Oxytocin analogues

Oxytocin is a naturally occurring posterior pituitary octapeptide hormone which acts both centrally and peripherally
It is an agonist at the oxytocin G-protein coupled receptor in the uterus, promoting calcium release and increase contractility
Peripheral actions therefore include uterine contraction, as well as the let-down reflex involved in breastfeeding
Therefore both short- (syntocinon) and long- (carbetocin) acting oxytocin analogues may be used in obstetric anaesthetic practice

As a colourless solution stored in glass vials containing 5IU/ml of syntocinon for use as an IV bolus or infusion
As a colourless solution stored in glass vials containing 100μg/ml of carbetocin for use as an IV bolus
In combination with ergometrine as syntometrine

For the induction or augmentation of labour (IV infusion)
To increase uterine tone following delivery to reduce bleeding inc. in the management of PPH

Oxytocin: as either slow IV boluses (up to 10IU) or via infusion (40IU over 4hrs)
Carbetocin: diluted up to 10ml in 0.9% NaCl and given as a slow IV bolus of up to 100μg
Syntometrine: IM

The ED₉₀ of oxytocin is reportedly only 0.35IU, so our 5IU boluses are more than enough to cause contraction

Rapid bolus administration may cause hypotension with marked compensatory tachycardia and increase in cardiac output
May also cause ECG changes including ST segment, T-wave and QTc changes
Due to its similarity with vasopressin it has a weak anti-diuretic effect that is only apparent with prolonged use
May cause nausea and vomiting
Increases renal blood flow in animal models
Rash and/or anaphylaxis possible
Overdose or toxicity can cause hyperstimulation ± uterine rupture

Absorption

Inactivated by chymotrypsin when given orally so only available via parenteral routes

Metabolism

Blood products as these may contain plasma oxytocinase which inactivate the drug
Suxamethonium as it may reduce efficacy and increase dose requirements

Excretion

Oxytocin has a short elimination half-life of 4mins (1 - 7mins), hence infusions may be required
Carbetocin has a duration of action of up to 7x that of oxytocin