Comparison of stroke volume estimation for non-steady-state and steady-state graded exercise testing

The stroke volume (SV) during exercise is an important index of the heart's functional capacity. A new method has been developed for the non-invasive estimation of exercise SV (SVex). It requires the determination of the slope for the oxygen uptake versus heart rate relationship in the steady state of graded exercise testing (GXT). The product of the slope and a constant (reciprocal of an assumed value of the arterial oxygen content) equals an estimated value for SVex. It was validated in a previous study using invasive measurements while subjects were performing steady-state GXT. However, currently the most commonly used GXT protocols are non-steady state, e.g. protocols with 1-min increment durations. We tested the hypothesis that SVex is the same for steady-state and non-steady-state GXT. A total of 30 subjects (15 males and 15 females) served as subjects for the study. Each subject performed two GXTs on different days with different increment durations - 1 and 4 min. Ventilation and gas exchange were measured with the Vacumed metabolic cart. For the male subjects, the mean (SD) SVex values for the 1- and 4-min GXTs were 155.4 (39.5) and 134.6 (27.5) ml, respectively. The corresponding values for the female subjects were 151.6 (37.6) and 134.3 (36.4) ml. Paired t-test analysis demonstrated that for both genders the mean SVex for the 1-min GXT was significantly larger than the 4-min GXT mean value (P<0.05). Hence, the commonly used 1-min GXT does not yield the same values for SVex as the steady-state GXT.