Preoperative medication is usually administered parenterally; however a more acceptable delivery route, particularly for children, is being sought. Rectal administration of midazolam produced a satisfactory sedative action 30 minutes after administration in children129. Rectal instillation of a solution of midazolam hydrochloride (5 g.L-1:0.3 mg.kg-1) in healthy volunteers produced a bioavailability of about 50%, however metabolic studies suggested that complete rectal absorption of the parent drug had occurred with substantial first-pass metabolism130. Absorption was rapid, the mean T being 31 min and C
max max reaching 120 pg.L-1.
Diazepam may also be used for preoperative medication. In adult patients, one study reported that premedication with a rectal diazepam solution was considered less effective than oral dosing. However the oral dose used was 50% higher than the rectal dose (15 vs. 10 mg)131. Rectally administered diazepam has been used in children for sedation for dental operations132.
The use of rectal methohexital to induce anaesthesia in children has received considerable interest in recent literature. In children aged between 2 and 7 years, anaesthesia was induced with rectal administration of 15 mg.kg-1 of methohexital solution. Peak plasma concentrations ranged between 1 and 6 mg.L-1 and were reached in 7 to 15 min. indicating very rapid absorption133. Although the bioavailability ranged between 8 and 32%, satisfactory induction of anaesthesia was reached in 90% of children. It has been suggested that the variability in methohexital plasma levels is due the depth to which the drug is inserted, thereby altering the amount of drug which avoids first-pass metabolism. No clear correlation exists between rectal pH and absorption, suggesting that the variability in the extent of first-pass metabolism has a greater effect than the effect of luminal pH on drug uptake134.
Atropine is administered prior to inhalation anaesthetics to reduce salivation and the production of bronchopulmonary secretions. Absorption from a rectal saline solution was slower and less complete than with intramuscular administration135. Atropine sulphate in a HPMC base had a bioavailability of approximately 30%, but absorption was fast, reaching a T after 15 minutes136.
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