What are the target pH and PaCO2 (partial pressure of carbon dioxide) levels for patients with Acute Respiratory Distress Syndrome (ARDS) on mechanical ventilation?

PaCO2 and pH Targets for Mechanical Ventilation in ARDS

Target a pH ≥7.20 with permissive hypercapnia when using lung-protective ventilation in ARDS patients, accepting elevated PaCO2 levels to maintain plateau pressures ≤30 cmH2O and tidal volumes of 6 mL/kg predicted body weight. 1

Primary pH and PaCO2 Targets

• pH target: ≥7.20 is the consensus lower limit when employing permissive hypercapnia 1
• PaCO2 range: Accept elevation to 60-70 mmHg or higher as long as pH remains ≥7.20 1, 2
• The upper limit for PaCO2 has not been definitively established, though levels up to 158 mmHg have been tolerated in clinical series 2
• Do not attempt to normalize arterial blood gases if doing so requires injurious ventilator settings 1, 3

Rationale for Permissive Hypercapnia

Permissive hypercapnia is the accepted strategy to prevent ventilator-induced lung injury (VILI) when maintaining lung-protective ventilation parameters. 1, 2

• Maintaining plateau pressure ≤30 cmH2O takes absolute priority over normalizing PaCO2 1
• Tidal volume of 6 mL/kg predicted body weight (range 4-8 mL/kg) is mandatory and may necessitate accepting hypercapnia 1, 4
• A pH above 7.20 is well tolerated and reduces mortality compared to strategies targeting normal blood gases 1
• Clinical series demonstrate mortality rates of only 26.4% using permissive hypercapnia with pH as low as 6.79, significantly better than predicted mortality 2

Specific Clinical Scenarios

Severe ARDS (PaO2/FiO2 <150)

• Target pH 7.20-7.40 with permissive hypercapnia if plateau pressure exceeds 28-30 cmH2O 1
• Consider ECMO when pH <7.20 persists for ≥6 hours despite optimized ventilation 1, 4
• Prone positioning should be implemented immediately for severe ARDS, which may improve both oxygenation and ventilation 1, 4

Moderate ARDS (PaO2/FiO2 100-200)

• Same pH target of ≥7.20 applies 1
• Higher PEEP levels (typically >10 cmH2O) should be used, which may help with both oxygenation and CO2 elimination 1, 4

Obstructive Disease Component

• Target pH 7.2-7.4 with permissive hypercapnia if inspiratory airway pressure >30 cmH2O 1
• Use prolonged expiratory time and reduced minute ventilation to limit dynamic hyperinflation 1
• Respiratory rate should be 10-15 breaths/min with I:E ratio of 1:2 to 1:4 1

Important Caveats and Contraindications

Permissive hypercapnia should be used cautiously or avoided in specific conditions:

• Increased intracranial pressure: Hypercapnia causes cerebral vasodilation and raises ICP 1
• Severe myocardial dysfunction: May compromise myocardial contractility 1
• Severe pulmonary hypertension: Though some data suggest THAM may mitigate CO2-induced increases in pulmonary vascular resistance 5

Monitoring Requirements

• Arterial blood gases should be checked 1 hour after ventilator changes and then every 4-6 hours initially 4
• Plateau pressure must be measured and maintained ≤30 cmH2O 1, 4
• Driving pressure (plateau pressure minus PEEP) should be targeted ≤15 cmH2O 4
• Do not use bicarbonate to buffer respiratory acidosis from permissive hypercapnia 2

When Hypercapnia Becomes Unacceptable

If pH falls below 7.20 for ≥6 hours despite optimized lung-protective ventilation, consider:

• Extracorporeal CO2 removal (ECCO2R) with blood flow of approximately 400-450 mL/min 6
• Venovenous ECMO for combined refractory hypoxemia and hypercapnia 1, 4
• These interventions allow further reduction in tidal volume to 4 mL/kg or less while maintaining acceptable pH 6

Oxygenation Targets (Related)

• Target SpO2: 88-95% to avoid hyperoxia while maintaining adequate oxygenation 4
• Target PaO2: 70-90 mmHg 4
• For obstructive disease: SpO2 88-92% 1
• For asthma specifically: SpO2 >96% 1