27.02.2 Suppression of Early Labour

A number of drugs are used as uterine relaxants. These drugs, known as tocolytics, are particularly useful in the suppression of early or premature labour affording time for further foetal development and prolonging foetal survival. Several drug classes have been shown to possess tocolytic properties. These include b2 adrenergic receptor agonists, oxytocin antagonists and calcium channel blockers.

Included in the b2 adrenoceptor agonist class are terbutaline, ritodrine, salbutamol, fenoterol and adrenaline. These drugs work by directly activating adrenergic receptors that mediate smooth muscle relaxation terminating uterine contractions. However, these drugs can have a number of adverse effects on both the mother and the foetus due to their ability to act on the respiratory and cardiovascular systems. Common maternal adverse effects include cardiac arrhythmias, altered thyroid function and tachycardia whilst tachycardia, hypoglycaemia, hypotension and myocardial ischaemia have been noted in the foetus and neonate.

Atosiban is a peptide analogue of oxytocin which antagonises the uterine contracting properties of the hormone at its receptor resulting in cessation of preterm labour. It is preferred over the b2 adrenergic agonists due to its lower incidence of adverse effects although reactions to the drug such as nausea, vomiting and hyperglycaemia have been noted.

Calcium channel blockers such as nifedipine, which has previously been discussed for its use as an antihypertensive in pre-eclampsia is also has tocolytic properties.