The degree to which the CardOut vs. V? o dos relationship is nonlinear has not been rigorously investigated. In addition to knowing the relative frequency of finding a nonlinear relationship in the general population, it would be interesting to see whether nonlinearity might be associated with relative fitness (e.g., V? o 2 max/kg). One could argue that, as relative fitness level increases, the ability of the heart to increase stroke volume (SV) at higher levels of exercise might become limited as filling time per unit of venous return is reduced.
2 in a group of healthy individuals, not selected for athleticism but covering a range of fitness levels. We sought to answer several specific questions: 1) in what proportion of individuals is there a statistically significant curvature in the CardOut vs. V? o 2 relationship? 2) Is curvature of CardOut vs. V? o 2 max related to fitness level as indicated by V? o dos max/kg or other subject characteristics? The purpose of the study focused solely on the CardOut vs. V? o 2 relationship and its relationship to fitness level. Although, as pointed out above, negative curvature in the CardOut vs. V? o 2 relationship might indicate a kind of cardiac limitation to exercise, determining the overall integrated determinants of V? o 2 maximum, which could include limitations in any component of the O2 transport or utilization chain (20), is not an end point of this study. To answer these questions, we utilized an open-circuit acetylene uptake method to measure CardOut (9) furfling coupon. Because the method does not involve altered breathing patterns or rebreathing, with inevitable buildup of CO2 and depletion of O2 during the maneuver, it does not inherently limit exercise performance, so it is well suited to the questions posed above.
Healthy subjects were recruited from the population of Rochester, MN, by advertisement in the local media and within the Mayo Clinic. Subjects signed written, informed consent to participate in an exercise evaluation, which had been approved by institutional review board of Mayo Clinic. Subjects were excluded if they had clinical histories of cardiac or pulmonary diseases, including asthma and tobacco use. For all subjects, ?2-adrenergic receptor subtyping was performed for purposes of studying differences in exercise response based on ?-receptor subtype, which is the subject of a separate report (18). Subjects were not excluded based on genotyping. Thus this group of subjects can be considered to be a sample from the general population of Rochester, MN, with weighted representation from Mayo Clinic employees.
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Subjects participated in two maximal exercise protocols on separate days. For the first study, 12-lead ECG and noninvasive cuff blood pressures were monitored during a 2-min incremental protocol on a stationary exercise cycle, each increment ranging from 15 to 30 W, depending on preexercise assessment of the fitness level of the subject. Breath-by-breath gas exchange was monitored using commercial software (CPX/D, Medical Graphics, St. Paul, MN) interfaced to a respiratory mass spectrometer (15). Exercise intensity increased until volitional fatigue. The second exercise study was performed to evaluate CardOut response to exercise using a noninvasive open-circuit technique (9) following an identical protocol as the first study. CardOut measurements were made the last 30 s of every exercise intensity up to maximal exertion. At the start of the protocol, the subject was seated on the cycle ergometer with mouthpiece in place while three CardOut assessments were completed with 2–4 min between each to allow inert-gas washout from the lungs. We report here the results of the Cardout data from the second study merged with the V? o 2 data from the first evaluation averaged over the last 30 s of each exercise level.