Does postexercise modelled capillary blood flow accurately reflect cardiovascular effects by different exercise intensities?

Blood flow (BF) in exercising muscles is an important factor for exercise capacity. Recently, a non-invasive method to estimate capillary BF (Q ) was introduced. Using this method, the Fick principle is re-arranged by using relative differences in deoxygenated haemoglobin (ΔHHb) as a surrogate for arteriovenous O difference and pulmonary oxygen uptake (VO ) instead of muscular oxygen uptake. The aim of this study was to examine (I) the relationship between Q and exercise intensity during and following exercise, and (II) to critically reflect the Q approach. Seventeen male subjects completed six bouts of cycling exercise with different exercise intensities (40-90% peak oxygen uptake, VO ) in randomized order. VO and ΔHHb were monitored continuously during the trail. Q was modelled bi-exponentially, and mean response time (MRT) was calculated during recovery as well as the dissociation of modelled VO and Q recovery kinetics (MRT/τVO ). End-exercise Q increased continuously with exercise intensity. This also applied to MRT. Postexercise MRT/τVO increased from 40 to 60% VO but remained stable thereafter. The results show that Q response to exercise is linearly related to exercise intensity. This is presumably due to vasoactive factors like shear-stress or endothelial-mediated vasodilation. MRT/τVO shows that postexercise Q is elevated for a longer period than VO , which is representative for metabolic demand following exercise ≥70% VO . This is a hint for prolonged local vasodilation. According to previous studies, Q could not be modelled properly in some cases, which is a limitation to the method and therefore has to be interpreted with caution.