I85PREP
⌘ K
CCRN (Adult) study materials
CCRN (Adult) study resources

ARDS Day 3: When Lung-Protective Ventilation Is Not Enough

CCRN (Adult) · Put your knowledge to work in context.

All case studies

ARDS Day 3: When Lung-Protective Ventilation Is Not Enough

Ms. K, 42, has ARDS from influenza A and is on day 3 of mechanical ventilation. Settings: volume control at 6 mL/kg predicted body weight, PEEP 14 cmH2O, FiO2 0.70, rate 28. ABG: pH 7.22, PaCO2 58 mmHg, PaO2 62 mmHg, HCO3 24. Plateau pressure 32 cmH2O, total PEEP 15, driving pressure 17. SpO2 90%. Respiratory system compliance is 24 mL/cmH2O. She is sedated with propofol and fentanyl, RASS -3, with no spontaneous effort. MAP 68 on norepinephrine 3 mcg/min, lactate 1.8. Two days ago she was on FiO2 0.50.

1. Are lung-protective targets being met? Interpret the gas exchange and pressure data.

Read the worked answer

Pressure targets

  • Plateau 32 cmH2O exceeds the usual target of 30 cmH2O or less.
  • Driving pressure (plateau minus total PEEP) is 17 cmH2O, above the 15 cmH2O threshold associated with worse outcomes.
  • Compliance of 24 mL/cmH2O confirms a stiff, small lung.

Oxygenation

  • P/F = 62 / 0.70 = 89, which is moderate-to-severe impairment and meets criteria for prone positioning (P/F under 150).

Ventilation

  • pH 7.22 with PaCO2 58 is permissive hypercapnia. It is acceptable if it is not new, the patient tolerates it hemodynamically, and there is no contraindication such as elevated intracranial pressure. It is a reason for caution, not a reason to abandon lung protection.
  • Bicarbonate of 24 means she has not had time to compensate; the acidosis is largely acute respiratory.

Bottom line: she is on a lung-protective tidal volume but has not met the pressure targets, and her oxygenation has deteriorated substantially over 48 hours. The tidal volume is not the only lever left.

2. What is your next major intervention, and what will you measure to decide whether it worked?

Read the worked answer

Prone positioning, targeted for at least 16 hours, is the highest-yield next step given P/F 89 on FiO2 0.70 with no contraindication documented.

Why prone rather than a reflex PEEP increase or a tidal volume reduction:

  • Reducing tidal volume below 6 mL/kg with a pH already at 7.22 risks dangerous acidosis without reliably lowering plateau.
  • Going higher on PEEP without a recruitability assessment can overdistend and raise plateau.
  • Proning recruits dorsal lung, improves ventilation-perfusion matching, and has a mortality benefit in moderate-severe ARDS when applied early.

Measure whether it worked

  • Driving pressure and plateau pressure before and 1 hour after proning.
  • PaO2/FiO2 and the FiO2/PEEP needed to hold SpO2 88-95%.
  • Respiratory system compliance.
  • Hemodynamics and vasopressor requirement.
  • Complications: pressure injuries, endotracheal tube displacement, line dislodgement, facial edema, enteral feed intolerance.

Plan for failure: if there is no improvement after a reasonable trial, consider neuromuscular blockade for 48 hours, reevaluate for a recruitment maneuver with extreme caution, and reassess volume status and the possibility of a superimposed process such as ventilator-associated pneumonia or pulmonary embolism.

3. In the first hour of proning, SpO2 falls to 78% and peak airway pressure rises from 30 to 42 cmH2O. Walk through your assessment and decision.

Read the worked answer

First, assume a mechanical problem, not treatment failure. Transient desaturation during the turn is common; sustained desaturation is usually something fixable.

Rapid systematic check

  • Endotracheal tube: depth, patency, secretions, whether it is kinked or against the bed.
  • Circuit: disconnection, water in the tubing, accidental PEEP loss during the turn.
  • Chest wall and abdomen: is she lying on lines, is the abdomen compressed, is the face and airway supported properly with the head turned and eyes protected?
  • Lungs: pneumothorax signs (asymmetric chest rise, sudden pressure rise, falling BP), mucus plug, bronchospasm.
  • Hemodynamics: is this a perfusion problem rather than an oxygenation problem?

Actions

  • Increase FiO2 to 1.0 temporarily and suction.
  • Reposition the head and airway, relieve abdominal pressure.
  • If SpO2 does not recover within a few minutes after these measures, or if there is any suggestion of an airway emergency or pneumothorax, return her to supine and stabilize. Proning is not worth an airway disaster.
  • Document the trial time; a failed short trial does not mean proning is contraindicated, but she needs to be stable before another attempt.