Diagnostic Value of the Transient Response of Oxygen Consumption to Exercise in Cardiac Patients: Random Noise Approach

Abstract

Although peak oxygen uptake (Vo₂) at maximal exercise stress is useful for assessing functional capacity in patients, various factors, including the need to impose strenuous load and the subjectivity of the endpoint, make its utility somewhat limited. To circumvent the limitations we developed a technique where we focused on the transient response of Vo₂ to exercise. In patients with chronic heart failure (NYHA class II-III), we intermittently imposed mild bicycle exercise while measuring breathby-breath Vo₂ After determining the transfer function from the workload to Vo₂, we computed a Vo₂ response against a hypothetical step exercise. The Vo₂ step response showed an initial immediate increase followed by a slow monotonic rise toward a steady-state level. The amplitude of the response was smaller in patients than in control subjects (182 ± 50 versus 262 ± 58 ml/min at 120 s; P < 0.001). The time constant of the slow monotonic rise was longer in patients than in control subjects (48 ± 37 versus 31 ± 8 s; P < 0.005). In patients with mitral stenosis the time constant was longer (62 ± 23 s; P < 0.005) with a small amplitude (195 ± 76 ml/min; P < 0.05). Successful percutaneous transvenous mitral commissurotomy (PTMC) shortened the time constant (40 ± 10 s; P < 0.05) without increasing the amplitude. These immediate effects of PTMC were unidentifiable by the peak Vo₂. We conclude that the dynamic Vo₂ response is useful and sensitive for quantitating the functional status of the cardiocirculatory system.