VAGAL MODULATION OF HEART RATE DURING EXERCISE: EFFECTS OF AGE AND PHYSICAL FITNESS 814

Vagal modulation of heart rate reduces with aging and with poor physical fitness. It is not well known whether age-related impairment in cardiac vagal activity is related to cardiovascular aging itself or is due to reduced in physical activity, and there is little information about effects of age and physical fitness on cardiac vagal modulation during exercise. Analysis of instantaneous beat-to-beat RR interval variability (SD1) provides an accurate measure of vagal tone during exercise (Am. J. Physiol. 271: H244-H252, 1996). SD1 was compared at rest and during exercise between subjects with age > 40 years (n=47) and < 40 years (n=44) matched with Vo2max, and between subjects with Vo2max > 40 ml/kg/min (n=40) and < 40 ml/kg/min (n=35) matched with age SD1 was lower at rest in subjects with age> 40 years than those < 40 years (24±10 vs. 39±21 ms, p<0.001), but the age-related difference in SD1 was smaller during the exercise (i.e. 5±3 vs. 6±3 ms, p<0.05, at the level of 100 W). In age-matched subjects with good and poor Vo2max the SD1 did not differ at rest (33±19 vs. 27±16 ms, p=ns), but became significantly different during the exercise (i.e. 7±3 vs 3±3 ms, p<0.001, at the level of 100 W). These data show that aging itself results predominantly in reduced vagal modulation at rest, but poor physical fitness is more specifically related to impairment of cardiac vagal modulation during exercise. Analysis of vagal modulation during exercise is more sensitive than analysis performed at rest in assessing the effects of physical fitness on cardiac autonomic function.