Muscle capillary blood flow kinetics estimated from pulmonary O₂ uptake and near-infrared spectroscopy

Abstract

. Muscle capillary blood flow kinetics estimated from pulmonary O2 uptake and near-infrared spectroscopy. J Appl Physiol 98: 1820 -1828, 2005. First published January 7, 2005 doi:10.1152/japplphysiol.00907.2004.-The near-infrared spectroscopy (NIRS) signal (deoxyhemoglobin concentration; [HHb]) reflects the dynamic balance between muscle capillary blood flow (Q cap) and muscle O2 uptake (V O2m) in the microcirculation. The purposes of the present study were to estimate the time course of Q cap from the kinetics of the primary component of pulmonary O2 uptake (V O2p) and [HHb] . However, there was no significant difference between MRT of Q cap and P-V O2 for both intensities (P ϭ 0.99), and these parameters were significantly correlated (M and H; r ϭ 0.99; P Ͻ 0.001). In conclusion, we have proposed a new method to noninvasively approximate Q cap kinetics in humans during exercise. The resulting overall Q cap kinetics appeared to be tightly coupled to the temporal profile of V O2m. exercise; skeletal muscle; oxygenation INSIGHTS ON THE CONTROL OF exercising muscle blood flow (Q m ) can be gained from the investigation of its response in the transitional phase (i.e., kinetics) (21, 27), but because of methodological constraints the kinetics of Q m in humans have been studied primarily in larger vessels (1,16,22,31,37,42,44).The difficulty in obtaining measurements with a time resolution that allows reliable kinetic analysis during large muscle mass exercise (e.g., cycling or running) has led to a predominant use of knee extensor or forearm exercise with measurements of blood flow made by Doppler ultrasound (11,22,31,37,42,44). These investigations have shown that the Q m response is biphasic with an initial fast phase determined by the combined effects of muscle contraction (muscle pump) (41) and possibly rapid vasodilation (48) followed by a second slower phase that appears to match O 2 delivery and utilization (43).Several studies have addressed the relationship between Q m and muscle O 2 uptake (V O 2m ) kinetic response after the onset of exercise (5,12,14,16,22,31 (27). To date, for technical and ethical reasons, assessing the kinetics of muscle capillary blood flow (Q cap ) in humans has been problematic. Resolution of this discrepancy in Q m kinetics relative to those of V O 2m is crucial to advancing our understanding of the mechanisms that govern the control of both Q m and V O 2m in health and disease.Near-infrared spectroscopy (NIRS) provides a noninvasive measure of muscle oxygenation (or O 2 extraction) in the microcirculation. Although distinction between hemoglobin (Hb) and myoglobin (Mb) with regard to absorption of the near-infrared light cannot be made, the deoxygenated Hb/Mb (deoxy-Hb/Mb) signal obtained by NIRS has been used as an index of local O 2 extraction reflecting the V O 2m -to-Q m ratio in the capillaries (9, 15). The time course of deoxy-Hb/Mb after the onset of exercise resembles qualitatively and quantitatively the arteriovenous O 2 difference [(a-v)O 2 ] observed in separate...

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

“…2, A-C), the dynamic adjustment of blood flow during the first moderate-intensity exercise transition could not be systematically characterized in all subjects since the Q cap time course was actually nonexponential in one-third of the them. While conformity of the Q cap response to an exponential profile is not necessary in order for it to be considered "physiologically reasonable" ("priming" exercise may make this less likely), this response profile has been reported previously (4,7). Indeed, in the subjects in whom an exponential characterization of the response was feasible, a faster adjustment of microvascular blood flow (i.e., a lower MRT of the Q cap response) was associated with a smaller…”

“…The profile of muscle capillary blood flow (Q cap) was estimated using the methods described by Ferreira et al (7). Briefly, the Qcap response was derived on a second-by-second basis from the kinetic profiles of V O2p (l/min) and [HHb] (M) with the assumption that the kinetics of the primary component of V O2p during constant work-rate exercise approximates that of the muscle V O2 (V O2m) (9) and that the NIRS-derived [HHb] is a function of the balance between V O2m and muscle blood flow (5,8), thus representing a proxy measure for O2 extraction (and a-vO2diff).…”

“…When this method is used, the amplitude of Qcap is quantitatively uncertain because the actual contribution of arterial and venous blood to the [HHb] signal is unknown. However, the temporal characteristics of [HHb], and thus Q cap, should be preserved (7). The Qcap response to exercise, in 5-s averages, from the onset to the end of the exercise (360 s), was fitted by means of a single exponential model with a TD (as in Eq.…”

“…However, this method does not reflect microvascular blood flow as derived from the Fick equation. In this regard, an estimation of capillary blood flow (Q cap ) has been proposed (7,10). When this method is used, [HHb] is substituted directly into the Fick equation in place of the arterio-venous O 2 difference (avO 2diff ), and kinetic profiles of muscle V O 2 (V O 2m ) are derived using the parameters of pulmonary V O 2 (V O 2p ).…”

Relating pulmonary oxygen uptake to muscle oxygen consumption at exercise onset: in vivo and in silico studies

“…Consequently, the estimation of Q m by taking into account the dynamics of oxygen stores and muscle blood flow (Kemp 2005) is more accurate than that based on a Fick relationship (Barstow et al 1990;Ferreira et al 2005aFerreira et al , 2005b, which is strictly valid only at steady state (Stringer et al 2005). In our model, blood flow dynamics are approximated by a single exponential with time constant τ Q m similar to that of heart rate (τ HR ) determined from measurements of heart rate dynamics (DeLorey et al 2004;Miyamoto et al 1982).…”