Measuring hemoglobin spectra: searching for carbamino-hemoglobin

Significance: The arterial carbon dioxide ([Formula: see text]) partial pressure [Formula: see text] is a clinically relevant variable. However, its measurement requires arterial blood sampling or bulky and expensive transcutaneous [Formula: see text] meters. While the spectrophotometric determination of hemoglobin species—such as oxy-hemoglobin ([Formula: see text]) and deoxy-hemoglobin (HHb)—allowed for the development of pulse oximetry, the measurement of [Formula: see text] blood content with minimal discomfort has not been addressed yet. Aim: Characterizing human carbamino-hemoglobin ([Formula: see text]) absorption spectrum, which is missing from the literature. Providing the theoretical background that will allow for transcutaneous, noninvasive [Formula: see text] measurements. Approach: A tonometry-based approach was used to obtain gas-equilibrated, lysed, diluted human blood. Equilibration was performed with both [Formula: see text] , dinitrogen ([Formula: see text]), 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 [Formula: see text] 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 [Formula: see text] 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: [Formula: see text] 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 [Formula: see text] measurement seems extremely challenging.