Drug Interactions in Obstetric Anaesthesia

SUMMARY

Nearly all obstetric drugs may require a change in anaesthetic care. Anticholinergics increase the threat of aspiration. Sedatives, tranquillizers and narcotics all impair consent, potentially depress the mother and infant, and increase the risk of general and regional anaesthesia. Magnesium sulphate decreases acetylcholine release at the neuromuscular junction, reduces end-plate sensitivity and muscle membrane excitability, and potentiates depolarizing and non-depolarizing muscle relaxants. Reduced renal function during pregnancy-induced hypertension increases the possibility of magnesium toxicity. Placental transfer and fetal toxicity may also occur. Oxytocin is a potent vasodilator and can cause severe hypotension, while ergometrine produces direct vasoconstriction and may cause severe hypertension, β-Agonists used for tocolysis lower the serum potassium, produce vasodilation, increase the heart rate and increase myocardial irritability. Caution should be used when inducing both regional and general anaesthesia. Importantly, these drugs may also generate pulmonary oedema in previously healthy parturients. The aetiology of pulmonary oedema remains unknown, but all cardiac complaints in patients on β-adrenergic therapy must be considered real until proved to be otherwise.

Anaesthetic involvement in complicated obstetrics is increasing at a rapid rate. The anaesthesiologist participating in perinatal care must be educated in the pharmacology of obstetric as well as anaesthetic drugs. Only then can the parturient fully and safely utilize the skill and expertise the anaesthesiologist can bring to the obstetric suite.