PURPOSE: Acute change in pCO2 leads to an acute change in pH, but the pH change is not linearly related to pCO2 change. Hydrogen ion (H) does change linearly with pCO2, with an equation H= 0.75 * pCO2 +10. Although accurate, this equation is difficult mentally. Converting H to pH is not difficult using the 80% rule. Each successive pH change of 0.10 causes H to be 80% of the previous H. pH values 7.00 7.10 7.20 7.30 7.40 7.50 7.60 7.70 have H values 100 80 64 50 40 32 25 20. pH is 9 –log(H), acute respiratory H = 0.75 * pCO2 + 10, hence pH = 9 –log(.75 * pCO2 + 10) = 9 –log(.75 * (pCO2 + 13.3)) = 9 –log(pCO2 + 13.3) –log(.75) = 9 –log(pCO2 + 13.3) + 0.124. This result suggests the acute respiratory pH can be accurately estimated mentally by adding 13 to the pCO2, converting this number from H to pH, and adding 0.12. This method is called the Baker's Dozen Rule.
METHODS: Author examined all integer pCO2 values from 15 to 100, in random order. 13 was added to the pCO2, the result was mentally converted to pH, then 0.12 added to get acute respiratory pH. The result was compared to the acute pH computed using the equation H = .75 * pCO2 + 10. Example, pCO2 = 87, adding 13 gives 100, which converts to a pH of 7.00, and adding 0.12 gives an acute respiratory pH 7.12. Interpolation is used when an H is in between the 80% values above.
RESULTS: The average absolute difference between the mental estimate and the calculated value for acute respiratory pH was 0.004. The maximum absolute difference between the mental estimate and the calculated value for acute respiratory pH was 0.01.
CONCLUSION: Mental estimation of acute respiratory pH is possible using the Baker's Dozen Rule.
CLINICAL IMPLICATIONS: Mixed acid-base problems are detectable using the Baker's Dozen Rule.
DISCLOSURE: Terrence Fagan, No Financial Disclosure Information; No Product/Research Disclosure Information