The Henderson-Hasselbalch Equation (H-H)
H-H equation mathematically illustrates how the pH of a solution is influenced by the HCO3– to H2CO3 ratio (the bicarbonate buffer system); the base to acid ratio
H-H equation is written as follows:
pK is derived from the dissociation constant of the acid portion of the buffer combination
pK is 6:1 and, under normal conditions, the HCO3– to H2CO3 ratio is 20:1
Clinically, the dissolved CO2 (PCO2 x 0.03) can be used for the denominator of the H-H equations, instead of the H2CO3
This is possible since the dissolved carbon dioxide is in equilibrium with, and directly proportional to, the blood [H2CO3], the PaCO2 is easily measured via blood gas analysis and can easily be converted to mmol/L (same as mEq/L).
Thus, the H-H equation can be written as follows:
H-H Equation Applied During Normal Conditions
When the HCO3– is 24 mEq/L, and the PaCO2 is 40 mm Hg, the base to acid ratio is 20:1 and the pH is 7.4 (normal).
H-H equation confirms the 20:1 ratio and pH of 7.4 as follows:
The ratio is the important factor, not the individual concentrations.
A HCO3- of 48 and a PCO2 of 80 would still give a ratio of 20/1
H-H Equation Applied During Abnormal Conditions
When the HCO3– is 29 mEq/L, and the PaCO2 is 80 mm Hg, the base to acid ratio decreases to 12:1 and the pH is 7.18 (acidic)
H-H equation confirms the 12:1 ratio and the pH of 7.18 as follows:
In contrast, when the HCO3– is 20 mEq/L, and the PaCO2 is 20 mm Hg, the base to acid ratio increases to 33:1 and the pH is 7.62 (alkalotic)
H-H equation confirms the 33:1 ratio and the pH of 7.62 as follows: