Haemodynamic changes caused by oxytocin during caesarean section under spinal anaesthesia

Abstract

The haemodynamic effects of oxytocin receive scant attention in pharmacology texts, but may be clinically significant in vulnerable patients. Despite prescriber information recommending a dose of 5 international units by slow i.v. injection, it is the authors' experience that it is very common practice in the UK to give 10 units as a rapid injection. We therefore conducted a randomised, double-blind study of the haemodynamic changes induced by rapid bolus of 5 or 10 units of oxytocin in 34 healthy term parturients at caesarean section under spinal anaesthesia. There was a small but statistically significant (P < 0.05) reduction in mean arterial pressure from baseline 30 s after a 10-unit bolus. However, large, statistically significant increases in heart rate and cardiac output occurred 1 min after 5 units and 2 min after 10 units. These changes peaked 1 min after oxytocin administration and were greater in the 10-unit group (P < 0.05). The importance of these findings is that some women with hypovolaemia or cardiac disease may be unable to mount these compensatory responses and are therefore at risk of haemodynamic collapse after oxytocin boluses. This has been illustrated by a maternal death reported to the Confidential Enquiries into Maternal Deaths in the United Kingdom. The need to adhere to a dose regimen of 5 units by slow injection needs re-emphasis, but no evidence exists to claim that even this will be haemodynamically inert. We therefore recommend that oxytocin boluses be avoided in women with hypovolaemia or cardiac disease.