Arterial-Alveolar N2 Gas Pressure Differences Due to Ventilation-Perfusion Variations

Abstract

Whenever differences in ventilation-perfusion ratio exist among alveoli of the lung, differences in the inert gas pressures must also prevail, with some pressures higher and others lower than the inert gas pressure of the mixed venous blood. Thus, inert gas will either diffuse into or out of the perfusing blood, depending upon the pressure difference established in each alveolus, even though the net inert gas exchange is zero. This condition has interesting consequences. 1) If the solubility of the inert gas is large, it will affect the shape of the ventilation-perfusion curve. 2) The mean arterial inert gas pressure is greater than that of the mean alveolar gas. Such an arterial-alveolar N₂ difference is demonstrated in anesthetized dogs. A method is described for the measurement of the total gas pressure in arterial blood from which the arterial N₂ pressure can be calculated if the O₂ and CO₂ pressures are determined by conventional methods. Submitted on November 15, 1956