![]() ![]() The equation can be estimated as PAO2 = 150 - (PaCO2/0.8). Subtracting the partial pressure of oxygen in the air by the partial pressure of oxygen consumed by the body gives you the partial pressure of oxygen in the alveoli. The partial pressure of carbon dioxide divided by the respiratory quotient yields an estimate of the proportion of oxygen consumed by the body. ![]() This is done by the partial pressure of oxygen in the inspired air subtracted by the partial pressure of carbon dioxide in the arteries first divided by the respiratory quotient. Shunt and anatomical deadspace caused some inaccuracy, although they are unlikely to prevent the clinical usefulness of this formula.The alveolar gas equation is used to calculate the alveolar partial pressure of oxygen (PAO2). The relationship between (Pa-E'CO2)/PaCO2 and VDalv/VTalv (best fit: VDalv VTalv = 1.135 x (Pa-E'CO2)/PaCO2-0.005) during normal physiological conditions was approximately linear and less influenced by physiological variation. The relationship between Pa-E'CO2 and VDalv/VTalv was non-linear and was affected significantly by all the factors except anatomical deadspace. The relationships were observed while pulmonary shunt, anatomical deadspace, ventilatory minute volume and metabolic rate were varied. The aim of the study was to examine the relationship between the arterial to end-tidal PCO2 gradient (Pa-E'CO2) and VDalv/VTalv and between (Pa-E'CO2)/PaCO2 and VDalv/VTalv using the Nottingham Physiology Simulator, an original, validated physiology simulation. The alveolar deadspace as a fraction of alveolar ventilation (VDalv/VTalv), while technically difficult to measure, is an objective monitor of pulmonary disease progression and a predictor of successful weaning from mechanical ventilation. ![]()
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