Assessment of parasympathetic reactivation after exercise

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Abstract
The objective of this study was to evaluate whether heart rate variability (HRV) can be used as an index of parasympathetic reactivation after exercise. Heart rate recovery after exercise has recently been shown to have prognostic significance and has been postulated to be related to abnormal recovery of parasympathetic tone. Ten normal subjects [5 men and 5 women; age 33 ± 5 yr (mean ± SE)] exercised to their maximum capacity, and 12 subjects (10 men and 2 women; age 61 ± 10 yr) with coronary artery disease exercised for 16 min on two separate occasions, once in the absence of atropine and once with atropine (0.04 mg/kg) administered during exercise. The root mean square residual (RMS), which measures the deviation of the R-R intervals from a straight line, as well as the standard deviation (SD) and the root mean square successive difference of the R-R intervals (MSSD), were measured on successive 15-, 30-, and 60-s segments of a 5-min ECG obtained immediately after exercise. In recovery, the R-R interval was shorter with atropine ( P < 0.0001). Without atropine, HRV, as measured by the MSSD and RMS, increased early in recovery from 4.1 ± 0.4 and 3.7 ± 0.4 ms in the first 15 s to 7.2 ± 1.0 and 7.4 ± 0.9 ms after 1 min, respectively ( P < 0.0001). RMS (range 1.7–2.1 ms) and MSSD were less with atropine ( P < 0.0001). RMS remained flat throughout recovery, whereas MSSD showed some decline over time from 3.0 to 2.2 ms ( P < 0.002). RMS and MSSD were both directly related ( r2 = 0.47 and 0.56, respectively; P < 0.0001) to parasympathetic effect, defined as the difference in R-R interval without and with atropine. In conclusion, RMS and MSSD are parameters of HRV that can be used in the postexercise recovery period as indexes of parasympathetic reactivation after exercise. These tools may improve our understanding of parasympathetic reactivation after exercise and the prognostic significance of heart rate recovery.