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 Q(cap) analysis may not be valid and should be interpreted with caution when assessing microvascular delivery of O2.