The effects of increased blood viscosity on pulmonary vascular resistance.

The isolated left lower lobes of 15 dogs' lungs were perfused by means of a roller pump with blood at hematocrit values ranging from 31 to 80 per cent. Pressure-flow curves were constructed at blood flow rates from one half to three times the normal flow for the left lower lobe at each hematocrit level. The perfusion pressure was normalized with reference to the normal hematocrit(38 to 48 per cent) and normal blood flow for the left lower lobe (20 ml. per kilogram per minute). From these normalized pressure-flow curves, normalized resistance-flow curves were constructed at different mean hematocrit levels. Regression lines were drawn relating normalized pulmonary vascular resistance to hematocrit at different rates of pulmonary blood flow which might be found in patients with congenital heart disease. It was found that pulmonary vascular resistance rose in an exponential fashion as the hematocrit was increased, and that the blood viscosity determined both the shape of the resistance-flow curve and magnitude of the increase in resistance to pulmonary blood flow, especially when the pulmonary blood flow was less than normal and the hematocrit was greater than 54 per cent. The family of regression lines relating pulmonary vascular resistance to hematocrit at different flow rates may be used clinically in patients with congenital heart disease and polycythemia to determine if an elevated pulmonary vascular resistance is due to increased blood viscosity or obstructive pulmonary vascular disease. It is concluded that an increased blood viscosity due to polycythemia significantly alters the pulmonary hemodynamics of patients with congenital heart disease with either increased or decreased pulmonary blood flow. Increased blood viscosity may play an important part in the early initiation and development of pulmonary arteriosclerosis in patients with transposition of the great arteries.