Henderson-Hasselbalch Equation for ABG Interpretation
The Henderson-Hasselbalch equation is the fundamental formula used to calculate plasma bicarbonate from measured pH and pCO2 in arterial blood gas analysis: pH = pK' + log([HCO3-]/[0.03 × pCO2]), where pK' is approximately 6.1 1.
How the Equation Works in Clinical Practice
The equation allows blood gas analyzers to calculate bicarbonate concentration from two directly measured values:
- pH (measured by electrode)
- pCO2 (measured by electrode)
- Bicarbonate [HCO3-] is then calculated, not directly measured
The constant pK'1 (the apparent first dissociation constant for CO2 in serum) is functionally fixed at 6.10 ± 0.018 in clinical practice, with variability explained by measurement error alone 1. This validates the reliability of calculated bicarbonate in most clinical situations.
Clinical Application for Acid-Base Interpretation
Using this equation, you can categorize four primary acid-base disturbances 2:
- Respiratory acidosis: Elevated pCO2
- Respiratory alkalosis: Decreased pCO2
- Metabolic acidosis: Decreased bicarbonate/base excess
- Metabolic alkalosis: Increased bicarbonate/base excess
Important Context from Guidelines
ABG measurement is essential prior to and following initiation of non-invasive ventilation in acute hypercapnic respiratory failure 3. The guidelines specify that NIV should be started when pH <7.35 and pCO2 >6.5 kPa persist despite optimal medical therapy in AECOPD 3.
Critical Caveats and Limitations
When the Traditional Approach May Be Insufficient
The Henderson-Hasselbalch approach works well when serum total protein, albumin, and phosphate concentrations are approximately normal 2. However:
- Poor agreement exists between calculated and enzymatically measured bicarbonate in critically ill patients, with bias of -1.6 mmol/L and limits of agreement from -5.85 to 2.65 mmol/L 4
- When protein, albumin, or phosphate are markedly abnormal, the equation may provide erroneous information about the cause of acid-base disturbances 2
What "Bicarbonate" Actually Includes
The calculated "HCO3-" may include CO3, carbamate, various ion pairs, and other bound CO2—not just free bicarbonate ions 5. This is clinically acceptable for routine interpretation but explains some measurement discrepancies.
Practical Clinical Interpretation
Compensated vs. Acute Respiratory Acidosis
High pCO2 with high bicarbonate and normal pH indicates "compensated respiratory acidosis"—common in stable severe COPD 6. However, during acute exacerbations, patients develop "acute-on-chronic" respiratory acidosis because the bicarbonate level equilibrated with the previous CO2 and cannot buffer the sudden further increase 6.
Key Physiological Principle
Approximately 20% of patients with AECOPD requiring hospital admission have respiratory acidosis 6. The equation helps identify when acidosis (pH <7.35) persists despite medical therapy, triggering escalation to NIV 3.
Alternative Approaches
While some advocate for the Stewart-Fencl strong ion approach in critically ill patients with marked protein/electrolyte abnormalities 2, 7, the Henderson-Hasselbalch equation remains the standard clinical tool and is sufficient for routine ABG interpretation when protein and electrolyte levels are near normal 2.