Pulmonary blood flow distribution in standing horses is not dominated by gravity

Abstract

Recent studies using microspheres in dogs, pigs and goats have demonstrated considerable heterogeneity of pulmonary perfusion within isogravitational planes. These studies demonstrate a minimal role of gravity in determining pulmonary blood flow distribution. To test whether a gravitational gradient would be more apparent in an animal with large vertical lung height, we measured perfusion heterogeneity in horses (vertical lung height = approximately 55 cm). Four unanesthetized Thoroughbred geldings (422-500 kg) were studied awake in the standing position with fluorescent microspheres injected into a central vein. Between 1,621 and 2,503 pieces (1.3 cm3 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 heterogeneity was seen in each isogravitational plane. The relationship between blood flow and vertical height up the lung was characterized by the slope and correlation coefficient of a least squares regression analysis. The slopes within each horse ranged from -0.052 to +0.021 relative flow units/cm height up the lung, and the correlation coefficients varied from 0.12 to 0.75. A positive slope, indicating that flow increased with vertical distance up the lung (opposite to gravity), was observed in three of the four horses. In addition, blood flow was uniformly 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 gradient to pulmonary blood flow and there is a considerable degree of perfusion heterogeneity, indicating that gravity alone does not play the major role in determining blood flow distribution.

Intrapulmonary arteriovenous shunts of >15 μm in diameter probably do not contribute to arterial hypoxemia in maximally exercising Thoroughbred horses

“…Our selection of 15-m-diameter microspheres for the present study was influenced by previous reports examining the distribution of pulmonary blood flow in resting and exercising horses with this technique (2,6,21). However, in these reports, intrapulmonary arteriovenous shunting of 15-m-diameter microspheres was not examined.…”

“…However, in these reports, intrapulmonary arteriovenous shunting of 15-m-diameter microspheres was not examined. Although, in these experiments, cardiac (2,6,21) and other tissues may have been removed at necropsy, investigators did not report on the absence/presence of intravenously injected microspheres in the heart and/or other tissues.…”

Prone positioning acute respiratory distress syndrome patients

“…The latter mechanism was considered as true as intuitively it was thought that the change in gravity direction will affect the lung perfusion in the same way, i.e., less perfusion towards dorsal lung regions, now non-dependent, in the prone position. Several experiments found that the dorsal lung regions when in the prone position still had the highest amount of blood flowing through them (2)(3)(4)(5)(6)(7)(8). Therefore, this unexpected finding argued against the second mechanism to explain the reduction in intra-pulmonary shunt.…”

Gas Exchange in the Normal Lung : Experimental studies on the effects of positive end-expiratory pressure and body position

“…With similar techniques yielding greater spatial resolution Glenny et al (Glenny et al, 1991) were able to quantify the contribution of gravitational heterogeneity to overall perfusion heterogeneity and concluded that gravity was a minor factor in the distribution of Q. Hlastala et al (Hlastala et al, 1996) using the same technique found that differences in flow at the same gravitational level could be 10 times greater than the difference in flow at different levels. Pulmonary perfusion has been studied in both humans and animals under microgravity conditions induced by parabolic flight.…”