Normal pH and Definitions of Acidosis and Alkalosis
Normal arterial blood pH is approximately 7.35-7.45, with acidosis defined as pH ≤7.35 and alkalosis as pH >7.45. 1, 2
Normal pH Range
- Arterial blood pH normally ranges from 7.35 to 7.45, representing a tightly regulated acid-base balance that is critical for cellular function 2
- The commonly cited "normal" value of 7.40 represents the midpoint of this physiologic range 3
- Normal pleural pH is approximately 7.6 due to bicarbonate accumulation in the pleural cavity 1
Acidosis Definition
Acidosis is defined as arterial pH ≤7.35, which represents a clinically significant accumulation of hydrogen ions 1, 2
Severity Classification of Acidosis
- Mild acidosis: pH 7.30-7.35 1
- Moderate acidosis: pH 7.25-7.35 (in respiratory contexts) 1
- Severe acidosis: pH <7.25 1
- Critical acidosis: pH <7.20 indicates severe physiologic derangement 1
Clinical Significance by pH Threshold
- pH <7.26 is predictive of poor outcomes in acute exacerbations of COPD and represents a critical threshold for intervention 1
- pH <7.2 in pleural fluid (with normal blood pH) indicates complicated effusions including empyema, rheumatoid disease, or malignancy 1
- **pH <7.3 in malignant pleural effusions** predicts poor survival (median 2.1 months vs 9.8 months for pH >7.3) 1
Alkalosis Definition
Alkalosis is defined as arterial pH >7.45, though the provided evidence focuses primarily on acidotic states 2
- The upper limit of the normal range (7.45) serves as the threshold above which alkalosis is diagnosed
- Chronic respiratory acidosis may be associated with compensatory metabolic alkalosis due to renal bicarbonate retention 2
Important Clinical Context
- The development of acidosis in patients with chronic hypercapnia indicates acute-on-chronic respiratory failure, not simply baseline status 1
- Respiratory acidosis specifically occurs when PaCO₂ rises above 6.1 kPa (46 mmHg) with pH falling below 7.35 2
- In acute settings, aim for PaO₂ ≥6.6 kPa without pH falling below 7.26 when managing oxygen therapy 1
Common Pitfall to Avoid
Do not confuse chronic compensated hypercapnia (elevated CO₂ with normal pH due to metabolic compensation) with acute respiratory acidosis (elevated CO₂ with pH ≤7.35). The presence of acidosis, not just elevated CO₂, defines the acute pathologic state requiring urgent intervention 1, 2