Does twice the volume of antacid have twice the effect in pregnant women at term?

The clinical effect of doubling the administered volume of a nonparticulate antacid (0.3M sodium citrate) was investigated in pregnant females using a radiotelemetry technique. The subjects swallowed a pH radio pill and the signal was detected transabdominally by an aerial connected to a radio receiver. In vitro studies showed that twice as much acid would be titrated to pH 2.0 when the volume of sodium citrate was doubled. The in vitro studies indicated that there was no significant difference between 15 ml and 30 ml of sodium citrate. The median duration of action of 15 ml of sodium citrate was 30.5 min compared with 42.1 min for 30 ml of sodium citrate. The area under the pH vs time curve was used as a measure of antacid efficacy. The median value for 15 ml of sodium citrate was 103.0 pH X min and for 30 ml of sodium citrate was 126.5 pH X min. The most likely explanation for the difference between in vitro and in vivo studies is that gastric emptying is the predominant factor influencing antacid effect in patients.