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 firstmet free app.
Dashed line highlights extraction out of 18 ml O
the fick equation expresses the mass balance between whole body O2 consumption (V? o 2), cardiac output (CardOut), and the difference in O2 content between mixed venous and arterial blood ( ? ):
The partnership anywhere between cardiac returns (CardOut) and you may fresh air practices (V? o
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 O2 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 O2 extraction, with the line for O2 extraction of 18 ml O2/100 ml blood indicated as a solid line.
Fig. 1.Model of cardiac output (CardOut) vs. O2 consumption (V? o 2) relationship. Lines of constant O2 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 O2 extraction of ?14 ml O2/100 ml blood. With CardOut still increasing linearly and O2 extraction <16 ml O2/100 ml blood, this person would likely not have reached V? o 2 maximum. By extrapolation, if this person could continue to an extraction of 18 ml O2/100 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, O2 extraction is near a maximal value of 18 ml O2/100 ml blood, indicating that exercise limitation is likely due to attainment of maximal O2 extraction, although the increase in O2 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 O2 extraction.
