_{dos}) during exercise has generally been assumed to be linear. To test this assumption, we studied 72 healthy subjects using a graded, 2-min cycle-ergometry exercise test to maximum while measuring gas exchange continuously and CardOut at the end of each stage, the latter using an open-circuit gas technique. Data for V? o _{2} and CardOut at each stage were fit to a quadratic expression y = a + (b·V? o _{2}) + (c·V? o _{2} 2 ), and statistical significance of the quadratic c term was determined in each subject. Subjects were then divided into two groups: those with statistically significant negative quadratic term (“negative curvature group,” n = 25) and those with either nonsignificant quadratic term or c significantly > 0 (“non-negative curvature group,” n = 47, 2 with c significantly > 0). We found the negative curvature group had significantly higher maximal V? o _{2}/kg (median 37.9 vs. 32.4 ml·min ?1 ·kg ?1 ; P = 0.03) higher resting stroke volume (SV; median 77 vs. 60 ml; P = 0.04), lower resting heart rate (HR; median 72 vs. 82 beats/min, P = 0.04), and higher tissue oxygen extraction at maximal exercise (17.1 ± 2.2 vs 15.5 ± 2.1 ml/100 ml; P < 0.01), with tendencies for higher maximal CardOut and SV. We also found the HR vs. V? o _{2} relationship to be negatively curved, with negative curvature in HR associated with the negative curvature in CardOut (P < 0.05), suggesting the curvature in the CardOut vs. V? o _{2} relationship was secondary to curvature in HR vs. V? o _{2}. We conclude that the CardOut vs. V? o _{2} relationship is not always linear, and negative curvature in the relationship is associated with higher fitness levels in normal, non-elite-athletic subjects.

the fick equation expresses the mass balance between whole body O_{2} consumption (V? o _{2}), cardiac output (CardOut), and the difference in O_{2} content between mixed venous and arterial blood ( ? ):

Although it is generally assumed that CardOut increases linearly with V? o _{2}, the pattern of variation in V? o _{2} and CardOut as maximal O_{2} extraction is approached has not been extensively investigated and ong individuals. This is illustrated in Fig. 1, which shows three patterns of increase in CardOut with increasing V? o _{2}. Dashed lines indicate isopleths of constant O_{2} extraction, with the line for O_{2} extraction of 18 ml O_{2}/100 ml blood indicated as a https://datingranking.net/oasis-dating-review/ solid line.

Fig. 1.Model of cardiac output (CardOut) vs. O_{2} consumption (V? o _{2}) relationship. Lines of constant O_{2} extraction, derived from the Fick equation, are indicated. For explanations of lines 1–3, see text. _{2}/100 ml blood.

In this example, all three patterns of increase in CardOut start with the same initial slope, ?5.2 l·min ?1 ·(l/min of V? o _{2}) ?1 and are distinguished by the amount of downward curvature. Curve 1, top, intersects a CardOut of 15 l/min at a V? o _{2} of ?2 l/min with O_{2} extraction of ?14 ml O_{2}/100 ml blood. With CardOut still increasing linearly and O_{2} extraction <16 ml O_{2}/100 ml blood, this person would likely not have reached V? o _{dos maximum}. By extrapolation, if this person could continue to an extraction of 18 ml O_{2}/100 ml blood, his/her V? o _{2 maximum} would be well over 3.5 l/min. The second curve indicates a steadily decreasing slope in CardOut vs. V? o _{2}, suggesting a developing CardOut limitation. At a V? o _{2} of 2.7 l/min, O_{2} extraction is near a maximal value of 18 ml O_{2}/100 ml blood, indicating that exercise limitation is likely due to attainment of maximal O_{2} extraction, although the increase in O_{2} extraction was accelerated relative to the linear curve because of a relative reduction in CardOut at higher exercise intensities. Curve 3 indicates a true limitation in CardOut, with the slope of CardOut vs. V? o _{2} reaching 0 at the point of maximal O_{2} extraction.