Assessment of gastric acidity in intensive care patients: Intermittent pH registration cannot replace continuous pH monitoring

Objective. To test the accuracy of colour-scaled indicator papers to measure pH values and to study the correlation between this method of measuring gastric juice pH once daily and 24-h continuous intragastric pH monitoring in intensive care patients. Design. The accuracy of indicator papers was tested in the laboratory using colourless solutions and aspirated gastric juice and was then verified with a laboratory pH meter. Continuous intragastric pH monitoring was performed in mechanically ventilated ICU patients. Percentages of time with a pH value < 3.0 and median pH values by 24-h continuous intragastric pH monitoring were compared to pH values measured once daily with indicator paper. Setting: A mixed ICU. Patients: A total of 150 measurements were taken by continuous pH monitoring in 91 mechanically ventilated ICU patients. Measurements and results: The correlation between the pH measured with the indicator paper and subsequently verified with a laboratory pH meter in colourless solutions was 0.96 [regression coefficient (RC) 0.98, 95% confidence interval (CI) 0.91-1.05]. Measured in gastric juice it was 0.95 (RC 0.95, 95% CI 0.88-1.01). The correlation between median pH values, determined with 24-h continuous intragastric pH monitoring, and values measured with indicator papers was 0.39 (RC 0.43, 95% CI 0.26-0.59). The mean difference in pH, as determined by the analysis of Bland and Altman [22], was 0.9 with a SD of 4.7. The correlation between the percentage of time with pH < 3.0, as obtained with continuous registration, and median gastric pH values (also obtained with continuous registration) was - 0.94 (RC - 0.06, 95% CI - 0.06- - 0.05); the correlation between the time and gastric pH values (measured with indicator paper) was - 0.40 (RC - 0.02, 95% CI - 0.03- - 0.02). Conclusion: The colour-scaled indicator paper is an accurate method of measuring pH values, but there is a poor correlation between gastric pH values measured once daily and a total measurement derived from 24-h continuous intragastric pH monitoring. Changes in intragastric pH values cannot be accurately studied when measuring acidity once daily. The influence of various treatment regimens on intragastric acidity in relation to the development of gastric colonization and nosocomial pneumonia should be investigated either with continuous intragastric monitoring or with frequent measurements in aspirated gastric juice.