Normal Arterial Blood pH
The normal pH range of arterial blood in humans is between 7.35 and 7.45 units, regardless of whether the patient has severe metabolic acidosis and hypoxemia. 1
Understanding pH in Context
The normal reference range remains constant at pH 7.35-7.45 (or hydrogen ion concentration [H+] of 35-45 nmol/L), which applies universally to all patients. 1 This is the physiologic standard against which all acid-base disturbances are measured.
Defining Acidosis
Acidosis is defined as any pH below 7.35 (or [H+] >45 nmol/L), regardless of the underlying cause. 1 In your patient with severe metabolic acidosis and hypoxemia, the arterial pH would be expected to fall below this normal range, but the normal reference values themselves do not change based on pathology.
Clinical Context for Metabolic Acidosis
Venous vs. Arterial pH Monitoring
- Venous pH can be used to monitor acidosis resolution and is typically 0.03 units lower than arterial pH. 1 This is particularly useful in diabetic ketoacidosis management where repeat arterial blood gases may be unnecessary.
Severity Thresholds in Practice
pH <7.15 in lactic acidemia represents a critical threshold where bicarbonate therapy may be considered, though the Surviving Sepsis Campaign suggests against routine bicarbonate use at pH ≥7.15 for improving hemodynamics. 1
pH <6.9 represents severe acidosis where bicarbonate therapy may be beneficial, particularly in diabetic ketoacidosis. 1
Resolution Criteria
- For diabetic ketoacidosis resolution, venous pH must exceed 7.3, along with serum bicarbonate ≥18 mEq/L and glucose <200 mg/dL. 1 This demonstrates that pH 7.3 represents the minimum threshold for adequate acid-base correction in this specific condition.
Important Clinical Caveats
Metabolic acidosis with normal or low PaCO2 requires investigation for causes beyond respiratory compensation, as this pattern suggests primary metabolic derangement. 1 The expected respiratory compensation should lower PaCO2 by approximately 1 mmHg for every 1 mmol/L fall in serum bicarbonate. 2
In patients with combined metabolic acidosis and hypoxemia, the normal pH range (7.35-7.45) remains the target for correction, though the approach to achieving this must account for both the acid-base disturbance and oxygenation deficit simultaneously. 1