Magnesium trisilicate mixture BP. Its physical characteristics and effectiveness as a prophylactic.
Study of samples of magnesium trisilicate mixture BP obtained commercially has shown that redistribution of the mixture into a reasonably well-dispersed suspension becomes increasingly difficult the longer the mixture is stored. This process of compacting is hastened in response to an increase of environmental temperature. It is possibly for this reason that to administer the mixture at some time later than shortly after it has been prepared contravenes the recommendation of the British National Formulary, that 'it should be recently prepared.' These findings were reflected in the contrast between gastric aspirates obtained from anaesthetised patients who had received the commercially-obtained antacids and those whose antacid had been prepared by the hospital pharmacy. There was a higher incidence of patients 'at risk' because of aspirates with pH value less than 3.0 in the former group. Among the latter group of 131 patients, only one was found to have been 'at risk'. The importance of administering a final dose of antacid within the 15-minute period before induction of anaesthesia is emphasised. The results of assays of the magnesium concentration in the gastric aspirates varied widely, and bore no systematic relationship to the total dose of antacid administered, the time elapsing since the last dose was given, or the pH value of the same aspirate.