Gravity is an important but secondary determinant of regional pulmonary blood flow in upright primates

Original studies leading to the gravitational model of pulmonary blood flow and contemporary studies showing gravity-independent perfusion differ in t he recent use of laboratory animals instead of humans. We explored the dist ribution of pulmonary blood flow in baboons because their anatomy, serial d istribution of vascular resistances, and hemodynamic responses to hypoxia a re similar to those of humans. Four baboons were anesthetized with ketamine , intubated, and mechanically ventilated. Different colors of fluorescent m icrospheres were given intravenously while the animals were in the supine, prone, upright (repeated), and head-down (repeated) postures. The animals w ere killed, and their lungs were excised, dried, and diced into similar to 2-cm(3) pieces with the spatial coordinates recorded for each piece. Region al blood flow was determined for each posture from the fluorescent signals of each piece. Perfusion heterogeneity was greatest in the upright posture and least when prone. Using multiple-stepwise regression, we estimate that 7, 5, and 25% of perfusion heterogeneity is due to gravity in the supine, p rone, and upright postures, respectively. Although important, gravity is no t the predominant determinant of pulmonary perfusion heterogeneity in uprig ht primates. Because of anatomic similarities, the same may be true for hum ans.