A density method to quantify pulmonary microvascular hematocrit.

We perfused the left lower lobe of a dog with autologous blood having a hematocrit Ha. When the vascular pressure perfusing the lobe was elevated, we observed a transient increase in the density of venous blood. Converting the density increase to a rise in hematocrit, we could calculate a volume (Vr) of red blood cells (RBC) over their normal outflow that was released by the lobe as a result of the elevation. We measured the weight gain of the lobe to determine the increase in pulmonary vascular volume, V' - V. We found that the ratio, Vr/Ha/(V' - V), is 0.11 +/- 0.02. To determine the implication of this ratio, we divided the lobular vasculature into an arterial, microvascular, and venous compartment. Due to the Fahraeus effect, the tube hematocrit in the microvascular compartment (Hc) is lower than that of two macrovascular compartments, Ha. An analysis on the balance of RBC and plasma flows through the lobe identified the volume Vr as (Vc' - Vc) (Ha - Hc) with Vc' - Vc being the volumetric increase of the lobular microvascular compartment. Based on the reported volumetric fractional change of microvascular compartment, we estimated that the microvascular (tube) hematocrit in pulmonary capillaries is 80% (ranging from 78 to 82%) of the hematocrit perfusing the lobe. Since the additional RBC volume (Vr) being released from the lobe cannot be accounted for by transcapillary filtration or capillary recruitment, we conclude from this analysis that the measurement of the transient density change in pulmonary outflow can be used to quantify the microvascular hematocrit of the lung.