Obstetrics Simplified - Diaa M. EI-Mowafi

Tocolytic Drugs

These are drugs that inhibit uterine contractions.

Beta-sympathomimetics

Action

Relaxation of the smooth muscle fibres by stimulating the beta receptors present on the cell membrane.

Examples:

Ritodrine (Yutopar):

Dosage: 50 mg of ritodrine in 500 ml of 5% glucose solution. Start by 10 drops per minute and increase by 5 drops every 10 minutes until uterine contractions cease.

• The infusion should be continued for 12-48 hours after cessation of contractions.
• The treatment is then maintained by oral therapy as one tablet (10 mg) every 8 hours after meal to reduce its side effects.
• Maternal pulse and blood pressure as well as foetal heart rate should be monitored during treatment to control the dose.

Side effects

• Maternal:
  • Tachycardia,
  • hypotension (relaxation of the smooth muscle fibres in the blood vessels wall),
  • flushing,
  • sweating
  • nausea,
  • vomiting,
  • headache,
  • anxiety,
  • tremors,
  • hyperglycaemia,  
  • hypokalaemia,
  • acidosis and
  • pulmonary oedema.
• Foetal:
  • Tachycardia,
  • arrhythmia,
  • loss of beat-to-beat variation,
  • neonatal hypotension and hypoglycaemia.

Contraindications

• Heart disease.
• Hypertension or hypotension.  
• Hyperthyroidism.
• Antepartum haemorrhage (dilatation of the uterine arteries may increase the bleeding).  
• Diabetes.

Other β-sympathomimetic drugs

• Terbutaline,
• Isoxuprine (Duvadilan - vasoxiprine) 20 mg 3-4 times daily.
• Salbutamol.

Calcium Antagonists

Action: Antagonise the action of calcium within the myometrial cells so reduce its contractility.

e.g. Nifedipine 10 mg oral tablet.

Magnesium Sulphate

Action: The intracellular calcium is displaced by magnesium ion leading to inhibition of the uterine activity.

Dosage: The initial dose is 40 cc of 10% solution given slowly IV. The subsequent doses depend upon the response and the development of MgSO4 toxicity so reflexes and respiratory rate should be observed.

Prostaglandin Inhibiting Agents

Action: Inhibition of uterine contractions by inhibiting prostaglandin synthesis.

Dosage: e.g. indomethacin 100 mg suppository initially, followed by 25 mg orally every 6 hours for up to 24 hours after contractions ceased.

Ethyl Alcohol

Action

• Inhibits the release of oxytocin from the posterior pituitary gland.
• Suppresses the myometrial activity directly.
• Inhibits prostaglandin F2 a synthesis.

Dosage

It is given IV and the dose is adjusted to maintain blood alcohol level of 0.9-1.6 mg/litre.

Side effects

• Nausea, vomiting and depression.
• Drunken mother and foetus.
• Maternal and foetal acidosis.

Links

• Premature labor, preterm delivery : Guidelines, reviews

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Edited by Aldo Campana,