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Reduced heart rate variability has been found in infants who later succumb to the sudden infant death syndrome (SIDS). To determine whether respiratory sinus arrhythmia, a major component of heart rate variability, is also reduced in SIDS victims, nighttime portions of eighteen 24-h recordings of ECG and respiration from infants who later died of SIDS and 52 recordings from control infants were assessed using spectral analysis. Two aspects of respiratory sinus arrhythmia were examined: "extent" (the absolute heart rate variation at the respiratory frequency) and "coherence" (the degree to which heart rate follows respiration regardless of the absolute amount of variation). Respiratory parameters were used to classify each 1-min epoch as quiet sleep, rapid eye movement sleep, waking, or indeterminate state. Median extent and coherence values across the night were then computed for each sleep-waking state. Two-way (group X state) repeated measures analysis of variance tests were then used to compare respiratory sinus arrhythmia values for 13 SIDS victims and 13 control infants matched by postnatal age, birth weight, sex, and gestational age. Extent of respiratory sinus arrhythmia was significantly lower in the SIDS victims across all sleep-waking states, a finding that persisted after adjusting for heart rate. Coherence values did not differ significantly. These results suggest that even before the time of maximal risk for the syndrome, SIDS victims, as a group, differ from controls in the extent to which cardiac and respiratory activity couple, and this difference is independent of basal heart rate.

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ABSTRACT. Development of heart rate variation in three frequency ranges was examined during sleep-waking states in normal infants over the first 6 mo of life. Extent of all three types of heart rate variation decreased from 1 wk to 1 mo of age. Extent of respiratory sinus arrhythmia increased from 1 mo to 6 mo during all sleep-waking states, with the increase most pronounced during quiet sleep. Variation in two bands of lower frequencies showed increases in extent from 1 to 3 mo, then a slowing or reversal of the increase between 3 and 4 mo of age. During rapid eye movement sleep, the two types of lower frequency heart rate variation decreased in extent from 3 through 6 mo of age. These results suggest that alterations in autonomic control of heart rate occur at several time periods over the first 6 mo of life and that these alterations may have an effect only on particular types of heart rate variation and only during particular sleep-waking states. The diminution of all three types of heart rate variation at 1 mo may indicate a reduction in vagal tone at this age. in infants: a high-frequency oscillation that is correlated with the respiratory cycle (RSA) (8-1 1) and lower frequency oscillations in a wide band of periods variously reported to range from 5 to 29 s (8-13). Radvanyi and Morel-Kahn (9) showed that the lower frequency variation is prominent in both QS and REM sleep before 37 wk gestational age. After 37 wk, RSA becomes the predominant type of heart rate variation in QS, and REM heart rate is characterized by RSA superimposed on lower frequency waves. Figure 1 shows heart rate variation during each sleepwaking state in one full-term infant at 2 mo postnatal age.Using spectral analytic techniques, Harper et al. (14) showed that RSA during QS decreases over the 1st mo of life and that RSA during all sleep-waking states increases from 1 to 6 mo of age. The maturational trends in the lower frequency variation identified in infants have yet to be examined. Characterization of the development of this variation in early life could provide clues to the maturation of the underlying processes in infants. In this study we use a time-domain technique to assess the maturational changes in RSA and lower frequency heart rate variation during each sleep-waking state in normal infants from 1 wk to 6 mo of age. MATERIALS AND METHODS QS, quiet skep REM, rapid eye movementPolygraphic recordings of EEG, ECG, electrooculogram, digastric electromyogram, and expired C 0 2 were obtained from 25 RSA, respiratory sinus arrhythmia normal infants. Each infant was recorded on six occasions: at 1 wk of age and at 1,2, 3,4, and 6 mo of age. Each recording was begun at 1900 h and terminated at 0700 h the next morning. The ECG signal was fed into a Schmitt trigger generator that In healthy mammals, heart rate is not fixed, but undergoes produced a pulse for each R wave. The resulting trigger event moment-to-moment variation; the changes in heart rate may be times were digitized along with the other signals. rapid or prolonged. One p...