

"There is no scientific rationale for using exhaled carbon dioxide to determine alveolar alcohol concentration. First, it is impossible to get air with the alveolar alcohol concentration at the mouth. Alcohol exchanges with the airway tissue so the alcohol concentration decreases about 20% by the time it reaches the mouth. Second, carbon dioxide exchanges in the alveoli with the pulmonary circulation while ethanol exchanges in the airways with the bronchial circulation. Third, the exhaled CO2 profile is not the same as the exhaled ethanol profile. Ethanol rises sooner during exhalation compared with CO2 which rises later in the exhalation. Fourth, the rise in CO2 is influenced by cardiac output and the heterogeneity of ventilation to perfusion matching in the lungs (worse with lung disease) whereas, the rise in ethanol is not. Using an arbitrary CO2 concentration to determine when alveolar air arrives at the mouth is not appropriate because alveolar CO2 changes as steady state ventilation changes relative to metabolic production of CO2 whereas ethanol does not change with changes in steady state ventilation.
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