There a few drugs which are only really used in obstetric anaesthesia which it is useful to know about. There is also an increased importance of pharmacological awareness here because of the additional impact of the drugs on the foetus, and the role of the placenta.
These drugs act to increase uterine tone. This may be for:
Reducing risk of PPH
Treatment of PPH
Augmentation of labour
Synthetic oxytocin is a nonapeptide that binds to myometrial cell receptors and increases their smooth muscle activity. It is manufactured in the UK as Syntocinon. It is the most common uterotonic used.
Dose. Given IV:
5 units after delivery of baby by caesarean section
A repeat 5 Unit bolus may be requested by the obstetrician to help with uterine tone
10 Unit/hour infusion (over 4 hours) as prophylaxis or treatment of PPH
It may also be used as an IV infusion by the obstetric team to augment labour. This is at a much lower dose of 1-2 miU/min. This is then titrated to effect (uterine contractions) to a max of 32 miU/min
Vascular smooth muscle relaxation - hypotension
Nausea & vomiting
Direct action of uterine smooth muscle through alpha-adrenergic pathways. It is a common second line uterotonic. It is usually given IM to obtund the notable side effects, though it still has rapid absorption.
Syntometrine is 500 micrograms ergometrine with 5 Units oxytocin
Increased vascular tone
Hypertension - can be profound in patients with hypertensive disease (including pre-eclampsia)
Nausea and vomiting - very emetogenic
These act due to the role that prostaglandins play in the physiology of delivery.
Carboprost Is a synthetic analogue of prostaglandin F2 alpha. It is given as a dose of 250 micrograms as an IM injection - this can be repeated after 15 minutes up to a maximum of 2mg. It’s role is mainly as a later pharmacological therapy in PPH.
Bronchospasm - relatively contraindicated in asthmatics
Misoprostol This is a synthetic analogue of prostaglandin E1. It can be used as treatment of (and prophylaxis against) PPH, though is also used for induction of labour and cervical ripening. Its dose in PPH is 800 micrograms sublingual (RCOG guidance). It can be given PR as well. It is important to note that the onset time is 1 to 2.5 hours.
Nausea & vomiting
Dinoprostone is a PGE2 used by obstetricians to aid cervical ripening at induction of labour.
Tocolytics are used to halt and relax uterine contraction. This may be in an effort to stop premature labour. Another indication is to stop uterine contraction in caesarean section to aid delivery, or to reduce foetal distress.
Beta agonists Beta 2 adrenergic agonists have a tocolytic effect through their adrenergic receptor activity. The side effects e.g. tachycardia mean that they are less commonly used. Nebulised salbutamol can be used in c-section. Terbutaline 250 mcg subcutaneously is another options in cases of uterine hyperstimulation.
GTN Again acting through its smooth muscle relaxant effects, can be used to aid delivery when uterine relaxation is needed. A dose of 800 mcg sublingually or titrated IV doses of 50 mcg have been used.
Volatile Anaesthetic Agents All the volatile anaesthetic agents result in some smooth muscle and uterine relaxation, and hence the increase risk of bleeding with c-section under general anaesthesia. This will occur at any MAC above 0.5. Once the MAC is higher than 0.9, the uterus loses its oxytocic response.
Magnesium Acting through its smooth muscle effects is also a useful tocolytic (though its main use is a seizure prophylaxis).
Nifedipine This is a calcium channel antagonist, with the result being a reduction in uterine smooth muscle activity. It is used to try and stall premature labour rather than as an acute tocolytic.
Atosiban This is an oxytocin antagonist. It is again used to aim to stall premature labour. Its side effects profile seem benign.
Links & References
Heazell, A. Clift, J (eds) Obstetrics for anaesthetists. Cambridge University Press. 2008.