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Systolic Time Intervals of the Fetal Cardiac Cycle

From the Department of Obstetrics and Gynecology, Los Angeles County-University of Southern California Medical Center.

Abstract

Simulanteous recordings of the fetal electrocardiogram (FECG) and Doppler cardiogram (DCG) were employed during labor to study the systolic time intervals of the fetal cardiac cycle. Clarity of the DCG signal was enhanced by means of a 600–2000 Hz bandpass filter. Experiments using a model to simulate valvular motions demonstrated that variations in time delay of the Doppler signal resulting from differences in the angle between ultrasonic beam and direction of valve motion were small and unlikely to invalidate comparisons based on the DCG. The Q-Ao interval, corresponding to the pre-ejection phase of the cardiac cycle, was found to be independent of heart rate. This interval appeared to increase with increasing fetal age. In normal fetuses of 38–40 weeks' menstrual age, the Q-Ao interval was 70.0 ± 2.4 msec. The Ao-Ac interval, corresponding to ventricular ejection time, increased as heart rate decreased from 180 to 115 beats/min in normal fetuses. There was no further prolongation of Ao-Ac as the heart rate fell below 115 beats/min. The Q-Ao interval was prolonged in each of 8 fetuses with abnormal FHR patterns.