EXERCISE INDUCED FATIGUE DECREASES RESTING HEART RATE VARIABILITY AND PLASMA VOLUME EXPANSION.

Abstract

1740 Residual fatigue following intense training is associated with decreased resting heart rate variability (HRV) and baroreceptor reflex sensitivity. Decreased baroreflex sensitivity may also partially explain the mechanism of exercise-induced plasma volume (PV) expansion (where normally an acute infusion of the same volume would induce a diuresis), and investigation of this phenomenon was the aim of the study. Changes in resting HRV, blood pressure (BP) and PV were recorded following 2 bouts of fatiguing exercise separated by 24 hr in a hot ambient environment (30 C, 55% RH). BP, ECG for HRV, hematocrit and hemoglobin (Hb) concentrations were recorded 60 minutes after each fatiguing bout, then 24, 48 72 hr and one week post exercise. The protocol was repeated one week later to assess reproducibility of the physiological responses. Six healthy male volunteers (22.6 ± 2.0 yrs, 177.6 ±2.6 cm, 72.2 ±2.7kg, VO2 peak 56.1 ± 1.8 ml·kg-1 min-1, mean ± SEM) rode on a cycle ergometer for 1.5 hr on two consecutive days at an average intensity of 90% HRmax. Hb mass (CO rebreathing method) was unchanged between week 1 and week 2(week 1, 13.9 ±0.7 mmol; week 2, 13.9 ± 0.9 mmol). PV expansion was 8.8 ± 2.0% (24 hr post) and 6.2 ± 3.1% (48 hr post) during week 1 and returned to baseline one week post exercise; similar responses were recorded in week 2. Resting baroreceptor sensitivity was reduced (-38 ± 7% at 24 hr post and -27 ± 13% at 48 hr post), similar responses were recorded in week 2. Sustained reductions in HRV (total spectral power) were clearly evident 48 hr after completion of the exercise. Post exercise resting HR remained elevated for 72 hr and correlated highly (r=0.654, p<0.05) with changes in the low frequency to high frequency ratio. Mean resting BP remained unchanged. Decreased baroreceptor reflex sensitivity was temporally related to the PV expansion (r=0.742, p<0.05). These results support the hypothesis that reduced baroreceptor reflex sensitivity (assessed by HRV) partially accounts for the maintenance of exercise-induced PV expansion.