Whether blood oxygen (O2) carrying capacity plays a substantial role in determining cardiorespiratory fitness, a strong predictor of mortality, remains uncertain in women and elderly individuals due to the scarcity of experimental investigations. This study experimentally assessed the role of blood O2 carrying capacity on cardiorespiratory fitness in middle-aged and older individuals.
Healthy women and men (n=31, 35-76 yr) matched by age and fitness were recruited. Transthoracic echocardiography, central hemodynamics and O2 uptake were assessed throughout incremental exercise in (i) control conditions and (ii) after a 10 % reduction of blood O2 carrying capacity via carbon monoxide administration, in a blinded manner. Effects on cardiac function, blood pressure, peak O2 uptake (VO2peak), and effective hemoglobin (Hb) were determined with established methods.
Blood O2 carrying capacity, represented by effective Hb, was similarly reduced in women (11.8±0.6 vs. 10.7±0.6 g·dl-1, P<0.001) and men (13.0±0.9 vs. 11.7±0.6 g·dl-1, P<0.001) (P for sex effect=0.580). Reduced O2 carrying capacity did not induce major effects on cardiac function and hemodynamics during exercise, except for a 10-15 % decrement in peak systolic blood pressure in both sexes (P≤0.034). VO2peak decreased from 35±6 to 31±6 ml·min-1·kg-1, P<0.001) in women and from 35±9 to 32±9 ml·min-1·kg-1 (P=0.024) in men in approximate proportion to the reduction of O2 carrying capacity, an effect that did not differ between sexes (P=0.778).
Blood O2 carrying capacity stands out as a major determinant of cardiorespiratory fitness in healthy mature women and men, with no differential effect of sex.

Copyright © 2021 American College of Sports Medicine.

Author