Significance: The arterial carbon dioxide (CO 2) partial pressure Pa CO 2 is a clinically relevant variable. However, its measurement requires arterial blood sampling or bulky and expensive transcutaneous Ptc CO 2 meters. While the spectrophotometric determination of hemoglobin species-such as oxy-hemoglobin (O 2 Hb) and deoxy-hemoglobin (HHb)-allowed for the development of pulse oximetry, the measurement of CO 2 blood content with minimal discomfort has not been addressed yet. Aim: Characterizing human carbamino-hemoglobin (CO 2 Hb) absorption spectrum, which is missing from the literature. Providing the theoretical background that will allow for transcutaneous, noninvasive Pa CO 2 measurements. Approach: A tonometry-based approach was used to obtain gas-equilibrated, lysed, diluted human blood. Equilibration was performed with both CO 2 , dinitrogen (N 2), and ambient air. Spectrophotometric measurements were carried out on the 235-to 1000-nm range. A theoretical background was also derived from that of pulse oximetry. Results: The absorption spectra of both CO 2 Hb and HHb were extremely close and comparable with that of state-of-the-art HHb. The above-mentioned theoretical background led to an estimated relative error above 30% on the measured amount of CO 2 Hb in a subject's blood. Auxiliary measurements revealed that the use of ethylene diamine tetraacetic acid did not interfere with spectrophotometric measurements, whereas sodium metabisulfite did. Conclusions: CO 2 Hb absorption spectrum was measured for the first time. Such spectrum being close to that of HHb, the use of a theoretical background based on pulse oximetry theory for noninvasive Pa CO 2 measurement seems extremely challenging.
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