Interpretation of the Arterial Oxygen and Carbon dioxide Partial Pressures

A Simplified Approch for Bedside Use

¹ Professor of Medicine, Boston University School Of Medicine

Arterial Po2 and Pco2 values give valuable information regarding gas exhange function of the lungs and oxygen transport by the blood. Alveolar ventilation varies inversly as arterial Pco2; the letter measurement, therefor, is an indication of the adequacy of CO2 eliminarion by the lings. The alveolar Po2 can be readily calculated using the arterial Pco2 as a measure of alveolar Pco2 and either a simplified version of the alveolar air equation or an O2-CO2 diagram. An abnormally widened alveolar-arterial Po2 difference indicates impaired oxygen exchange function of the lungs, although whether the abnormality is due to anatomic tight-to-left shunting, ventilation/perfusion imbalance or diffusion abnormality is not reveales. A normal alveolar-arterial Po2 difference inthe presence of hypoxemia indicates alveolar hypoventilation or a decresed inspired Po2. Saturation of hemoglobin with O2 can be calculated from the arterial Po2 and the O2 hemoglobin dissociation curve. Oxygen content can be calculated from the product of O2 saturation and hemoglobin O2 capacity; the latter is directly related to hemoglobin concentration. Cardiac output and regional circulatory factors are also of great importance in O2 delivery to tissues.