Carbon monoxide reduces near-infrared spectroscopy determined 'total' hemoglobin: a human volunteer study.

Carbon monoxide (CO) increases middle cerebral artery mean flow velocity (MCAV(mean)), but the effect of CO on the near-infrared spectroscopy (NIRS) determined cerebral oxygenation (ScO2) is not detailed. In our study, 11 non-smoking subjects breathed 100% O-2 through a closed circuit. A CO2 scrubber with CO (1.5mLkg(-1)) was added to the circuit. Two NIRS systems (NIRO-200NX and INVOS-5100) assessed ScO2 as the ratio of oxygenated to deoxygenated hemoglobin, while venous blood samples were analyzed for carboxyhemoglobin (COHb). After CO/O-2 rebreathing COHb increased to 8.7% (IQR; 7.9-9.4; p=.004) vs. normoxia, but MCAV(mean) remained stable (55.6cms(-1); 53.1-69.7) compared to inhalation of O-2 (54.6cms(-1); 48.4-62.9; p=.178) and normoxia (54.1cms(-1); 44.5-66.9; p=.055). Also, INVOS-5100 determined ScO2 increased during CO/O-2 (74.4 +/- 7.5%) and O-2 inhalation (73.1 +/- 7.2%) compared to normoxia (68.9 +/- 6.9%; p<.001). In contrast, NIRO-200NX determined ScO2 remained unchanged during CO/O-2 and O-2 inhalations but oxygenated and deoxygenated hemoglobin decreased (by 19.7M (median; IQR 2.8-34.8; p=.016) and 37.3M (30.8-46.6; p=.004), respectively) during inhalation of CO/O-2 compared to inhalation of O-2. Therefore, NIRO-200NX determined total' hemoglobin (sum of O(2)Hb and HHb) decreased (by 62.1M; 44.5-78.2; p=.001). In conclusion, exposure to CO did not increase MCAV(mean), and neither NIRO-200NX nor INVOS-5100 detected a change in ScO2 when CO was added to inhalation of oxygen. Unaffected ScO2 after exposure to CO reflected a similar decrease in oxygenated and deoxygenated hemoglobin suggesting that detection of exposure to CO by NIRS should focus on total' hemoglobin rather than on ScO2.