Noninvasive estimation of microvascular O₂ provision during exercise on-transients in healthy young males

Two methods for estimating changes in microvascular O₂ delivery during the on-transient of exercise were evaluated. They were tested to assess the role of the adjustment of the estimated microvascular O₂ delivery in the speeding of VO₂ kinetics during a Mod1-Hvy-Mod2 protocol (Mod, moderate-intensity exercise; Hvy, heavy-intensity "priming" exercise), in which Mod2 is preceded by a bout of Hvy. Mod pulmonary VO₂ (VO_2p) and deoxy-hemoglobin [HHb] data were collected in 12 males (23 ± 3 yr); response profiles were fit with a monoexponential. Signals were also 1) scaled to a relative % of the response (0-100%) to calculate the [HHb]/VO₂ ratio for each individual and 2) rearranged in the Fick equation for estimation of capillary blood flow (Q_cap). A transient [HHb]/VO₂ "overshoot" observed in Mod1 (1.06 ± 0.05; P < 0.05) was absent during Mod2 (1.01 ± 0.06; P > 0.05); reductions in the [HHb]/VO₂ ratio (Mod1 - Mod2) were related to reductions in phase II τVO_2p (r = 0.82; P < 0.05). For Q_cap, a near-exponential response was observed in 8/12 subjects in Mod1 and only in 4/12 subjects in Mod2. The Q_cap profile was shown to be highly dependent on the [HHb] baseline-to-amplitude ratio. Thus, accurate and physiologically consistent estimations of Q_cap were not possible in most cases. This study confirmed that priming exercise results in an improved O₂ delivery as shown by the decreased [HHb]/VO₂ ratio that was related to the smaller τVO₂ in Mod2. Additionally, this study suggested that Qcap analysis may not be valid and should be interpreted with caution when assessing microvascular delivery of O₂.