INFLUENCE OF TIDAL VOLUME AND POSITIVE END-EXPIRATORY PRESSURE ON INSPIRATORY GAS-DISTRIBUTION AND GAS-EXCHANGE DURING MECHANICAL VENTILATION IN HORSES POSITIONED IN LATERAL RECUMBENCY

Objective-To study effects of intermittent positive-pressure ventilati on (IPPV) with large tidal volumes and addition of positive end-expira tory pressure (PEEP) on maldistribution of ventilation in anesthetized horses positioned in lateral recumbency. Animals-6 healthy adult hors es. Procedure-Anesthesia was induced by IV infusion of thiopental sodi um and guiafenesin and was maintained with supplemental doses of thiop ental and IV infusion of chloral hydrate. Functional separation of the lungs was achieved, using a tube-in-tube intubation technique. Interm ittent positive-pressure ventilation of both lungs with air was done b y use of an anesthetic circle system and a ventilator. Data were colle cted during spontaneous respiration and during IPPV, using increasing tidal volumes with and without PEEP of 10 and 20 cm of H2O. Results-Un even distribution of inspired gas between the lungs that existed durin g spontaneous respiration was not altered by IPPV and large tidal volu mes. Addition of PEEP caused a significant and reversible shift of ins pired gas to the dependent lung and preferentially increased functiona l residual capacity of the nondependent lung. This was accompanied by significant increase in Pac,. With IPPV, the combined ef fects of PEEP and large tidal volume caused an increase of the fractional distribut ion of inspired gas to the dependent lung from 34% to 50%, accompanied by an increase in Pa-O2 and alveolar dead space of both lungs. Conclu sions and Clinical Relevance-Use of PEEP during IPPV changes distribut ion of inspired gas. increased in Pao, can be attributed to improved v entilation-perfusion, especially in the dependent lung, in which previ ously collapsed lung units might have been reopened and participated a gain in gas exchange after redistribution of inspired gas. The most pr onounced effects of IPPV and PEEP were associated with high airway pre ssures, which are likely to offset the beneficial effects of the incre ase of Pac, on total oxygen availability to the tissues because of the expected negative effects on cardiac output.