Prophylactic use of oxytocin at caesarean section: where are the guidelines?

Oxytocin, a polypeptide hormone produced by the posterior pituitary gland, was first synthesised by Vincent du Vigneaud, an American biochemist in the 1950s [1]. He was subsequently awarded the Nobel Prize for Chemistry for this and other work. Syntocinon , its synthetic analogue, was introduced into medical practice in the late 1950s and its dose, administration and clinical applications have been the subject of debate ever since. The findings of a UK-wide survey into the administration of oxytocin, published in this issue of the Journal, highlights continued variation in practice throughout the UK, and the authors call for national protocols to guide dosing [2]. Postpartum haemorrhage remains a significant cause of maternal morbidity [3] and the use of oxytocin has reduced maternal morbidity worldwide [4]. However, use of oxytocin is not without its own problems as it can cause significant maternal side effects [5, 6]. Hypotension and tachycardia are common, and electrocardiographic changes suggestive of myocardial ischaemia may occur [7]. Maternal death related to oxytocin administration is documented in the Confidential Enquiries into Maternal Deaths, 1997–99 [8]. The varied oxytocin dosing regimens used throughout the UK have been a subject of increased interest over the last decade. In 2001, a national survey found that 87% of responding UK lead obstetric anaesthetists were using a dose of 10 IU oxytocin, 50% by rapid intravenous bolus [9]. A repeat of the same survey a year later indicated that only 15% of respondents were using a 10-IU dose, and only 23% of these by rapid bolus [9]. A welcome finding of this current survey by West et al. is that an initial 10-IU oxytocin bolus has completely disappeared from UK practice. Current guidelines from the Royal College of Obstetricians and Gynaecologists (RCOG) [10] and the British National Formulary [11] recommend the use of 5 IU oxytocin as a ‘slow bolus’, although the timescale over which it should be administered is not specified. Bolus dosing is associated with significantly more cardiovascular sideeffects than if administered over 5 minutes, for both 5 IU and 3 IU oxytocin [12, 13]. Reducing the dose of the oxytocin bolus may not be the key to reducing side-effects: even doses between 0.5 IU and 3 IU have been shown to produce hypotension in 20-30% of patients when given as a bolus [14]. It may be that reducing the speed of drug delivery is the most important factor for reducing potentially dangerous side-effects, and this is especially important in those at high risk due to cardiac disease or hypovolaemia. Some centres do not use a traditional ‘bolus’ of oxytocin but commence a prophylactic oxytocin infusion. A Canadian study of 40 patients found that the ED90 of oxytocin infusion required to produce adequate uterine contraction three minutes after delivery was 0.29 IU.min 1 in a low-risk population undergoing elective caesarean delivery [15]. What evidence base is there to guide oxytocin doses? In the mid2000s, two studies published data on a range of oxytocin doses used at caesarean section and estimated a minimum effective dose (ED90) for oxytocin after both elective caesarean section [16] and caesarean after labour [17]. The primary endpoint used was uterine tone, subjectively assessed by a blinded obstetrician. The ED90 of oxytocin at elective caesarean was calculated as 0.35 IU