Glyceryl Trinitrate and Acute Uterine Relaxation: A Literature Review

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Abstract
The use of nitro-vasodilators for achieving rapid uterine relaxation in the resolution of obstetric emergencies has been documented for nearly 120 years. Glyceryltrinitrate (GTN) is the most commonly used nitro-vasodilator for this purpose, with the presumed mechanism of action being via nitric oxide and cyclic guanosine monophosphate (cGMP) mediated processes. GTN is known to release nitric oxide to effect smooth muscle relaxation and some dose response data is available for its vasodilator activity. Human myometrium is known to synthesize and respond to nitric oxide, with changes in the production of and sensitivity to nitric oxide being subject to the cyclical and gestational state of the uterus. Experimental data on the efficacy of GTN in reliably producing uterine relaxation is conflicting and inconsistent. A total of 32 studies and case reports on the use of GTN in achieving rapid uterine relaxation have appeared in the English language literature. Case reports are subject to reporting bias and prospective randomized controlled trials are not without design flaws. Indications for the use of GTN in achieving rapid uterine relaxation cover the antepartum, intrapartum and postpartum periods. The safety of GTN during obstetric emergencies appears high, with no adverse maternal or neonatal outcomes. To establish the efficacy of GTN in reliably achieving uterine relaxation, well designed randomized controlled trials in labouring women are required.