Evaluation and Comparison of the Distributions of Gastric pH and Hydrogen Ion Concentration

The shapes of the distributions of gastric pH and hydrogen ion concentration [H⁺] were determined for each of 68 groups of patients scheduled for elective surgery under general anesthesia. The 68 groups comprised a total of 1,326 patients who had served as subjects in 13 of the authors' previously published studies. In general, the results showed that neither pH nor H⁺ was normally distributed; most of the pH distributions (47 of 68 = 69%) and most of the H⁺ distributions (53 of 68 = 78%) showed significant departure from the normal distribution. Moreover, the shapes of the distributions varied, depending upon the conditions under which gastric acidity was assessed. Groups receiving no medication for gastric acidity had positively skewed pH distributions (nonsymmetrical distribution with tail pointing to right and majority of cases in lower range), and groups receiving medications for the reduction of acidity had negatively skewed pH distributions (nonsymmetrical with tail pointing to left and majority of cases in upper range). The medications produced an inverse relationship between mean pH and skewness such that the skewness of the groups decreased from positive to negative as mean pH increased. For H⁺, all groups had positively skewed distributions, but the distributions were more positively skewed for groups receiving medications for gastric acidity. Again, the medication conditions produced an inverse relationship between mean acidity and skewness such that the groups became more positively skewed as the mean H⁺ decreased. Thus, a blanket recommendation of either of the two measures of gastric acidity based on the assumption that the measure has an underlying normal distribution is not warranted by the findings of this study. Based on the argument that pH measures the chemical potential of the hydrogen ion and, thus, is directly related to chemical reactions and biological activity, gastric pH is recommended as the measure of choice for gastric acidity.