Dynamics of cardiovascular and baroreflex readjustments during a light-to-moderate exercise transient in humans.

Purpose: We hypothesised that, during a light-to-moderate exercise transient, compared to an equivalent rest-to-exercise transient, (1) a further baroreflex sensitivity (BRS) decrease would be slower, (2) no rapid heart rate (HR) response would occur, and (3) the rapid cardiac output (CO) response would have a smaller amplitude (A1). Hence, we analysed the dynamics of arterial baroreflexes and the HR and CO kinetics during rest-to-50 W (0-50 W) and 50-to-100 W (50-100 W) exercise transients.

Methods: 10 subjects performed three 0-50 W and three 50-100 W on a cycle ergometer. We recorded arterial blood pressure profiles (photo-plethysmography) and R-to-R interval (RRi, electrocardiography). The former were analysed to obtain beat-by-beat mean arterial pressure (MAP) and stroke volume (SV). CO was calculated as SV times HR. BRS was measured by modified sequence method.

Results: During 0-50 W, MAP transiently fell (- 9.0 ± 5.7 mmHg, p < 0.01) and BRS passed from 15.0 ± 3.7 at rest to 7.3 ± 2.4 ms mmHg at 50 W (p < 0.01) promptly (first BRS sequence: 8.1 ± 4.6 ms mmHg, p < 0.01 vs. rest). During 50-100 W, MAP did not fall and BRS passed from 7.2 ± 2.6 at 50 W to 3.3 ± 1.3 ms mmHg at 100 W (p < 0.01) slowly (first BRS sequence: 5.3 ± 3.1 ms mmHg, p = 0.07 vs. 50 W). A1 for HR was 9.2 ± 6.0 and 6.0 ± 4.5 min in 0-50 W and 50-100 W, respectively (p = 0.19). The corresponding A1 for CO were 2.80 ± 1.54 and 0.91 ± 0.55 l∙min (p < 0.01).

Conclusion: During 50-100 W, with respect to 0-50 W, BRS decreased more slowly, in absence of a prompt pressure decrease. BRS decrease and rapid HR response in 50-100 W were unexpected and ascribed to possible persistence of some vagal tone at 50 W.