Ventilation-perfusion inequality and overall gas exchange in computer models of the lung

The effects of increasing ventilation-perfusion inequality on overall gas exchange were studied in digital computer models of the lung. Ventilation/unit volume and perfusion/unit volume were distributed log normally with respect to lung volume. Ventilation-perfusion inequality was found to affect the transfer of carbon dioxide nearly as much as oxygen. The reasons for the misconception that inequality does not interfere with carbon dioxide transfer are discussed. The effects of changing overall ventilation, blood flow, inspired oxygen, hemoglobin concentration, and the acid-base status of the blood were investigated when oxygen uptake and carbon dioxide output were held constant at normal values. In general, mismatch of ventilation and blood flow in a lung caused the arterial oxygen tension to fall and the carbon dioxide to rise; increasing overall ventilation rapidly restored blood carbon dioxide tensions to normal but improved oxygen tensions little. Considerable improvement in oxygen tensions occurred, however, if the oxygen dissociation curve was made linear. In spite of large increases in inspired oxygen, severe hypoxemia and large alveolar-arterial oxygen differences remained when the inequality was severe. The results help to explain how the lung maintains oxygen and carbon dioxide transfer in the face of ventilation-perfusion ratio inequality and may be useful in characterizing the degree of ventilation-perfusion inequality in real situations.