How is bicarbonate (HCO3-) calculated on a Venous Blood Gas (VBG)?

Calculation of Bicarbonate on Venous Blood Gas (VBG)

On a venous blood gas (VBG), bicarbonate (HCO3-) is calculated using the Henderson-Hasselbalch equation based on the measured pH and PCO2 values.

The Calculation Process

• Bicarbonate is calculated rather than directly measured on blood gas analyzers using the Henderson-Hasselbalch equation 1:

  pH = 6.1 + log([HCO3-]/[0.03 × PCO2])

• When rearranged to solve for bicarbonate:

  [HCO3-] = 0.03 × PCO2 × 10^(pH - 6.1) 1

• This calculation uses the measured venous pH and PCO2 values from the blood gas analyzer 1

Modern Blood Gas Analyzers

• Modern blood gas analyzers automatically perform this calculation and report the bicarbonate value alongside the directly measured parameters (pH and PCO2) 1

• The calculation accounts for the relationship between CO2, water, carbonic acid, and bicarbonate as represented by the equation: CO2 + H2O ⟶ H2CO3 ⟶ H+ + HCO3- 1

Venous vs. Arterial Bicarbonate

• Venous bicarbonate values are typically slightly higher than arterial values, with studies showing a mean difference of approximately 0.8-2.3 mmol/L 2, 3

• Regression equations have been developed to predict arterial bicarbonate from venous values:

  • Arterial HCO3- = 2.34 + 0.82 × venous HCO3- 4
  • Arterial HCO3- = 0.513 + 0.945 × venous HCO3- 2

• Some clinicians use a simple correction factor where arterial HCO3- ≈ venous HCO3- × 0.90 4, 3

Clinical Considerations

• Bicarbonate values calculated from VBG samples show strong correlation with those from arterial samples (correlation coefficients of 0.91-0.97) 2, 4, 3

• The calculated bicarbonate value is used in assessing acid-base status, particularly in identifying metabolic acidosis or alkalosis 1

• In patients with respiratory acidosis, the bicarbonate level may be elevated as a compensatory mechanism, especially in chronic cases 1

• When interpreting calculated bicarbonate values, clinicians should be aware that:

  • Organic acids can cause a positive bias in some measurement methods 5
  • The pK' value used in the Henderson-Hasselbalch equation may vary in certain clinical conditions, particularly metabolic acidosis 5

Limitations and Caveats

• The calculated bicarbonate should not be substituted for arterial bicarbonate when calculating arterial pH or PaCO2 using the Henderson-Hasselbalch equation 6

• The accuracy of the calculated bicarbonate depends on the precision of the pH and PCO2 measurements 5

• Blood gas samples should be analyzed promptly to avoid errors due to ongoing metabolism in the sample 6

• In hemodynamically unstable patients, the correlation between venous and arterial values may be less reliable 3