Changes in cardiac autonomic regulation after prolonged maximal exercise

Harmful cardiac events occurs frequently after exercise. However, the cardi ac autonomic regulation after vigorous exercise is not well known. This stu dy was designed to assess heart rate (HR) variability before and after a 75 km cross-country skiing race. HR variability was assessed by using standar d statistical measures along with spectral and quantitative Poincare plot a nalysis of HR variability in 10 healthy male subjects (age 36 +/- 11 years) . The average HR was at the same level 1 day after the race as before the r ace, but on the second day, HR was significantly lower (P<0.001) compared w ith the prerace and 1 day after values. The normalized high-frequency (HF) spectral component of HR variability (nuHF) was lower (P<0.01) on the first day after the maximal exercise compared with the pre-exercise values but r eturned to or even exceeded the prerace level on the second day (P<0.01). T he changes in short-term R-R interval variability analysed from the Poincar e plot were similar to those observed in the HF spectral component. The nor malized low-frequency (LF) spectral component of HR variability (nuLF) was higher (P<0.01) on the first day after the exercise compared with the prera ce levels and it also returned to the preexercise level or even dropped bel ow it on the second day after the race. The mean time it took the HF spectr al component to return to the pre-exercise level was 4.2 +/- 4.2 h (ranging from 0 to 12 h). This recovery time correlated inversely with the maximal oxygen consumption (VO2max) measured during the bicycle exercise test befor e the skiing race (r = -0712, P<0.016). The cardiac vagal outflow is blunte d for several hours after prolonged vigorous exercise. The recovery time of reduced vagal outflow depends on individual cardiorespiratory fitness and there is an accentuated rebound of altered autonomic regulation on the seco nd day after prolonged exercise.