Arterial Oxygen (Pao2) and Carbon Dioxide (Paco2) Tensions Are Measured to Evaluate Gas Exchange
A systemic arterial blood sample is essential to evaluate pulmonary gas exchange because this blood has just passed through the lung. A venous blood sample is inadequate because its composition varies depending on the blood IlowZmetabolism ratio of the tissue of origin.
Arterial blood gas tensions are the end result of the individual processes involved in gas exchange and thus are affected by the composition of inspired air, alveolar ventilation, alveolocapillary diffusion, and ventilation/perfusion matching. AU these factors must be considered when evaluating a blood gas result.Inspired air usually contains 21 % oxygen (FIo2 = 0.21), but during anesthesia or oxygen therapy, the administration of oxygen increases FIo2, which causes an increase in inspired oxygen tension (PIo2). Although the daily fluctuations in PB cause only trivial changes in PIo2, the decrease in PB that occurs at higher altitudes results in a major decrease in PIo2. As a result, there is a decrease in PAo2 and thus a decrease in Pao2 as animals ascend in altitude. In the appropriate geographic locations, altitude-induced changes in Pao, must always be considered when blood gas tensions are evaluated.
Adequacy of alveolar ventilation is assessed by examination of Paco2. It is elevated above the normal value of 40 mm Hg when animals hypoventilate and is decreased during hyperventilation. At the same time, hypoventilation decreases PAo2 and Pao2, and hyperventilation increases these tensions.
Diffusion abnormalities and VZQ mismatching impair the transfer of oxygen from the alveolus to arterial blood, increase the AaDo2, and reduce Pao2. Paco2 is rarely elevated by these problems for two reasons. First, the high solubility of carbon dioxide allows easy diffusion across the remaining healthy lung. Second, the hypoxemia caused by lung disease stimulates ventilation. The resultant increase in alveolar ventilation keeps Paco2 normal or even reduces it below normal.
In animals with normal lungs, administering oxygen (increasing FIo2) elevates Pao2. As VZQ mismatching becomes more extreme, oxygen administration increases Pao2 only modestly, especially in the presence of right-to-left shunts. Concurrently, the alveolar-arterial oxygen difference widens. The Pao2 response to oxygen is a good way to evaluate the severity of lung disease.
Pao2 tends to be lower in newborn animals than in adults. This is because there is greater mismatching of ventilation and blood flow in the immature lungs.