Effects of a height and weight adjusted dose of local anaesthetic for spinal anaesthesia for elective Caesarean section.

In this prospective, randomised, double-blind study, we compared the effects of two dosage regimens. Pregnant patients at term were randomly assigned to two groups to be given diamorphine 0.4 mg in hyperbaric bupivacaine 0.5% 2.4 ml or diamorphine 0.4 mg in a volume of hyperbaric bupivacaine 0.5% adjusted according to the patient's height and weight. Adequate anaesthesia was provided in all patients in both groups. The onset of the sensory block for cold and pinprick was faster with the fixed dose regimen (p = 0.01). There were more spinal blocks to above the first thoracic dermatome in the fixed dose group (17.1% vs. 2.2%, p = 0.022). Hypotension occurred in 71.7% vs. 50.0% of patients in the fixed dose and adjusted dose groups respectively (p = 0.035). In the fixed dose group, more patients required ephedrine to treat hypotension (79.5% vs. 56.8%, p = 0.022) and a larger median dose was administered (9 mg vs. 6 mg, p = 0.042). The decrease in mean (SD) arterial pressure was less in the adjusted group (35.0 (16.4) mmHg vs. 28.0 (13.5) mmHg, p = 0.036).