FRACTIONAL CHANGES IN LUNG CAPILLARY BLOOD-VOLUME AND OXYGEN-SATURATION DURING THE CARDIAC CYCLE IN RABBITS

Changes in local pulmonary capillary blood volume (Vc) and oxygen satu ration (S) have been difficult to measure in Live animals. By utilizin g the differences in absorption of light at two wavelengths (650 and 8 00 nn), we estimated the fractional change in Vc and S during the cour se of the cardiac cycle in eight anesthetized, ventilated rabbits at l ow and high lung volumes. Observations were made of the pattern of dif fusely backscattered light, from an similar to 1-cm(3) volume of lung illuminated with a point source placed on the pleural surface through a thoracotomy. At low lung volume, the fractional change in Vc was sim ilar to 13%, the change in S was similar to 4.6%, and the mean S was c lose to 77%. The fluctuations in Vc and S lagged behind peak systemic blood pressure by about one-fifth and three-fifths of a cycle, respect ively. At high lung volume, there were no important fluctuations in Vc or S, and the mean S was similar to 82%. These results are consistent with fluctuations in pulmonary capillary pressure and gas exchange ov er the cardiac cycle, and with decreasing capillary compliance with in creasing lung volume.