20 Myometrial Relaxants
These drugs are sympathomimetics and relax uterine muscle, hopefully preventing premature labour. Their main use is to delay delivery until corticosteroid therapy is complete. Some are used as antagonists to oxytocin, which can cause hyperstimulation of the uterus during the induction or augmentation of labour. Their use is indicated between 24 and 34 weeks’ gestation in uncomplicated cases. Other terminology used for this group of drugs is tocolytics (RCOG, 2011).
The student should be aware of:
what constitutes premature labour
local protocols for the management of premature labour
the sequelae of the action of these drugs on the mother and fetus.
BP
Terbutaline
Proprietary
Bricanyl® injection (AstraZeneca UK Ltd)
Bricanyl® 5 mg tablets (AstraZeneca UK Ltd)
Bricanyl® 5 mg tablets (AstraZeneca UK Ltd)
Group
Myometrial relaxant/bronchodilator
Uses/indications
Selective β2-adrenergic agonist for the relief of bronchospasm in bronchial asthma and other bronchopulmonary disorders
To arrest labour between 24 and 33 weeks of gestation in patients with no medical or obstetric contraindication to tocolytic therapy. The main effect of tocolytic therapy is a delay in delivery of up to 48 h
To arrest labour between 24 and 33 weeks of gestation in patients with no medical or obstetric contraindication to tocolytic therapy. The main effect of tocolytic therapy is a delay in delivery of up to 48 h
Type of drug
POM
Presentation
Ampoules, tablets
Dosage
Injection – 0.5 mg/mL
Tablets 5 mg
As per local protocols or the manufacturer’s recommendations – use of syringe pump or controlled infusion device essential
Tablets 5 mg
As per local protocols or the manufacturer’s recommendations – use of syringe pump or controlled infusion device essential
Route of admin
IVI, S.C., IM, oral
Contraindications
Pre-existing ischaemic heart disease or with significant risk factors for ischaemic heart disease
Hypersensitivity
Any condition of the mother or fetus in which prolongation of the pregnancy is hazardous, e.g. severe toxaemia, antepartum haemorrhage, intrauterine infection, intrauterine infection, severe pre-eclampsia, abruptio placentae, threatened abortion during first and second trimester, or cord compression
Hypersensitivity
Any condition of the mother or fetus in which prolongation of the pregnancy is hazardous, e.g. severe toxaemia, antepartum haemorrhage, intrauterine infection, intrauterine infection, severe pre-eclampsia, abruptio placentae, threatened abortion during first and second trimester, or cord compression
Side effects
Tachycardia tremor, headache, palpitations paradoxical bronchospasm, an increased tendency to bleeding in connection with peripheral vasodilatation, caesarean section, nausea, myocardial ischaemia, hypokalaemia, hypersensitivity reactions including angio-oedema,
bronchospasm, hypotension and collapse, arrhythmias, e.g. atrial fibrillation, supraventricular tachycardia and extrasystole, symptoms of pulmonary oedema, mouth and throat irritation, sleep disorder and behavioural disturbances, such as agitation and restlessness, hyperactivity, hyperglycaemia, muscle spasms, hyperlactacidaemia, urticaria, rash
Interactions
Beta-blocking agents (including eye drops), especially the non-selective ones such as propranolol, may partially or totally inhibit the effect of β stimulants. Therefore, Bricanyl® preparations and non-selective beta-blockers should not normally be administered concurrently
Use with caution in patients receiving other sympathomimetics
Hypokalaemia may result from β2-agonist therapy and may be potentiated by concomitant treatment with xanthine derivatives, corticosteroids and diuretics
Use with caution in patients receiving other sympathomimetics
Hypokalaemia may result from β2-agonist therapy and may be potentiated by concomitant treatment with xanthine derivatives, corticosteroids and diuretics
Pharmacodynamic properties
Selective β2-adrenergic stimulant that inhibits uterine contractility
Fetal risk
Administer with caution during the first trimester of pregnancy
Breastfeeding
Secreted into breast milk, but any effects on the infant are unlikely at therapeutic doses
Transient hypoglycaemia has been reported in newborn
Transient hypoglycaemia has been reported in newborn
BP
Salbutamol
Proprietary
Ventolin™ for IV infusion (Allen & Hanburys Ltd) Salbutamol 4 mg tablets (Actavis UK Ltd)
salbutamol (non-proprietary, see BNF for details)
salbutamol (non-proprietary, see BNF for details)
Group
Myometrial relaxant/bronchodilator
Uses/indications
Relief of severe bronchospasm
Management of premature labour; to arrest uncomplicated labour between 24 and 33 weeks of gestation in patients with no medical or obstetric contraindication to tocolytic therapy
Management of premature labour; to arrest uncomplicated labour between 24 and 33 weeks of gestation in patients with no medical or obstetric contraindication to tocolytic therapy
Type of drug
POM
Presentation
Ampoules (5 mg/5 mL), tablets (4 mg)
Dosage
Pre-term labour: Syringe pump or controlled infusion device essential during infusion; the maternal pulse rate should be monitored and rate adjusted to avoid excessive heart rate above 140 beats/min
The volume of fluid infused must be minimized to avoid the risk of pulmonary oedema, hence strict fluid balance records must be kept
Regimen: start 10 mcg/min and increase at 10-min intervals to max 10–45 mcg/min depending on contraction strength, frequency or duration. Then, slowly reduce to cessation of contractions
Maintained at the same level for 1 h and then reduced by half at 6-hrly intervals
If labour progresses despite treatment, the infusion should be stopped
Maintenance once contractions cease: SalbutamolTM tablets 4 mg given three or four times daily in divided doses
The volume of fluid infused must be minimized to avoid the risk of pulmonary oedema, hence strict fluid balance records must be kept
Regimen: start 10 mcg/min and increase at 10-min intervals to max 10–45 mcg/min depending on contraction strength, frequency or duration. Then, slowly reduce to cessation of contractions
Maintained at the same level for 1 h and then reduced by half at 6-hrly intervals
If labour progresses despite treatment, the infusion should be stopped
Maintenance once contractions cease: SalbutamolTM tablets 4 mg given three or four times daily in divided doses
Route of admin
Oral, IM, IV/infusion (in 5% dextrose – can use NaCl with diabetic patients)
Contraindications
Threatened abortion
Hypersensitivity to any of the components
Pre-existing ischaemic heart disease or those with Significant risk factors for ischaemic heart disease
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Hypersensitivity to any of the components
Pre-existing ischaemic heart disease or those with Significant risk factors for ischaemic heart disease
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