The effect of cigarette smoking on fetal heart rate characteristics*1

OBJECTIVE: To evaluate the effect of repeated cigarette smoking on fetal heart rate (FHR) characteristics. METHODS: Fifteen chronic smokers who were between 28 and 36 weeks' gestation were evaluated during an 8-hour smoking session. Baseline FHR and reactivity were evaluated before an initial cigarette and 4 hours later (after the fourth cigarette), when the effects of smoking on FHR were expected to be maximal. Plasma nicotine was measured at baseline and repeated at times of fetal monitoring. RESULTS: Subjects smoked a mean +/- standard deviation of 22 +/- 6 cigarettes per day. They abstained from smoking for 9.2 +/- 3.2 hours before evaluation. The initial baseline FHR was 134 +/- 9 beats per minute versus 135 +/- 11 beats per minute after the fourth cigarette (P =.17). Plasma nicotine increased from 2.6 +/- 5.6 ng/mL to 24 +/- 9.9 ng/mL after the fourth cigarette (P <.001). The initial nonstress test was reactive in 12 of 15 (80%) of the fetuses. After the fourth cigarette, only four of 15 (27%) of the nonstress tests were reactive. A majority of tracings (eight of 15) were initially reactive before smoking and became nonreactive after smoking. Some tracings remained nonreactive (three of 15), and some tracings remained reactive (four of 15) at both assessments. None of the tracings that were initially nonreactive became reactive. The change in reactivity was significant (P =.013). CONCLUSION: Acute, repeated smoking decreases FHR reactivity.