Fig. 1.

Heart rate variability. A: time domain measure of high-frequency heart period variability [root mean square of successive differences in RR intervals (RMSSD), means ± SE] at four levels of fetal breathing activity (in %time: none = 0, low = 1–33, mid = 34–67, high = >67) at 139–145 days gestation (dg). Dashed lines, nicotine-exposed fetuses (n = 5); solid lines, controls (n = 5). B: RMSSD vs. breathing at 146–152 dg. C: relationship between amount of nicotinic acetylcholine receptor (nAChR) subunit binding (³H-epibatidine) in the dorsal motor nucleus of the vagus (DMX) and RMSSD during periods of high fetal breathing activity (>67% of time) during late gestation (146–152 dg).