Effects of Preoperative Sublingual Misoprostol on Uterine Tone During Isoflurane Anaesthesia for Caesarean Section

Volatile anaesthetics including sevoflurane, desflurane, and isoflurane are often used during general anaesthesia for caesarean delivery. The cost effectiveness of isoflurane anaesthesia1 for caesarean delivery is widely used at many centres including the authors' centre, where general anaesthesia is commonly used for caesarean deliveries than did regional anaesthesia because of the refusal of many ladies for the later, especially in the light of evidence-based equivocal maternal or neonatal outcomes of both techniques. 2 However, isoflurane in similar to other inhalational anaesthetics have been shown a dose-dependent (from 0.5 to 2.35 minimum alveolar concentration (MAC)) induced myometrial relaxation in 25% of parturients with added risks of postpartum haemorrhage, 3-5 which may be mediated through the inhibition of the oxytocin-induced contraction,6 decrease in intracellular concentration of free calcium,7 inhibition of voltage-dependent calcium channels activity,8 and activation of adenosine triphosphate-sensitive potassium channels (K (ATP))9 of pregnant uterine smooth muscle.

Several uterotonics such as oxytocin reduce postpartum haemorrhage by inducing uterine contraction, but with added risks of haemodynamic adverse effects. 10 Sublingual or rectal misoprostol, a prostaglandin E1 analogue, in doses of 100 to 800 µg is safe and as effective as intravenous infusion of oxytocin in reducing blood loss and the need for additional oxytocin after caesarean delivery under either spinal or general anaesthesia, with occurrence of transient side effects such as nausea, shivering and pyrexia. 11-17 Misoprostol possess several advantages over oxytocin, including long shelf life, stability at room temperature, and possible buccal, rectal and sublingual administration. 11, 16The later has many advantages such as rapid uptake, long-lasting duration of effect, and greatest bioavailability, compared with other routes of misoprostol administration. 18 Up to our best knowledge, this trial was the first one studied the inhibitory effects of misoprostol on the uterine atonic effects of inhalational anaesthetics.

Based upon previous published data, 19 blood loss after caesarean delivery was normally distributed with standard deviation 560 ml. A priori power analysis indicated that 174 patients in each group would be sufficient to detect a 20% reduction in blood loss after caesarean delivery, with a type-I error of 0.05 and a power of 90%. The investigators added 10% more patients to account for patients dropping out during the study.

Patients were randomly allocated to receive sublingual 400 µg of misoprostol or identical placebo two tablets after tracheal intubation before surgery.

Anaesthetic management was standardized in all studied patients. Oral ranitidine 150 mg was given the night before and on the morning of surgery, with 0.3 mol/L sodium citrate (30 mL) given 15 min before induction. In the operating theatre women were positioned supine on the operating table with 15° firm rubber wedge under the right hip to effect left uterine displacement. A slow 500-mL i.v. infusion of lactated Ringer's solution was given to all subjects over 20 min.