Characterization Of The Cardiac Response To Step Increases And Decreases In Exercise Intensity

The dynamic cardiac output (CO) response at exercise onset and offset is achieved through an integrative system of feedforward and feedback mechanisms that are not fully understood. PURPOSE: Characterize the dynamic response of CO and its determinants, stroke volume (SV) and heart rate (HR), during increases and decreases in work rate to gain insight into the nature of cardiac control mechanisms in exercise. METHODS: Seven healthy male participants (20.9 ± 1.7 y) performed three upright cycling trials consisting of four work rate transitions determined by HR: rest to 90 bpm (R-90), 90 to 120 bpm (90-120), 120 to 90 bpm (120-90) and 90 bpm to rest (90-R). Finger photoplethysmography continuously recorded CO, SV and HR. 1-second interpolated trials were averaged. Exponential fits quantified gains, time delays (TD) and time constants (Tau). RESULTS: Data mean ± SD. R-90 vs 90-R: Gains were not different (P > 0.05). TD (s) for HR was shorter (-0.8 ± 1.1 vs 9.6 ± 8.4, P < 0.05) and Tau (s) for HR, SV and CO were faster (HR 5.5 ± 3.4 vs 31.0 ± 14.9, P < 0.05; SV 7.7 ± 5.8 vs 32.3 ± 19.7, P < 0.05; CO 5.5 ± 3.2 vs 29.4 ± 15.1, P < 0.05). 90-120 vs 120-90: Gains, TD and Tau were similar for HR, SV and CO (P > 0.05). 90-120 vs R-90: Gains for SV (ml) and CO (L/min) were smaller (SV 9.3 ± 6.1 vs 34.1 ± 4.7, P < 0.05; CO 3.5 ± 0.9 vs 5.1 ± 1.2, P < 0.05) and Tau for HR and CO were slower (HR 20.5 ± 11.2 vs 5.5 ± 3.4, P < 0.05; CO 29.4 ± 15.1 vs 22.5 ± 12.1, P < 0.05). CONCLUSION: At exercise onset, muscle contractions are thought to translocate blood centrally, increasing cardiac filling and therefore SV. Fast SV increase in R-90 is consistent with this. The slower SV decrease with removal of muscle contractions (90-R) supports elevated venous return as an additional key contributor to SV magnitude. Slower HR decline in 90-R vs increase in R-90 suggests slower cardiac chronotropic inactivation compared to activation at exercise onset. In contrast, within exercise-to-exercise transitions, the CO response is similar with increasing and decreasing exercise intensity suggesting the mechanisms driving cardiac output increase and decrease similarly. Finally, the slower HR response and smaller SV gain in 90-120 vs R-90 are consistent with slow-acting sympathetic activation and reduced ventricular filling reserve slowing cardiac output adjustment when the muscles are already contracting.

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