Ventilation-perfusion relationships and gas exchange.

The most common cause of hypoxemia at sea level is mismatched ventilation and per­ fusion. Common clinical problems such as pneumonia, pulmonary embolism, obstruc­ tive lung disease and many other diseases produce regional hypoventilation as com­ pared to perfusion, resulting in incompletely oxygenated blood leaving the pulmonary capillary in hypoventilated regions and re­ turning to the left heart. The relationship of ventilation to perfu­ sion or Va/Qc refers to the turnover of air in the alveoli (V A) compared to the flow of blood through pulmonary capillaries (Qc). A V a/Q c of one would consist of one L of air ventilating the alveoli and one L of blood flowing through the pulmonary capil­ lary bed during the same period of time. The measurement of overall VA/Q e in the human subject requires measurement of alveolar ventilation by the collection of expired air and measurement of arterial p C 0 2 as well as measurement of cardiac output. This relationship is usually around 0.8. When total alveolar ventilation is in­ adequate for the needs of the patient, then hypoventilation causes hypoxemia and hypercapnia. In the clinical states mentioned, the authors are interested in regional mis­ match of ventilation and perfusion usually referred to as V/Q mismatch. Regional V/Q mismatch or regional hypoventilation com­ pared to perfusion produces hypoxemia and low or normal pC 02; in some instances it may also produce hypercapnia. Until the development of radioactive gas techniques for measurement of regional ventilation and perfusion by Knipping in 1953,3, V/Q ratios for the entire lung could be obtained but no methods were available to estimate these relationships regionally. Since that time, extensive research has been carried out by many investigators who have used these techniques both for clinical de­ scription and for better understanding of basic physiology of the lung. West and others5 have described a V/Q mismatch in the normal upright subject where V/Q is higher at the apex than at the base. The net effect of this mismatch is not sufficient to alter arterial p0 2 significantly, however. These studies do, however, point out the variation in regional perfusion dependent on body position and change in ventilation required to maintain adequate matching of ventilation to perfusion. Such matching avoids overventilation of portions of the lung that are poorly perfused. Recent evi­ dence has been obtained showing that measurements of ventilation and perfusion at resting lung volumes using clearance of perfused 133Xenon during normal breathing shows V/Q to be more evenly matched than was observed previously. 2 The prac­ tical clinical application of these methods