Carbon monoxide reduces near-infrared spectroscopy determined “total” hemoglobin: a human volunteer study Near-infrared spectroscopy determined brain and muscle oxygenation during head-up tilt induced central hypovolemia Mads Niemann1, Henrik Sørensen1, Christoph Siebenmann2, Carsten Lundby2, Niels H. Secher1 1Department of Anesthesia, Rigshospitalet, University of Copenhagen, Denmark 2Zurich Centre for Integrative Human Physiology, University of Zürich, Switzerland Introduction Carboxyhemoglobin (COHb) rapidly increases during exposure to carbon monoxide (CO). We evaluated whether inhalation of CO can be detected by near-infrared spectroscopy (NIRS) of the brain. We hypothesized that even though NIRS focuses on detection of oxygenated (O2Hb) and deoxygenated hemoglobin (HHb), NIRS might also be used for non-invasive assessment of exposure to CO by demonstrating a decrease in “total” hemoglobin (tHb; sum of the detected O2Hb and HHb). Figure 1: Changes in NIRS and TCD values during inhalation * * †* Methods 11 non-smoking subjects (10 males; 74.2 ± 8.5 kg) rested 20 min before breathing 100% O2 for 4 min. The circuit was then switched for 10 min to a closed re-breathing circuit with O2 and a 1.5 ml/kg body mass of chemically pure CO.   COHb-levels were assessed by ABL800 (Radiometer, Denmark) and MCAVmean by Transcranial Doppler (n=9). Non-invasive cerebral oxygenation saturation (ScO2) (n=11) was determined by two NIRS devices: INVOS-5100 and NIRO-200NX to evaluate the effect of COHb on ScO2. ScO2 is calculated as the ratio between cerebral tissue O2Hb and HHb or between O2Hb and total hemoglobin (O2Hb plus HHb). The INVOS-5100 utilizes near-infrared light at wavelengths at 730 and 810 nm, while the NIRO-200NX uses light at 735, 810, and 855 nm. In addition, the NIRO-200NX displays ΔO2Hb, ΔHHb, and ΔtHb. Figure 1. Mean blood flow velocity in the middle cerebral artery (MCAVmean) determined by transcranial Doppler (TCD) and near-infrared spectroscopy determined cerebral oxygenation (ScO2) by INVOS-5100 and NIRO-200NX during inhalation of CO/O2, O2 and normal air. Values are mean ± SD. † = P<0.05 compared to O2 inhalation. * = P<0.05 compared to normoxia. Figure 2: Changes in NIRO-determined hemoglobin species p=0.001 p=0.004 Results COHb increased to 8.7% (7.9 - 9.4) during CO/O2 inhalation compared to 1.3% (1.0-1.4; p=0.004) during normoxia. MCAVmean increased during CO/O2 inhalation (60.1±8.7 cm s-1) compared to O2 inhalation (55.3±9.3 cm s-1; p=0.028) and normoxia (55.1±10.8 cm s-1; p=0.009). INVOS-5100 determined ScO2 increased during CO/O2 (74.4±7.5%) and O2 inhalation (73.1±7.2%) compared to normoxia (68.9±6.9%; p<0.001) but the NIRO-200NX determined ScO2 remained unchanged. NIRO-200NX reported a decrease in HHb by 37.3 µM (30.8 to 46.6; p=0.004) and O2Hb by 19.7 µM (2.8 to 34.8; p=0.016) during CO/O2 inhalation compared to inhalation of O2. Thus, tHb decreased by 62.1 µM (44.5 to 78.2; p=0.001) between the two inhalations. p=0.016 Figure 2. Changes from O2 inhalation to CO/O2 inhalation for oxygenated (O2Hb), deoxygenated (HHb), and total (tHb) hemoglobin. µM = 10-6 mol/l. Conclusions Exposure to CO increases MCAVmean INVOS-5100 and NIRO-200NX detected “falsely” elevated ScO2 during CO inhalation NIRO-200NX detected a decrease in ΔO2Hb, ΔHHb and ΔtHb during CO/O2 inhalation Detection of moderate exposure to CO by current NIRS apparatus should focus on the reported “total” hemoglobin rather than on ScO2. Mads Niemann, Medical student Email: mads.jacob.niemann@regionh.dk Niels H. Secher, Professor, MD, DMSc Email: niels.secher@regionh.dk