Skin blood flow influences cerebral oxygenation measured by near-infrared spectroscopy during dynamic exercise

Purpose

Near-infrared spectroscopy (NIRS) is widely used to investigate cerebral oxygenation and/or neural activation during physiological conditions such as exercise. However, NIRS-determined cerebral oxygenated hemoglobin (O₂Hb) may not necessarily correspond to intracranial blood flow during dynamic exercise. To determine the selectivity of NIRS to assess cerebral oxygenation and neural activation during exercise, we examined the influence of changes in forehead skin blood flow (SkBF_head) on NIRS signals during dynamic exercise.

Methods

In ten healthy men (age: 20 ± 1 years), middle cerebral artery blood flow velocity (MCA V _mean, via transcranial Doppler ultrasonography), SkBF_head (via laser Doppler flowmetry), and cerebral O₂Hb (via NIRS) were continuously measured. Each subject performed 60 % maximum heart rate moderate-intensity steady-state cycling exercise. To manipulate SkBF_head, facial cooling using a mist of cold water (~4 °C) was applied for 3 min during steady-state cycling.

Results

MCA V _mean significantly increased during exercise and remained unchanged with facial cooling. O₂Hb and SkBF_head were also significantly increased during exercise; however, both of these signals were lowered with facial cooling and returned to pre-cooling values with the removal of facial cooling. The changes in O₂Hb correlated significantly with the relative percent changes in SkBF_head in each individual (r = 0.71–0.99).

Conclusions

These findings suggest that during dynamic exercise NIRS-derived O₂Hb signal can be influenced by thermoregulatory changes in SkBF_head and therefore, may not be completely reflective of cerebral oxygenation or neural activation.