A very simple formula to compute pCO2 in hemodialysis patients

To identify mixed acid-base disorders, clinicians must estimate the value of partial pressure of carbonic dioxide (pCO2), complying with the reduced plasma bicarbonate concentration (HCO3). What is the most appropriate equation relating the two quantities in chronic hemodialysis patients remains unknown. Chronic hemodialysis patients remains unknown, which motivates our study.

Among a large database of blood gas analysis from chronic hemodialysis patients, we selected 291 blood samples showing HCO3 < 24 mmol/L and, among these, we further selected a subset of samples claimed for pure metabolic acidosis. A linear approximation based upon the least-square criterion was adopted to derive the best-fit equation. The differences between this and other commonly used formulas were computed in terms of root mean square (RMS) errors.

In chronic hemodialysis patients, the reduction in pCO2 due to metabolic acidosis is better predicted multiplying by 1.2 the reduction in HCO3, or by using the expression pCO2 = HCO3 + 15; the two approaches lead to almost the same results. In contrast, the equation pCO2 = 1.5 × HCO3 + 8, known as Winters' formula, exhibits larger errors.

The easy-to-use expression pCO2 = HCO3 + 15 seems suitable for the daily clinical practice in hemodialysis patients. However, if HCO3 value is lower than 12 mmol/L, a threshold at which different formulas return almost the same value, also Winters' formula, derived in the 60's from patients with low values of HCO3, could be used.