Quantifying The Inter-Relationships Amongst Muscle Blood Flow, O2 Uptake And Muscle Deoxygenation Kinetics From Elevated Baseline Metabolic Rates And Increasing Work Rate Amplitudes

Lorenzo Love, The University of Western Ontario

Abstract

Upon an instantaneous increase in work rate (WR), pulmonary O₂ uptake (V̇O_2p) increases in an exponential-like manner towards a new steady-state. To support this increase in V̇O_2p there is an increase in blood flow (BF; O₂ delivery) to the muscle. Previous research has shown that the rate of this increase in V̇O_2p (which defines V̇O_2p kinetics) becomes progressively slower with increasing baseline WR (WR_bl). Given the paucity of research on blood flow dynamics and underlying muscle metabolic rate, this thesis examined the effect of metabolic rate on the dynamics and relationships amongst V̇O_2p, BF, and muscle deoxygenation ([HHb]). Using mass spectrometry and volume turbinometry, Doppler ultrasound, and near-infrared spectroscopy (NIRS), the response of V̇O_2p, BF, and [HHb] were measured, respectively, in response to step transitions in alternate-leg, knee-extension exercise from: i) a common WR_bl (3 W) to increasing ΔWR (21, 30, 42, 51, 63 W); and ii) increasing WR_bl (3, 12, 24, 33, 45 W) with a common ΔWR (21 W). As the BF signal is distorted by fluctuations in arterial pressure (from the heart rate (HR) cycle) and intramuscular pressure (from the contraction-relaxation (CR) cycle), it was necessary to first determine an acceptable technique for integrating the BF signal. From integrating BF by the CR or HR cycle over 1, 2, or 5 cycles using either ‘binning’ or ‘rolling’ averages, it was found that averaging the signal over a single CR cycle (CR1) was preferred due to its low variability. In analyzing the kinetic response to the various exercise transitions, it was found that: i) BF kinetics were similar across different ΔWR’s, but became progressively slower with increasing WR_bl before significantly speeding at the highest WR_bl; ii) V̇O_2p kinetics became progressively slower with both increasing ΔWR and WR_bl; iii) the increase in BF relative to the increase in metabolic demand (ΔBF/ΔV̇O_2p) became progressively smaller with both increasing ΔWR and increasing WR_bl; iv) [HHb] kinetics became progressively faster with increasing ΔWR, but became progressively slower with increasing WR_bl. These findings suggest that with an increasing ΔWR, the muscle has an attenuated increase in bulk BF and a larger reliance on O₂ extraction to meet the O₂ requirements of the muscle, but with an increasing WR_bl the muscle relies less on O₂ extraction and more on BF redistribution within the muscle microvasculature.