Relation of Beat-to-Beat Variability to Heart Rate in Normal Sleeping Infants

Abstract

SummaryThe relationship of beat-tebeat heart rate variability (delta RR) and instantaneous heart rate was studied in eight normal infants while asleep during the first four months of life. The sleep state (REM or quiet) was determined using neurophysiologic and behavioral criteria. The results of regression analyses indicated that the delta RR values were positively correlated with the instantaneous heart rate (RR intervals). The correlation coefficient range was 0.49 to 0.92 in quiet sleep and 0.50 to 0.93 in REM sleep. Regression analyses supported-a linear approximation of the delta RR to RR relation over the RR range investigated (400 to 520 msec). The median slope was 0.124 k REM Pad 0.117 in quiet sleep. The slopes of these linear functions were similar in both sleep states and at all ages. If beat-tebeat variability is to be used as an index of the integrity of the autonomic nervous system, these results suggest that delta RR be corrected for RR. A model is presented which relates the demonstrated positive correlation of delta RR to RR with the physiology of cardiac output control. SpeculationThe positive correlation of delta RR with RR is consistent with the view that beat-tebeat heart rate variability is a homeostatic mechanism required for control of cardiac output.Beat-to-beat or short-term heart rate variability (delta RR) is used for identification of fetal distress and is believed to reflect the integrity of the autonomic nervous system in the fetus and newborn (6, 11,18). Autonomic dysfunction may be manifested by decreased short-term variability as seen in hypoxic newborn infants at delivery (15) and in infants of mothers given parasympatholytic drugs (7,8,13,14).Short-term vrcriability may also prove to be a useful parameter for monitoring autonomic function during early infancy. However, as in the fetus (7, 8), short-term variability of the heart rate might be a function of the instantaneous heart rate (instantaneous heart rate defined as the inverse of the RR interval of the electrocardiotrocardiograms done after each study were normal. The procedures and objectives of the study were explained to the parents. and informed consent was obtained. A total of four or five studies, lasting one to two hr, were performed on each infant while it was asleep. Serial studies were performed during the first two wk of life and subsequently at monthly intervals through four months of age. Each study was done in a quiet laboratory environment after a usual midmorning feeding. MEASUREMENT OF THE RR INTERVALSElectrocardiogram surface electrodes were positioned on the chest to obtain a unidirectional polygraph recording with a single prominent R wave which had a sharp peak. Each RR interval of this wide-band electrocardiogram signal (0.5 to 2000 Hz) was measured with an accuracy of 200 p e c using a digital preprocessor designed in this laboratory (17). The data was carefully reviewed, and less than 0.1% of the 10,000 to 15,000 of each study RR intervals were rejected as artifact. SLEEP STAGINGSurface electrodes...

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