Determination of Single Breath Carbon Monoxide Alveolar-Capillary Diffusion
Miller, Marilea Kay
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The determination of pulmonary diffusing capacity of the transfer factor requires the use of gases that combine chemically with hemoglobin and which are considerably more soluble in the blood than in the alveolar-capillary membrane. Two such gases are knovn -- oxygen and carbon monoxide. The latter has been clinically more useful, and the following discussion will be limited to a survey of the variables involved in and the applications of a certain carbon monoxide method for determining diffusing capacity. The oxygen method tor determining diffusing capacity and the difficulties associated with such measurement have been summarized by Canroe et. al. elsewhere. Carbon monoxide is favorable for use since it has approximately 210 times more affinity for hemoglobin than does oxygen. Carbon monoxide is consequently taken up from respiratory gas for considerable periods of time without there being a significant change in the pulmonary capillary carbon monoxide pressure; thus the "escaping tendency" or plasma tension, or back pressure of carbon monoxide in the pulmonary capillary blood is usually considered negligible and the partial pressure of CO in the a1veoli is therefore considered a primary force responsible for CO transport across the pulmonary membrane. In addition, the transfer of CO is limited to an extent by its rate of diffusion across the erythrocytic membrane and by the rate of the intraerythrooytic chemical reaction between hemoglobin and carbon monoxide.