PULMONARY BLOOD-FLOW DISTRIBUTION IN SHEEP - EFFECTS OF ANESTHESIA, MECHANICAL VENTILATION, AND CHANGE IN POSTURE

Background: Recent studies providing high-resolution images of pulmona ry perfusion have questioned the classical zone model of pulmonary per fusion. Hence the present work was undertaken to provide detailed maps of regional pulmonary perfusion to examine the influence of anesthesi a, mechanical ventilation, and posture, Methods: Pulmonary perfusion w as analyzed with intravenous fluorescent microspheres (15 mu m) in six sheep studied in four conditions: prone and awake, prone with pentoba rbital-anesthesia and breathing spontaneously, prone with anesthesia a nd mechanical ventilation, and supine with anesthesia and mechanical v entilation. Lungs were air dried at total lung capacity and sectioned into approximately 1,100 pieces (about 2 cm(3)) per animal, The pieces were weighed and assigned spatial coordinates. Fluorescence was read on a spectrophotometer, and signals were corrected for piece weight an d normalized to mean flow. Pulmonary blood flow heterogeneity was asse ssed using the coefficient of variation of flow data. Results: Pentoba rbital anesthesia and mechanical ventilation did not influence perfusi on heterogeneity, but heterogeneity increased when the animals were in the supine posture (P < 0.01), Gravitational flow gradients were abse nt in the prone position but present in the supine (P < 0.001 compared with zero). Pulmonary perfusion was distributed with a hilar-to-perip heral gradient in animals breathing spontaneously (P < 0.05). Conclusi ons: The influence of pentobarbital anesthesia and mechanical ventilat ion on pulmonary perfusion heterogeneity is small compared with the ef fect of changes in posture, Analysis of flow gradients indicate that g ravity plays a small role in determining pulmonary blood flow distribu tion.