Acute Tocolysis - Clinical Tree

Introduction

The uterine myometrium is one of the most powerful muscles in the body, designed to ‘push out’ a 3.5–4.5-kg fetus during birth, and to expel the placenta after birth. However, any deviation from the normal uterine activity may adversely affect fetal and maternal health, because excessive uterine activity may result in repetitive and sustained compression of the umbilical cord and progressive reduction in uteroplacental circulation leading to fetal hypoxia and acidosis. Similarly, contraction of the lower uterine segment prior to the expulsion of the placenta may lead to retention of the placenta, requiring manual removal. Therefore, an ‘acute’ relaxation of the uterine myometrium may be useful in such situations to improve maternal and fetal outcomes.

Definition

Use of pharmacological agents to ensure an immediate relaxation of the uterine myometrium to improve fetoplacental circulation or to facilitate intrauterine manipulation.

History

The use of acute tocolysis was first described in the scientific literature in 1882 by Fancourt Barnes, who reported

‘I found it impossible to get my hand into the uterus to deliver the placenta. Bearing in mind the remarkable power which nitrite of amyl possesses in relaxing tension in the blood-vessels, I determined to test its action on the uterine spasms. The patient had three drops of the nitrite of amyl given her on a handkerchief to inhale. During the inhalation, the ring of muscular fibres around the os interna, which had been so rigid as to be absolutely undilatable, steadily yielded, until I could pass the whole hand into the uterus…’.

Fancourt Barnes

Hour-glass contraction of the uterus treated with nitrate of amyl. BMJ. 1882;1:377.

However, the use of acute tocolysis has been subsequently extended to improve fetal wellbeing during labour and to abolish uterine contraction in undiagnosed breech presentation and umbilical cord prolapse.

Pharmacological Agents for Acute Tocolytics

An understanding of normal physiology of contraction of smooth muscles and the individual medical conditions of the woman is essential to determine the right tocolytic in the given clinical circumstances. Nitric oxide donors, beta sympathomimetics, calcium channel blockers and oxytocin antagonists can counteract uterine activity. Commonly used tocolytics, their doses, duration of action and side effects are given in Table 11.1 .

TABLE 11.1

Pharmacological Agents Used in Acute Tocolysis.

Drug	Mode of Action	Dose	Onset/Duration of Action	Side Effects	Caution
Ritodrine	B-Agonist	6 mg mixed in 10 mL normal saline and administered intravenously over 3 min.	Onset of action is 3–5 mins. Usually betamimetics abolish uterine contractions for 15–30 min.	Palpitation, transient maternal and fetal tachycardia, hypotension, hyperglycaemia in women with diabetes.	Cardiac arrhythmias or ischaemic heart conditions or maternal hypovolemia.
Terbutaline	B-Agonist	Terbutaline 250 μg subcutaneously (or in 5 mL saline, given IV slowly over 5 min).	Onset of action is 3–5 min. Usually betamimetics abolish uterine contractions for 15–30 min.	Palpitation, transient maternal and fetal tachycardia, hypotension, hyperglycaemia in women with diabetes.	Cardiac arrhythmias or ischaemic heart conditions or maternal hypovolemia.
Hexoprenaline	B-Agonist	5 μg in 10 mL normal saline intravenously over 5 min.	Onset of action is 3–5 min. Usually betamimetics abolish uterine contractions for 15–30 min.	Palpitation, transient maternal and fetal tachycardia, hypotension, hyperglycaemia in women with diabetes.	Cardiac arrhythmias or ischaemic heart conditions or maternal hypovolemia.
Nitroglycerine	Nitric oxide donor	Nitroglycerine comes in an ampoule containing 5 mg in 1 mL solution. If this is added to a 100 mL bag of normal saline it produces a solution of 50 μg per mL. Draw up 20 mL into a syringe – this allows the precise titration of 50 μg per mL administered.	Onset of action is within 90 s and the tocolytic effect lasts for a further 1–2 min. Sublingual administration via an aerosol spray (400 μg) is not recommended due to erratic and unpredictable mucosal absorption during pregnancy.	Maternal hypovolemia hypotension and tachycardia. Therefore, the dose should be titrated according to the clinical situation. For fetal entrapment a rapid response is required – 200 μg, repeating this at about 2-min intervals until appropriate uterine relaxation is achieved. For retained placenta or acute uterine inversion, hypovolaemia should be corrected and smaller initial doses (100 μg) may be given.	Maternal cardiac disease and maternal hypovolemia (e.g. uterine rupture or placental abruption).
Atosiban	Oxytocin antagonist	Mix 6.75 mg of atosiban in 5 mL normal saline and give IV over 1 min.	The half-life is about 12 min and it crosses the placenta, but umbilical vein levels are only 10% of those in the maternal uterine vein.	Atosiban has minimal cardiovascular side effects as opposed to other tocolytics. May increase the risk of postpartum haemorrhage.	Maternal hypotension.
Nifedipine	Calcium channel blocker	No evidence of usefulness in acute tocolytics. Used for inhibition of uterine contractions in threatened preterm labour.	N/A	N/A	N/A
Magnesium sulphate	Competitive blocker of voltage-gated calcium channels	No evidence of usefulness in acute tocolytics.	N/A	N/A	N/A

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