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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.

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