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

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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. 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. 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). 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.