Quantification of extra-cerebral and cerebral hemoglobin concentrations during physical exercise using time-domain near infrared spectroscopy

Abstract

Fitness is known to have beneficial effects on brain anatomy and function. However, the understanding of mechanisms underlying immediate and long-term neurophysiological changes due to exercise is currently incomplete due to the lack of tools to investigate brain function during physical activity. In this study, we used time-domain near infrared spectroscopy (TD-NIRS) to quantify and discriminate extra-cerebral and cerebral hemoglobin concentrations and oxygen saturation (SO₂) in young adults at rest and during incremental intensity exercise. In extra-cerebral tissue, an increase in deoxy-hemoglobin (HbR) and a decrease in SO₂ were observed while only cerebral HbR increased at high intensity exercise. Results in extra-cerebral tissue are consistent with thermoregulatory mechanisms to dissipate excess heat through skin blood flow, while cerebral changes are in agreement with cerebral blood flow (CBF) redistribution mechanisms to meet oxygen demand in activated regions during exercise. No significant difference was observed in oxy- (HbO₂) and total hemoglobin (HbT). In addition HbO₂, HbR and HbT increased with subject's peak power output (equivalent to the maximum oxygen volume consumption; VO₂ peak) supporting previous observations of increased total mass of red blood cells in trained individuals. Our results also revealed known gender differences with higher hemoglobin in men. Our approach in quantifying both extra-cerebral and cerebral absolute hemoglobin during exercise may help to better interpret past and future continuous-wave NIRS studies that are prone to extra-cerebral contamination and allow a better understanding of acute cerebral changes due to physical exercise.

Keywords: (170.0110) Imaging systems; (170.1470) Blood or tissue constituent monitoring; (300.6500) Spectroscopy, time-resolved.

Fig. 1

Schematic representation of the TD-NIRS probe. A fiber coupler light guide was centered and light was emitted at 690, 760, 810 and 840 nm. Four single photon counting avalanche photodiodes (D1 to D4) were placed at 10, 15, 25 and 30 mm.

Fig. 2

Boxplots of extra-cerebral and cerebral (A) oxy- (HbO₂), (B) deoxy- (HbR), (C) total hemoglobin (HbT), and (D) hemoglobin oxygen saturation (SO₂) in subjects (n_total = 15) at rest, 40% and 80% peak power output (PPO) exercise intensity. On each box, the central mark is the median, the black square is the mean, the edges of the box are the 25^th and 75^th percentiles, and the whiskers show the standard error of the mean. Empty circles denote outliers and significant statistical comparisons are indicated with their corresponding p-value.

Fig. 3

Pearson correlation coefficients (R) and corresponding p-values between extra-cerebral (A)–(C) and cerebral (D)–(F) hemoglobin concentrations, and subject’s peak power output (PPO) at rest, 40% and 80% intensity exercise. Oxy- (HbO₂, red circles), deoxy-(HbR, green squares) and total hemoglobin (HbT, blue triangles) are displayed with the corresponding linear fits (colored solid lines).

Fig. 4

Boxplots of extra-cerebral and cerebral (A) oxy- (HbO₂), (B) deoxy- (HbR), (C) total hemoglobin (HbT), and (D) hemoglobin oxygen saturation (SO₂) in men (M, n_men = 6) and women (W, n_women = 9) at rest, 40% and 80% peak power output (PPO) exercise intensity. On each box, the central mark is the median, the black square is the mean, the edges of the box are the 25^th and 75^th percentiles, and the whiskers show the standard error of the mean. Empty circles denote outliers and significant statistical comparisons are indicated with their corresponding p-value.