Inaccuracy of hourly sampled pH measurements in describing the effect of antisecretory drugs on circadian gastric acidity

This study was carried out in order to verify whether gastric pH measurements obtained with continuous monitoring are more accurate in assessing the pharmacodynamic properties of antisecretory drugs than those obtained with the traditional hourly nasogastric aspiration method. Accordingly, we compared the most commonly used acidity indexes (median of pHs and mean of [H+]s) and the total circadian time spent above 4.0 pH units, all of which were calculated from both raw data sampled at rates of a few seconds with modern apparatus and data obtained from the same profiles scanned punctually at a 60 minute rate. The analysis referred to 231 continuous gastric pH-metries which were performed over the circadian period to evaluate the acid inhibitory effects of different doses and dosage regimens of various H2-antagonists. In the population as a whole, the numbers and percentages of subjects, whose 60-minute sampled medians of pHs and means of [H+]s differed by less than +/- 10% from those obtained from raw fast-acquired data, were 107 (46%) and 34 (15%), respectively. There was even less agreement in the case of the circadian time above 4.0 pH units (26 cases = 11%). No statistical difference (P = .7) between the concordance rates of each variable in the three groups was observed. Considered simultaneously, there was concordance of the above three indexes in only seven out of the 231 cases. These results show that the hourly sampling rate fails to represent adequately gastric acidity changes induced by antisecretory drugs, due to the scarcity of experimental points. The acidity indexes calculated from these slowly sampled data are inaccurate, especially when the means of [H+]s are considered, and the duration of action of the drug often is defined randomly.
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