Determinants of skeletal muscle oxygen consumption assessed by near-infrared diffuse correlation spectroscopy during incremental handgrip exercise

Abstract

Near-infrared diffuse correlation spectroscopy (DCS) is a rapidly evolving optical imaging technique for the assessment of skeletal muscle O₂ utilization (mVO₂). We compared DCS-derived determinants of mVO₂ with conventional measures [blood flow by brachial artery Doppler ultrasound and venous O₂ saturation ($\text{S}{\text{V}}_{{\text{O}}_2}$)] in eight volunteers at rest and during incremental handgrip exercise. Brachial artery blood flow and DCS-derived blood flow index (BFI) were linearly related (R² = 0.57) and increased with each workload, whereas $\text{S}{\text{V}}_{{\text{O}}_2}$ decreased from 65.3 ± 2.5% (rest) to 39.9 ± 3.0% (light exercise; P < 0.01) with no change thereafter. In contrast, DCS-derived tissue O₂ saturation decreased progressively with each incremental stage (P < 0.01), driven almost entirely by an initial steep rise in deoxyhemoglobin/myoglobin, followed by a linear increase thereafter. Whereas seemingly disparate at first glance, we believe these two approaches provide similar information. Indeed, by plotting the mean convective O₂ delivery and diffusive O₂ conductance, we show that the initial increase in mVO₂ during the transition from rest to exercise was achieved by a greater increase in diffusive O₂ conductance versus convective O₂ delivery (10-fold vs. 4-fold increase, respectively), explaining the initial decline in $\text{S}{\text{V}}_{{\text{O}}_2}$. In contrast, the increase in mVO₂ from light to heavy exercise was achieved by equal increases (1.8-fold) in convective O₂ delivery and diffusive O₂ conductance, explaining the plateau in $\text{S}{\text{V}}_{{\text{O}}_2}$. That DCS-derived BFI and deoxyhemoglobin/myoglobin (surrogate measure of O₂ extraction) share the same general biphasic pattern suggests that both DCS and conventional approaches provide complementary information regarding the determinants of mVO₂.NEW & NOTEWORTHY Near-infrared diffuse correlation spectroscopy (DCS) is an emerging optical imaging technique for quantifying skeletal muscle O₂ delivery and utilization at the microvascular level. Here, we show that DCS provides complementary insight into the determinants of muscle O₂ consumption across a wide range of exercise intensities, further establishing the utility of DCS.

Keywords: Fick principle; blood flow; near-infrared spectroscopy; oxygen uptake.

Fig. 1.

A–C: conventional measures of muscle O₂ consumption. A: Doppler-derived brachial artery blood flow. B: arterial-venous O₂ difference. C: skeletal muscle O₂ consumption (V̇o₂). D–F: diffuse correlation spectroscopy (DCS)-derived determinants of relative muscle O₂ consumption.D: DCS-derived blood flow index. E: near-infrared spectroscopy-derived skeletal muscle tissue saturation. F: relative muscle V̇o₂. Rhythmic handgrip began with a fixed workload of 10 kg and progressed by 3 kg every 3 min thereafter. Arrows depict the onset of exercise. Data are reported as means ± SE; n = 8.

Fig. 2.

Near-infrared-derived oxygenated (Oxy; A), deoxygenated (Deoxy; B), and total (C) hemoglobin (Hb)/myoglobin (Mb) at rest and throughout the incremental handgrip exercise protocol. Rhythmic handgrip began with a fixed workload of 10 kg and progressed by 3 kg every 3 min thereafter. Data are reported as means ± SE; n = 8. Oxygenated hemoglobin/myoglobin remained unchanged from rest throughout exercise, whereas deoxygenated and total hemoglobin/myoglobin increased with graded exercise (P < 0.01 for both).

Fig. 3.

The Fick principle curve depicts muscle O₂ consumption (mVO₂) as a function of convective O₂ delivery (curved line) vs. venous partial pressure of O₂ (Po₂). Fick’s law of diffusion depicts mVO₂ as a function of venous Po₂, with the slope (straight line) representative of muscle O₂ diffusive conductance. The intersection of these lines determines the mVO₂ achieved. A: the transition from rest to 10 kg of rhythmic handgrip exercise was achieved by a greater increase in diffusive O₂ conductance compared with convective O₂ delivery. B: the increase in mVO₂ from 10 to 19 kg was achieved by an increase in both convective O₂ delivery and diffuse O₂ conductance that resulted in a minimal change in venous Po₂. C: the arterial (open bars) and venous (closed bars) O₂ transport at rest and at the end of each respective exercise workload. O₂ utilization (O₂ Util.) was calculated as the differences between arterial and venous O₂ transport, expressed in milliliters of O₂ per minute. Data are reported as means ± SE; n = 8.

Fig. 4.

A: relationship between Doppler-derived brachial artery blood flow and diffuse correlation spectroscopy (DCS)-derived blood flow index (BFI) across the incremental exercise test. B: relationship between DCS-derived BFI and near-infrared spectroscopy-derived deoxy hemoglobin (Hb) + myoglobin (Mb; an indicator fractional O₂ extraction). C: relationship between conventional muscle O₂ utilization (mVO₂) and DCS-derived metabolic rate of O₂ (MRO₂). Individuals are color coded across each panel to highlight intra-individual relationships, which ranged between 0.62 and 0.99 for A, 0.76 and 0.98 for B, and 0.54 and 0.98 for C.