Uterine Relaxants

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Chapter: Essential pharmacology : Oxytocin And Other Drugs Acting On Uterus

These are drugs which decrease uterine motility. They have been used to delay or postpone labour, arrest threatened abortion and in dysmenorrhoea. Suppression of labour may be needed to allow foetus to mature, to initiate glucocorticoid therapy for foetal lung maturation.


UTERINE RELAXANTS

(Tocolytics)

 

These are drugs which decrease uterine motility. They have been used to delay or postpone labour, arrest threatened abortion and in dysmenorrhoea. Suppression of labour may be needed to allow foetus to mature, to initiate glucocorticoid therapy for foetal lung maturation or to transfer the mother in labour to a centre with proper facilities, but no clearly satisfactory drug is available yet. An attempt to delay premature labour is likely to succeed only if cervical dilatation is < 4 cm and ‘taking up’ of lower segment is minimal. It should not be undertaken if membranes have ruptured, antepartum haemorrhage is occurring, in severe toxaemia of pregnancy, intrauterine infection or foetal death.

 

1. Adrenergic Agonists

 

Ritodrine, the β2 selective agonist having major uterine relaxant action is preferred to suppress premature labour and to delay delivery in case of some exigency or acute foetal distress. For dependable action it is started as 50 μg/min i.v. infusion, the rate is increased every 10 min till uterine contractions cease or maternal HR rises to 120/min. Contractions are kept suppressed by continuing i.v. infusion or by 10 mg i.m. 4–6 hourly followed by 10 mg oral 4–6 hourly. Delivery can be postponed in about 70% cases by few hours to few weeks. However, cardiovascular (hypotension, tachycardia, arrhythmia, pulmonary edema) and metabolic (hyperglycaemia, hyperinsulinaemia, hypokalaemia) complications and anxiety, restlessness, headache occur frequently. Use of ritodrine to arrest labour has been found to increase maternal morbidity. The neonate may develop hypoglycaemia and ileus. It should not be used if mother is diabetic, having heart disease, or receiving β blockers or steroids.

 

YUTOPAR, RITROD 10 mg/ml inj (5 ml amp), 10 mg tab. RITODINE 10 mg tab, 10 mg in 1 ml inj.

 

Isoxsuprine oral/i.m. has been used to stop threatened abortion, but efficacy is uncertain.

 

2. Calcium Channel Blockers

 

Because influx of Ca2+ ions plays an important role in uterine contractions, Ca2+ channel blockers (see Ch. No. 39) reduce the tone of myometrium and oppose contractions. These drugs, especially nifedipine, which has prominent smooth muscle relaxant action, can postpone labour if used early enough. Efficacy comparable to β2 adrenergic agonists has been shown in some reports. Oral nifedipine 10 mg every 20–30 min till uterine contractions subside, followed by 10 mg 6 hourly has been used. Tachycardia and hypotension are prominent at doses which suppress uterine contractions. Reduced placental perfusion causing foetal hypoxia is apprehended. However, in one multicentric trial, its use in preterm labour was found to produce fewer maternal side effects than ritodrine. Also, fewer babies delivered after nifedipine needed intensive care.

 

3. Magnesium sulfate

 

Given by i.v. infusion, it has been used for long to control convulsions and to reduce BP in toxaemia of pregnancy. As per WHO, it is the drug of choice for prevention and treatment of seizures in preeclampsia and eclampsia. An i.v. bolus (2–4 g over 10–20 min) is followed by 1 g/hr i.v. infusion regulated by the response. The international ’Magpie trial’ (2002) among >10,000 preeclamptic women found that it halved the risk of developing eclampsia and reduced maternal mortality. Adverse effects were minor both in the mother and the baby.

 

Magnesium sulfate (i.v.) also suppresses uterine contractions and has been used to delay preterm labour. The efficacy is rated similar to β2 agonists. However, a recent review (2002) has concluded that it is ineffective in preventing preterm birth. Higher infant mortality was noted in one trial after the use of mag. sulfate. It appears to increase perinatal mortality in low birthweight offsprings, though it may be safer at term. Toxicity of i.v. mag. sulfate includes cardiac arrhythmias, muscular paralysis, CNS and respiratory depression in the mother as well as the neonate. Thus, its use as a tocolytic appears risky.

 

4. Oxytocin Antagonist

 

Atosiban is a peptide analogue of oxytocin that acts as antagonist at the oxytocin receptors. In clinical trials, it has been found to suppress premature uterine contractions and postpone preterm delivery with fewer cardiovascular and metabolic complications than β2 adrenergic agonists. However, benefit in terms of infant survival is uncertain, because in one trial higher neonatal mortality was recorded in the group treated with atosiban compared to placebo.

 

5. Miscellaneous Drugs

 

Ethyl alcohol, nitrates, progesterone, general anaesthetics and PG synthesis inhibitors are the other drugs, which can depress uterine contractions. However, their effect is not dependable and they are rarely used clinically as tocolytics. Progesterone has been found to prevent miscarriage in some high risk cases.

 

Halothane is an efficacious uterine relaxant that has been used as the anaesthetic when external or internal version is attempted.


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