Winters Formula for Metabolic Acidosis Compensation Calculator

How does this Winters formula for metabolic acidosis compensation calculator work?

This is a tool designed to help clinicians and any medical personnel evaluate PCO₂ compensation in connection with the level of bicarbonate [HCO₃^-].

The form is very simple to use and only requires inputting bicarbonate in mEq/L and press calculate. The result will be displayed as an interval with the lower and upper values of partial CO2 pressure in mmHg.

The formula used by this metabolic acidosis compensation calculator is explained below:

Expected pCO₂ = 1.5 x HCO₃- + 8 +/- 2

Which means the interval between: 1.5 x HCO₃- + 6 and 1.5 x HCO₃- + 10

As a rule of thumb, there is a 1.2 mmHg PCO₂ reduction for every 1 mEq/L reduction of plasma bicarbonate but only to a minimum of 10 - 15 mmHg.

Taking an example, it shows that in order to compensate for a plasma concentration of HCO₃^- of 9 mEq/L it would be required a partial pressure of CO₂ between 19.5 and 23.5 mmHg.  

Interpreting the result:

The patient’s data is then compared to the computed value:

■ If the value retrieved in patient’s data corresponds, this means that the respiratory compensation is adequate.

■ If the value is higher this is indicative of primary respiratory acidosis and if the value is lower than the calculated value, it is indicative of primary respiratory alkalosis.

■ However, the Winters formula is used mostly for metabolic acidosis and in the second case a different set of equations should be used as provided below:

PCO2 = 0.7 x HCO₃^- + 20 +/- 5 mmHg meaning the interval between 0.7 x HCO₃^- + 15 and 0.7 x HCO₃^- + 25.

When to use Winters formula?

There are several cases involving compensation states such as in metabolic acidosis which in turn should compensate by developing respiratory alkalosis or in cases with mixed acidosis.

This formula was developed by Dr. RW Winters specifically to address such cases and to predict the level of respiratory compensation that would be appropriate to recover from acidosis. These results are also often used when analyzing acid base disorders along with the anion gap.

It is important to remember that in metabolic acidosis, the bicarbonate reduction may result even in extracellular imbalances and loss of bicarbonate in urine. The body responds to this by trying to restore the PCO₂/HCO₃^- ratio and does this by trying to reduce what is sees as excess PCO₂.

This is physically made possible with signals from the low arterial pH that stimulates chemoreceptors in charge with respiration that produce an increase in alveolar ventilation. This leads to Kussmaul respiration in the most severe cases.

References

1) Asch MJ, Dell RB, Williams GS, Cohen M, Winters RW. (1969) Time course for development of respiratory compensation in metabolic acidosis. J Lab Clin Med; 73(4):610-5.

2) Albert MS, Dell RB, Winters RW. (1967) Quantitative displacement of acid-base equilibrium in metabolic acidosis. Ann Intern Med; 66(2):312-22.