A biexponential approach for assessing parasympathetic reactivation after submaximal exercise

The occurrence of cardiac events and sudden death in Brugada syndrome (BrS) patients during sleep or rest, suggests that the parasympathetic activity of autonomic control might play a key role in this disease. In this study, the instantaneous heart rate during an exercise test was assessed in 115 BrS patients, of whom 25 were symptomatic. The recovery period lasted 6 minutes and included two phases of 3 minutes each called active and passive recovery. A bi-exponential function was used to model the shape of heart rate decay during recovery, from which the heart rate recovery (HRR) index was determined throughout the entire period. The time-course of HRR was investigated and compared by separating patients according to symptoms, sex and SNC5a mutation. Results showed that asymptomatic patients presented larger HRR values than the symptomatic group for most of the time. The temporal evolution of the p-value revealed that significant differences between these groups begin to appear after 2.5 minutes of recovery lasting about 0.5 minutes. The critical time instant was found at minute 2.85, being HRR= 46.0±10.9 beats/min (asymptomatic) and 41.6±9.7 beats/min (symptomatic), p=0.031. Regarding sex, relevant differences appeared from minute 1.5 to minute 1.7, with the critical timing found at minute 1.62 (p=0.043, HRR=0.9±8.2 beats/min for women and 33.9±9.5 beats/min for men). We concluded that assessing the HRR beyond classical intervals (1 or 2 min) might help to determine which asymptomatic patients are most at risk for developing future adverse cardiac events.