check tube depth against the documented mark and capnography; disconnection or partial withdrawal during the turn is the commonest cause. Check the whole breathing circuit and catheter mount for kinks under the patient's chest.
  3. Obstruction: suction through the tube; secretions and blood pooling after repositioning are frequent. If resistance is high, check for tube bite or kink with a suction catheter.
  4. Airway pressures: compare peak and plateau. A rise in both suggests airway/resistance or reduced compliance; a rise in peak alone suggests obstruction. Look at chest wall expansion and auscultate both sides.
  5. Pneumothorax: a sudden rise in airway pressure with desaturation and unilateral signs in a high-pressure ARDS lung is a pneumothorax until proved otherwise - consider needle decompression or ultrasound and call for a chest drain.
  6. Positioning: ensure the abdomen is free and hanging, the breasts are displaced laterally, the face and eyes are not compressed, and the head is turned so the tube is not kinked. Increased abdominal pressure reduces compliance.
  7. Return supine if SpO2 remains below 80–85% despite these measures or the patient becomes unstable; proning is not worth a cardiac arrest. Afterwards, debrief the turn with the team and record the cause.