The Two-Digit Rule and Winter’s Formula

Abstract

No abstract available. Article truncated after 150 words. I recently wondered, “How does the “Two-digit rule” for determining the adequacy of respiratory compensation for metabolic acidosis relate to Winter’s formula?” Many clinicians were taught this rule-of-thumb: the last two digits of the pH (those visible if you hold your thumb over the “7”) should provide an approximation of pCO2 (in mm Hg) in the presence of normal respiratory compensation. Although Winter’s formula is easy to remember and perform, the Two-digit rule is even easier. Winter’s formula and the Two-digit rule have both been derived empirically from clinical data (1,2). But the reason the Two-digit rule should work has never, to our knowledge, been adequately explained. We wondered how the two rules were related. We began with serum HCO3 values ranging from 4 -18 mmol/L and used them to calculate the corresponding predicted pCO2 values using Winter’s formula (3). Next, we used the Henderson-Hasselbalch equation to calculate the corresponding …

Keywords

• two-digit rule
• winter's formula
• ph
• metabolic acidosis
• serum bicarbonate
• henderson-hasselbalch equation
• pco2
• carbon dioxide
• graph
• ventilatory drive