Pharmacology Notes

Blockade of calcium channels causes relaxation of uterine smooth muscle. The most extensively studied calcium channel blocker for tocolysis or suppression of threatened or pre-term labour is nifedipine. Although studies of nifedipine have been inadequately powered, meta-analyses suggest that nifedipine is at least as effective as salbutamol in prolonging pregnancy for at least 48 hours or to beyond 34 weeks gestation. Nifedipine appears to be better tolerated than salbutamol, with fewer interruptions in therapy due to maternal adverse effects. Nifedipine may also provide improved neonatal outcomes with fewer cases of respiratory distress syndrome and admissions to neonatal intensive care units.

Nifedipine, in this setting, causes minimal effects on blood pressure in normotensive patients. In hypertensive patients, blood pressure fluctuations can be significant. Nifedipine should be avoided in cases of significant maternal heart disease. Due to risks of maternal hypotension and pulmonary oedema, nifedipine should not be combined with other tocolytic drugs such as intravenous salbutamol, other hypotensive drugs, or with magnesium sulfate, except under close supervision and expert advice. Hypotension associated with nifedipine may be managed by ceasing the drug and/or administering intravenous fluids. Fluid administration should be used cautiously as it may contribute to maternal pulmonary oedema.

Initially, during dose titration, nifedipine immediate-release tablets should be used. These may be chewed before swallowing to achieve faster absorption. If a maintenance regimen is needed, the patient may be switched to the controlled-release daily nifedipine tablets. The controlled-release formulation must not be chewed or crushed.v