Heart rate variation during fetal behavioural states 1 and 2

Fetal heart rate (FHR) variation was numerically assessed in relation to behavioural states in 34 normal near term fetuses, in order to study the normal ranges of FHR variation during state 1F and 2F and to determine possible factors influencing variation in these states. In all individuals 2-h recordings were made of fetal heart rate, and body and eye movements. FHR variation was quantified using the Sonicaid System 8000. During both 1F and 2F there was a large range in variation, with a wide overlap in variation between both states (mean 1-min range in pulse intervals 16-53 ms and 36-97 ms, respectively). In all fetuses variation was higher during 2F than during 1F. The large ranges in variation were mainly caused by differences in basal heart rate and the correlation coefficients between rate (beats/min) and variation were -0.68 and -0.49 for 1F and 2F, respectively. Other reasons for a relatively high FHR variation during 1F included 'pseudo' sinusoidal patterns related to fetal sucking or mouth movements and relatively flat periods sporadically interrupted by an acceleration. With the System 8000, periods of low and high variation are identified using fixed criteria. Because of the large overlap in variation between both states, these episodes poorly correlated with the underlying behavioural states. Objective identification of the heart rate patterns during states 1F and 2F requires incorporation of heart rate itself in the analysis. As yet, 'global' visual identification of heart rate patterns is superior to computer analysis in identifying the underlying fetal behavioural states.