Dynamic characteristics of cardiac R-R intervals during sleep and waking states

Traditional assessment of cardiac R-R intervals during sleep-wake states uses summary statistics such as mean and interquartile range. Such summary statistics are less able to provide information about instantaneous or dynamic aspects of beat-to-beat control. We examined state-dependent beat-to-beat patterning by plotting each cardiac R-R interval against the previous interval (Poincaré plots). This procedure provides an indication of the probability of occurrence of one interval from its predecessor and allows assessment of dynamic properties of R-R interval variation. Cardiac R-R intervals were determined over 5-12-minute periods of wakefulness (AW), quiet sleep (QS) and rapid eye movement sleep (REM) from six cats; each interval was plotted on the y-axis against the previous value on the x-axis resulting in an RRn + 1 vs. RRn plot. These plots provided evidence that the correlation between an interval and its successor was greater in REM than in QS, even though the overall range was greater in REM. The results suggest that short-term cardiac control during REM may be influenced by more closely regulated mechanisms than suggested by classic concepts.