Effects of calcium channel-blocker tokolysis on the foetal circulation

The Ca++-antagonist nifedipine has been successfully employed in the treatment of non-gravid hypertension, and was found to inhibit uterine contractions in the perimenstrual period, as well as during premature labour in animal models. The use of antihypertensive drugs in pregnancy introduces the possibility of iatrogenic foetal distress. It has been established that nifedipine crosses the placental barrier in the sheep and causes a fall in mean arterial pressure and tachycardia in both the ewe and the foetus. This paper examines the effects of nifedipine on the foetus when administered to the pregnant ewe. Catheters and electrodes were implanted by surgical procedures in 15 ewes and foetal lambs between days 118 and 122 of gestation. The redistribution of foetal blood flow was measured by the radioactive microsphere injection technique. The infusion of nifedipine caused a 9% increase in the combined ventricular output (CVO) from 446 to 509 ml/min/kg in the foetus. Foetal lung blood flow increased from 29 +/- 6 to 69 +/- 14 ml/min/kg while figures for the skeletal muscle flow were 109 +/- 34 and 141 +/- 41.6 ml/min/kg. Heart and brain blood flow, expressed as percentages of CVO showed variations of 4.3 and 5.6 per cent, respectively. Blood flow in the gut, placental membranes, skin, kidney and spleen was reduced. The present results show that nifedipine, in addition to its known effects causes a redistribution of the foetal circulation.