Normal Arterial Blood Gas pH Level
The normal arterial pH is 7.35-7.45, with 7.40 representing the physiologic optimum that reflects intracellular neutrality and provides evolutionary advantages for cellular metabolism. 1
Standard Reference Range
- Arterial pH: 7.35-7.45 is the universally accepted normal range used in clinical practice 2, 3
- A pH of 7.40 specifically represents the ideal physiologic value, derived from intracellular pH being close to neutrality, which optimizes enzymatic function and prevents ionized metabolic intermediates from crossing cell membranes 1
Clinical Significance of pH Thresholds
Acidemia Thresholds
- **pH <7.35** with elevated PaCO2 (>6.0-6.5 kPa or >45 mmHg) confirms acute respiratory acidosis requiring intervention 2, 3
- pH <7.26 represents significant acidemia associated with poor outcomes and increased mortality, requiring urgent treatment 3
- pH <7.25 is a critical threshold indicating severe acidemia with high mortality risk 3
- pH <7.20 is the lower safety limit when implementing permissive hypercapnia strategies in mechanically ventilated patients 2
Key Clinical Decision Points
- pH 7.26-7.35 with PaCO2 >6.5 kPa and respiratory rate >23 breaths/min indicates need for non-invasive ventilation after one hour of optimal medical therapy 2
- pH between 6.0-6.5 kPa with acidosis should prompt consideration (not automatic initiation) of NIV, as this level may not contribute substantially to acidosis 2
Important Caveats
- Arterial sampling is essential for accurate pH measurement in acute settings, as venous pH can differ significantly (mean 0.5% lower) and does not reflect tissue acid-base status during critical illness 4, 5
- During cardiopulmonary resuscitation, arterial pH may appear falsely normal (mean 7.41) while mixed venous pH is severely acidotic (mean 7.15), making arterial values misleading in this specific context 5
- pH alone has limited diagnostic value for distinguishing between pulmonary and non-pulmonary causes of dyspnea, but serves as an independent predictor of 12-month mortality in patients presenting with acute dyspnea 6