The contribution of red blood cell desaturation to exercising skeletal muscle vasodilation heterogeneity

Experimentally challenging muscle blood flow during exercise requires cardiovascular compensation to protect the matching of oxygen delivery to oxygen demand; however, certain individuals may fail to protect this matching due to an absence of compensatory vasodilation at the local vasculature. We investigated whether red blood cell (RBC) desaturation explains the underlying presence of compensatory vasodilation. Furthermore, earlier studies were completed in males, and we sought to elucidate whether biological sex impacts this relationship. Forty healthy individuals (age: 23 ± 5 yr, 50% female) completed 4 min of submaximal forearm handgrip exercise at 30% maximum voluntary contraction both with and without brachial artery compression reducing forearm blood velocity by 30% and challenging muscle blood flow. Measures of forearm blood flow (mL/min), blood pressure (mmHg), and RBC desaturation (skeletal muscle oxygen saturation, %) were obtained throughout. Forearm vascular conductance (FVC, mL/min/100 mmHg), following challenged exercise, greater than 1.96 times the coefficient of variation of unchallenged exercise represented the presence of compensatory vasodilation. Data are Δ from rest (means ± SD). Twenty-six individuals responded with compensatory vasodilation, whereas 14 individuals did not (49 ± 20 vs. 0 ± 4%FVC, <i>P</i> < 0.001). RBC desaturation was greater in individuals responding with compensatory vasodilation (-22 ± 9 vs. -5 ± 4%oxygen, <i>P</i> < 0.001) and was negatively correlated with FVC (<i>r</i>_s = -0.819, <i>P</i> < 0.001). Sex had no effect on the presence of compensatory vasodilation (<i>P</i> = 0.507) nor on the extent (<i>P</i> = 0.506) or rate (<i>P</i> = 0.857) of RBC desaturation during the muscle blood flow challenge. The presence/absence of compensatory vasodilation following a muscle blood flow challenge appears to be proportional between sex and associated with local RBC desaturation.<b>NEW & NOTEWORTHY</b> When challenging muscle blood flow during small muscle mass exercise, compensatory vasodilation may protect the matching of oxygen delivery to the muscle's oxygen demand. The present study extended previous work by revealing an absence of compensatory vasodilation in 35% of individuals. Vasodilation was correlated with red blood cell (RBC) desaturation and sex did not influence the occurrence of experiencing compensatory vasodilation nor the relationship between vasodilation and RBC desaturation.