Effect of lung volume on steady state pulmonary membrane diffusing capacity and pulmonary capillary blood volume.

Abstract

Although resting steady state measurements of pulmonary diffusing capacity of the lung are frequently used in the evaluation of patients, results from such tests are often quite variable because the steady state method is highly dependent upon alveolar ventilation; an increase in tidal volume or respiratory frequency produces an increase in the diffusing capacity. Although these changes have been attributed to an increase in alveolar volume, data obtained in this and other studies indicate that a change in end-expiratory lung volume does not affect steady state diffusing capacity. In addition, it was observed that the membrane diffusing capacity and pulmonary capillary blood volume remained unchanged. Further, it was shown that steady state diffusing capacity increases as a function of hyperventilation because of an artifactual increase in membrane diffusing capacity. Steady state membrane diffusing capacity is highly dependent not only on the alveolar ventilation but also on the method used to estimate mean alveolar carbon monoxide tension. Such factors, however, do not influence estimation of pulmonary capillary blood volume. In contrast to the estimation of membrane diffusing capacity, measurements of capillary blood volume by steady state methods compare favorably with those obtained by the single breath technique. The steady state method does not give a reliable estimate of membrane diffusing capacity, an observation consistent with calculations that indicate that diffusing capacity is unevenly distributed with regard to alveolar ventilation. Nevertheless, steady state estimates of diffusing capacity in normal resting subjects can have physiologic meaning if measurements are made at different alveolar oxygen tensions to calculate pulmonary capillary blood volume; however, because alveolar ventilation must be rigidly controlled in such studies, the single breath technique is far more practical.