1123: REEVALUATION OF EXISTING ACID-BASE COMPENSATION FORMULAS FOR SEVERE RESPIRATORY ACIDOSIS

Abstract

Learning Objectives: The Davenport diagram was designed in the 1970s as a visual tool to allow clinicians to describe expected compensation for acid base disturbances based on multiple experimental laboratory studies. According to the Davenport nomogram for chronic respiratory acidosis, there is an expected compensatory increase in serum bicarbonate by 3.5–4.0 mEq/L for every 10 unit increase in paCO2. However, the COPD population has changed substantially over time with advances in the treatment. Consequently, traditional formulae may not be applicable to patients with severe respiratory acidosis. Hypothesis: The existing formula for compensation for acid base disorders does not accurately predict bicarbonate in patients with severe respiratory acidosis (paCO2 ≥ 80 mmHg). Methods: We analyzed data from 312 adult patient observations from a single, northeastern Level I Trauma teaching hospital. The study period was from July 2015-July 2017. Inclusion criteria was a pH of 7.2–7.4 along with a paCO2 ≥ 80 mmHg. Measured bicarbonate was obtained from serum electrolyte results drawn within 12 hours of the corresponding blood gas draw. A linear regression model using pH and paCO2 as independent predictors to predict bicarbonate was compared to the traditional formulae for expected bicarbonate compensation in chronic respiratory acidosis; Formula #1: (paCO240)/10 *4 + 24 and Formula #2: (paCO240)/10 *3.5 + 24. Results: The mean and standard deviation of the pH, paCO2, and bicarbonate were 7.27 (0.05), 90.3 (8.9), and 37.9 (5.2) respectively. The regression formula from our data produced the equation: Bicarbonate = 88.64*pH + 0.34*paCO2 637.24. The coefficient of determination (R2) = 0.73. The mean absolute error (MAE; the average of the absolute values of observed – predicted serum bicarbonate) was 2.1 mEq/L. In contrast, the MAE was 6.9 for Formula #1 and 5.2 for Formula #2. The latter two formulas tended to overestimate bicarbonate values across the observed range. Conclusions: Traditional formulae overestimated bicarbonate values during chronic severe respiratory acidosis. A new formula; Serum Bicarbonate = 88.64*pH + 0.34*paCO2 637.24 better predicts bicarbonate in this population.