Pulmonary artery pressure responses to increased cardiac output in chickens with raised metabolic rate

Previous work has shown remarkable differences in the pressure–flow relations of the pulmonary circulation between birds and mammals. For example several studies suggest that the avian pulmonary blood vessels behave like rigid tubes, very different from the situation in mammalian lung. We therefore speculated that birds would develop high pulmonary artery pressures when the cardiac output was substantially increased during heavy exercise, for example during flight. However because of the technical difficulties of measuring pulmonary artery pressures in flight, the metabolic rate and cardiac output in anesthetized chickens were increased by infusing 2,4 Dinitrophenol (DNP) and the mean pressure was measured by means of a catheter in the pulmonary artery. Although the pulmonary artery pressure rose steadily as cardiac output increased, it remained below the high levels predicted from the previous studies for similar changes in pulmonary blood flow. Furthermore the increase in pressure was less than in mammals where recruitment and distension of pulmonary capillaries are known to occur. The reasons for this unexpected result are not clear.