The main physiological role of oxytocin is to stimulate uterine contractions and milk ejection. As subsidiary effects, it leads to selective vasodilatation and vasoconstriction in a variety of blood vessels. Vasodilatation occurs primarily in the subcutaneous vessels and results in flushing, with vasoconstriction occurring primarily in the splanchnic bed and coronary arteries. It exerts a cardio-acceleratory effect by stimulating myocardial receptors and enhancing atrioventricular conduction. Oxytocin, because of its antidiuretic effect, also leads to water retention and can result in water intoxication. At physiological levels, the action of oxytocin is primarily on the uterus and breast, but the subsidiary actions of oxytocin become prominent at pharmacologic doses.1 The predominant acute side effects of such doses are hypotension, tachycardia and myocardial ischaemia,1 plus nausea and vomiting. On average, the administration of 5 iu bolus oxytocin reduces the mean arterial pressure (MAP) by 27 mmHg.2 Prolonged use of high doses results in water intoxication.

Spinal anaesthesia is the most popular form of anaesthesia administered for caesarean deliveries. The most common and dangerous side effect of spinal anaesthesia is hypotension, which can lead to cardiovascular collapse. Although vasoconstrictors are of some benefit in counteracting these effects, prevention is always better than cure. Unfortunately, it is common practice for a stat bolus dose (usually given intravenously) of 5 or 10 iu of oxytocin to be given at caesarean delivery to minimise blood loss, despite the fact that the uterotonic efficacy of 2 iu of oxytocin is similar to that of 5 iu, whereas the haemodynamic side effects are significantly less with the reduced dose.3 Indeed, a bolus of as little as 0.35 iu may be all that is required for uterotonic effectiveness, and a low dose plus a continuing low dose infusion may be optimal.4 Unfortunately, obstetricians often seem unaware of the haemodynamic effects of oxytocin, whereas anaesthetists seem to think that higher doses of uterotonics are always better in terms of stimulating uterine contractions. It appears that mutual education, one group by the other, would be beneficial.


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