Observations on perinatal heart rate monitoring. II. Quantitative unreliability of Doppler fetal heart rate variability.

Fifty-three fetal heart rate (FHR) tracings obtained by direct ECG monitoring during labor and 24 simultaneous external Doppler FHR tracings were analyzed for the quantitative assessment of baseline FHR variability. The beat-to-beat difference in FHR subsumed by 90% of all beat-to-beat changes within a tracing was taken as the index of variability of that tracing. This index correlated well with the visual assessment of variability for both internal ECG records (Spearman rank correlation coefficient, rs, = 0.71) and external Doppler records (rs = 0.78). However, there was no correlation between the variability index of the Doppler FHR records and that of simultaneous ECG FHR records (r = 0.11, p greater than 0.6). This finding casts doubt on the use of the presence of "normal" FHR variability as a sign of fetal well-being during antepartum FHR monitoring with current Doppler equipment.