PULMONARY BLOOD-FLOW DISTRIBUTION IN STANDING HORSES IS NOT DOMINATEDBY GRAVITY

Recent studies using microspheres in dogs, pigs and goats have demonst rated considerable heterogeneity of pulmonary perfusion within isograv itational planes. These studies demonstrate a minimal role of gravity in determining pulmonary blood flow distribution. To test whether a gr avitational gradient would be more apparent in an animal with large ve rtical lung height, we measured perfusion heterogeneity in horses (ver tical lung height = similar to 55 cm). Four unanesthetized Thoroughbre d geldings (422-500 kg) were studied awake in the standing position wi th fluorescent microspheres injected into a central vein. Between 1,62 1 and 2,503 pieces (1.3 cm(3) in volume) were obtained from the lungs of each horse with spatial coordinates, and blood flow was determined for each piece. The coefficient of variation of blood flow throughout the lungs ranged between 22 and 57% among the horses. Considerable het erogeneity was seen in each isogravitational plane. The relationship b etween blood flow and vertical height up the lung was characterized by the slope and correlation coefficient of a least squares regression a nalysis. The slopes within each horse ranged from -0.052 to +0.021 rel ative flow units/cm height up the lung, and the correlation coefficien ts varied from 0.12 to 0.75. A positive slope, indicating that flow in creased with vertical distance up the lung (opposite to gravity), was observed in three of the four horses. In addition, blood flow was unif ormly low in three of the four horses in the most cranial portions of the lungs. We conclude that in lungs of resting unanesthetized horses, animals with a large lung height, there is no consistent vertical gra dient to pulmonary blood flow and there is a considerable degree of pe rfusion heterogeneity, indicating that gravity alone does not play the major role in determining blood flow distribution.