Acute effects of nicotine on fetal heart rate variability

Summary. The effects of nicotine on fetal heart rate (FHR) variability were studied in seven women in the second trimester and eight women in the third trimester of pregnancy by giving them nicotine-containing chewinggum. The possible effects of carbon monoxide or some other agents of nicotine-free smoke on FHR variability were tested in eight women in the second and eight in the third trimester of pregnancy by giving them herbal cigarettes to smoke. The gum caused an acute decrease in the interval index of FHR variability in both trimesters, as we have observed previously in association with tobacco smoking, and an acute decrease in the differential index in the second trimester but not in the third trimester. These effects differ from those associated with tobacco smoking, during which the differential index decreased in the third trimester but remained unchanged in the second trimester. In the second trimester the gum elevated the baseline FHR, as did tobacco smoking. In the third trimester, the gum decreased the baseline FHR in contrast to tobacco smoking, which had no effect on it. Smoking a herbal cigarette had no effect on FHR and no depressant effect on FHR variability, as had nicotine. The only fetal response was a transient increase in the interval index 5—10 min after smoking in the second trimester. We conclude that nicotine seems to be responsible for the depressant effect of tobacco smoke on the fetus, as manifested in lowered interval indices. The different responses of the differential index and baseline FHR to tobacco smoking and chewing nicotine-containing gum at different stages of gestation remain unexplained.