Transient oxygen uptake response to exercise characterizes functional capacity of the cardiocirculatory system in patients with chronic heart failure: a random stimulus approach

Reduced oxygen uptake increase to work rate increment (DeltaVO2/DeltaWR) is predictable by VO2 response to constant work rate exercise in patients with chronic heart failure

Patients with reduced peak oxygen uptake (VO2) due to chronic heart failure (CHF) exhibit abnormal VO2 kinetics even during mild to moderate exercise. This is characterized by a reduced ratio of the VO2 increase to the work rate increment (DeltaVO2/DeltaWR) during ramp exercise, and by a slow increase in VO2 during constant work rate exercise. Because the slow kinetics alone is unlikely to explain the reduced DeltaVO2/DeltaWR on theoretical grounds, we can postulate that the linearity between work rate and VO2 may be impaired when exercise is imposed in a ramp fashion. The present study was designed to address this issue. In 21 CHF patients and 17 normal controls, we performed both symptom-limited exercise testing (15 W. min(-1) ramp) and a constant work rate exercise test (0 W followed by 50-W step). The VO2 step response was used to mathematically derive the hypothetical VO2 ramp response by time integration. Although peak VO2 and work rate were both significantly lower in patients, the attenuation in peak VO2 was more prominent ( p<0.05), which could be explained by a lower DeltaVO2/DeltaWR in patients compared with controls [8.1 (SD 1.0) and 9.8 (0.5) ml. min(-1). W(-1), p<0.01]. The hypothetical DeltaVO2/DeltaWR derived from the VO2 step response was also significantly lower in patients [8.7 (1.0) and 10.0 (0.7) ml. min(-1). W(-1), p<0.01]. The hypothetical and measured DeltaVO2/DeltaWR were highly correlated ( r=0.78, p<0.001). Thus, we can reasonably predict the VO2 ramp response from the VO2 response to a step increase in work rate, indicating that linearity between VO2 and work rate is held constant irrespective of loading patterns. Additional studies, such as those including evaluation of anaerobic bioenergetics, are needed to further elucidate the precise mechanism(s) of this phenomenon.

Development of a servo-controller of heart rate using a cycle ergometer

In exercise training, precise control of exercise intensity would maximize the training efficacy while minimizing risks. To adjust work rate, heart rate (HR) has been used as a measure of exercise intensity. Thus, we developed a servo-controller of HR using a cycle ergometer. After estimating the transfer function from work rate to HR, we optimized feedback parameters for achieving a quick and stable HR response by means of a computer simulation. We then examined the performance of the servo-controller of HR in 55 healthy volunteers. We set the target HR at 60% and 75% of the age-predicted maximum HR. Times required for HR to reach 90% of the target HR were 136 +/- 33 and 137 +/-22s in the respective protocols. Standard deviations of the steady-state difference between the target and measured HRs were 2.5 +/- 0.6 and 3.8 +/- 1.1 beats/min. We conclude that the developed servo-controller makes it possible to precisely regulate HR and, thereby, exercise intensity.

Development of a servo-controller of heart rate using a treadmill

Although treadmill exercise involves a more familiar range of motions and is thus more physiological in terms of daily activity than cycle ergometer exercise, difficulties in controlling the exercise intensity have limited its utility. As heart rate (HR) has been used as a measure of exercise intensity, controlling HR should allow for the proper control of exercise intensity during treadmill exercise. Thus, a servo-controller framework was applied to regulate HR during treadmill exercise. After estimating an averaged transfer function from speed command to HR, feedback parameters were optimized via a computer simulation in order to achieve a quick and stable HR response. The performance of the servo-controller of HR was then examined in 10 healthy subjects. Standard deviations of the steady-state difference between the target and measured HRs were 2.7+/-0.9 and 5.0+/-1.4 beats/min in the stepwise and ramp target HR protocols, respectively. The rise time to reach 90% of the target HR was 93+/-20 s in the stepwise protocol. It was concluded that a treadmill implemented with a negative feedback mechanism made it possible to precisely regulate HR and thus exercise intensity.