The R-R interval and R-R variability in normal infants during sleep

Eighteen normal infants were studied in the first 2 wk of life during sleep and subsequently at their monthly birthdays for the first 4 months of life. The R-R interval was measured with an accuracy of 0.2 msec. Sleep staging was performed visually using electroencephalogram, electrooculogram, and electromyogram, and behaviroal criteria. Our results show that the R-R interval and the beat-to-beat variability are, in general, smaller in rapid eye movement than in quiet sleep. The two sleep states, however, are best differentiated by the overall variability which is characteristically higher in rapid eye movement sleep. The R-R interval as well as the overall and the beat-to-beat variability show minimal values at 1 month and maximal rates of increase between 2 and 3 months of age, indicating that the R-R interval and R-R variability are not simple linear functions of age.