In an adult mechanically ventilated patient with marked acidosis (pH ~7.2–7.3), PaO2 150 mm Hg on FiO2 0.40 and PEEP 5 cm H2O, to what PEEP level should I increase to improve ventilation and CO2 clearance while preserving oxygenation?

Ventilator Management for Acidotic Patient

Increase PEEP to at least 10 cm H₂O immediately, and consider titrating up to 12-15 cm H₂O while monitoring plateau pressures to stay below 30 cm H₂O. Your patient has adequate oxygenation (PaO₂ 150 mmHg on FiO₂ 0.40) but severe acidosis requiring urgent intervention to improve CO₂ clearance and prevent ventilator-induced lung injury.

Primary Ventilator Adjustments

PEEP optimization is your first priority:

• Increase PEEP from 5 to >10 cm H₂O as recommended for mechanically ventilated patients to maintain alveolar inflation, prevent atelectasis, and improve ventilation-perfusion matching 1
• Target PEEP of 10-15 cm H₂O is appropriate for most mechanically ventilated patients with acidosis, particularly when oxygenation is already adequate 1
• Monitor plateau pressures carefully—keep below 30 cm H₂O to avoid barotrauma and employ permissive hypercapnia if needed 1

Additional ventilator modifications:

• Ensure tidal volumes are 6-8 mL/kg ideal body weight to prevent ventilator-induced lung injury 1
• Increase respiratory rate to 15-20 breaths/min to enhance minute ventilation and CO₂ clearance 1
• Adjust I:E ratio to allow adequate expiratory time (1:2 to 1:4 depending on underlying pathology) 1

Target pH and Permissive Hypercapnia Strategy

Accept pH >7.2 as your therapeutic target:

• The consensus target for difficult-to-control pH is 7.2, which is well-tolerated and reduces mortality when attempting to normalize pH would require excessive airway pressures 1
• Permissive hypercapnia is indicated when peak airway pressure exceeds 30 cm H₂O, as this strategy reduces mortality in ARDS and acute respiratory failure 1
• Avoid rapid correction of CO₂—a large drop in PaCO₂ (>20 mmHg) within 24 hours is associated with intracranial hemorrhage and acute brain injury 1

Oxygenation Management

Your current oxygenation is actually excessive:

• PaO₂ of 150 mmHg represents mild hyperoxia, which is associated with mortality and poor neurological outcomes 1
• Reduce FiO₂ to target SpO₂ 92-97% (or 88-92% if chronic CO₂ retainer) to avoid hyperoxia-related complications 1, 2
• This allows you to focus ventilator adjustments on CO₂ clearance rather than oxygenation 1

Critical Monitoring Parameters

Reassess arterial blood gas within 1-2 hours:

• Check pH, PaCO₂, and plateau pressures after implementing PEEP changes 2
• If pH continues to worsen despite optimized ventilator settings with plateau pressure <30 cm H₂O, consider adjunctive therapies 1
• If pH fails to improve or worsens, or if plateau pressures exceed 30 cm H₂O, accept permissive hypercapnia with target pH >7.2 1

Important Caveats

PEEP can be harmful in specific circumstances:

• In obstructive lung disease (COPD, asthma), setting PEEP above intrinsic PEEP (iPEEP) can worsen hyperinflation and is deleterious 1
• If your patient has obstructive disease, measure iPEEP first and keep applied PEEP slightly below this level 1
• In restrictive disease or ARDS, higher PEEP assists lung recruitment and improves compliance 1

Address metabolic components separately:

• Your patient likely has mixed respiratory and metabolic acidosis given the severity 2
• Treat any metabolic causes (lactic acidosis from hypoperfusion, diabetic ketoacidosis, renal failure) concurrently 1
• Consider bicarbonate therapy only if severe metabolic acidosis persists despite treating underlying cause 1

Sedation requirements:

• Higher PEEP and controlled ventilation modes may require deeper sedation to prevent patient-ventilator dyssynchrony 1
• Adequate sedation improves tolerance of permissive hypercapnia strategy 1