Capillary-oxygenation-level-dependent near-infrared spectrometry in frontal lobe of humans

Brain function requires oxygen and maintenance of brain capillary oxygenation is important. We evaluated how faithfully frontal lobe near-infrared spectroscopy (NIRS) follows haemoglobin saturation (S_Cap) and how calculated mitochondrial oxygen tension (P_MitoO₂) influences motor performance. Twelve healthy subjects (20 to 29 years), supine and seated, inhaled O₂ air-mixtures (10% to 100%) with and without added 5% carbon dioxide and during hyperventilation. Two measures of frontal lobe oxygenation by NIRS (NIRO-200 and INVOS) were compared with capillary oxygen saturation (S_Cap) as calculated from the O₂ content of brachial arterial and right internal jugular venous blood. At control S _Cap (78%±4%; mean±s.d.) was halfway between the arterial (98%±1%) and jugular venous oxygenation (S_vO ₂; 61%±6%). Both NIRS devices monitored S_Cap (P<0.001) within ∼5% as S_vO₂ increased from 39%±5% to 79%±7% with an increase in the transcranial ultrasound Doppler determined middle cerebral artery flow velocity from 29±8 to 65±15 cm/sec. When S_Cap fell below ∼70% with reduced flow and inspired oxygen tension, P_MitoO₂ decreased (P<0.001) and brain lactate release increased concomitantly (P<0.001). Handgrip strength correlated with the measured (NIRS) and calculated capillary oxygenation values as well as with P_MitoO₂ (r> 0.74; P<0.05). These results show that NIRS is an adequate cerebral capillary-oxygenation-level-dependent (COLD) measure during manipulation of cerebral blood flow or inspired oxygen tension, or both, and suggest that motor performance correlates with the frontal lobe COLD signal.