Control of cardiac output in exercising dogs using different types of workload.

The system which controls cardiac output was studied in dogs during exercise on the treadmill. The aim was to investigate whether the pattern of the workload influences the control system. To measure cardiac output, electromagnetic flow probes were implanted at least 10 days before the exercise study. During the experiments cardiac output was computed on a beat-to-beat basis. We compared changes in cardiac output resulting from stepwise, sinusoidally and randomly varying workloads, obtained by changing treadmill velocity accordingly. The time constants found with sinusoidally and randomly varying workloads were 11.6 and 10.0s respectively. The time constants of the alteration in cardiac output resulting from a step function was 9.9s for the positive step and 15.6s for the negative step. However when the stepwise change in workload was between a velocity of 0.67 and 1.56 m.s-1 positive and negative steps yielded the same time constant (13.5 s). It is concluded that the pattern of the workload has no influence on the control system of cardiac output during exercise.