TRANSIENT OXYGEN-UPTAKE RESPONSE TO EXERCISE CHARACTERIZES FUNCTIONAL-CAPACITY OF THE CARDIOCIRCULATORY SYSTEM IN PATIENTS WITH CHRONIC HEART-FAILURE - A RANDOM STIMULUS APPROACH

The transient response of oxygen uptake ((V) over dot O-2) to submaxim al exercise, known to be abnormal in patients with cardiovascular diso rders, can be useful in assessing the functional status of the cardioc irculatory system, however, a method for evaluating it accurately has not yet been established. As an alternative approach to the convention al test at constant exercise intensity, we applied a random stimulus t echnique that has been shown to provide relatively noise immune respon ses of system being investigated. In 27 patients with heart failure an d 24 age-matched control subjects, we imposed cycle exercise at 50 W i ntermittently according to a pseudo-random binary (exercise-rest) sequ ence, while measuring breath-by-breath (V) over dot O-2. After determi ning the transfer function relating exercise intensity ((W)) over dot to (V) over dot O-2 and attenuating the high frequency ranges (> 6 exe rcise-rest cycles . min(-1)), we computed the high resolution band-lim ited (0-6 cycles . min(-1)) (V) over dot O-2 response (0-120 s) to a h ypothetical step exercise. The (V) over dot O-2 response showed a long er time constant: in the patients than in the control subjects [47 (SD 37) and 31 (SD 8) s, respectively, P < 0.05]. Furthermore, the amplit ude of the (V) over dot O-2 response after the initial response was sh own to be significantly smaller in the patients than in the control su bjects [176 (SD 50) and 267 (SD 54) ml . min(-1) at 120 s]. The averag e amplitude over 120 s correlated well with peak (V) over dot O-2 (r = 0.73) and Delta(V) over dot O-2/Delta(W) over dot (r = 0.70), both of which are well-established indexes of exercise tolerance. The data in dicated that our band-limited (V) over dot O-2 step response using ran dom exercise was more markedly attenuated and delayed in the patients with heart failure than in the normal controls and that it could be us eful in quantifying the overall functional status of the cardiocircula tory system.