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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 ventilation (IPPV) with large tidal volumes and addition of positive end-expiratory pressure (PEEP) on maldistribution of ventilation in anesthetized horses positioned in lateral recumbency.


ANIMALS
6 healthy adult horses.


PROCEDURE
Anesthesia was induced by i.v. infusion of thiopental sodium and guiafenesin and was maintained with supplemental doses of thiopental and i.v. infusion of chloral hydrate. Functional separation of the lungs was achieved, using a tube-in-tube intubation technique. Intermittent positive-pressure ventilation of both lungs with air was done by use of an anesthetic circle system and a ventilator. Data were collected during spontaneous respiration and during IPPV, using increasing tidal volumes with and without PEEP of 10 and 20 cm of H2O.


RESULTS
Uneven distribution of inspired gas between the lungs that existed during spontaneous respiration was not altered by IPPV and large tidal volumes. Addition of PEEP caused a significant and reversible shift of inspired gas to the dependent lung and preferentially increased functional residual capacity of the nondependent lung. This was accompanied by significant increase in PaO2. With IPPV, the combined effects of PEEP and large tidal volume caused an increase of the fractional distribution of inspired gas to the dependent lung from 34% to 50%, accompanied by an increase in PaO2 and alveolar dead space of both lungs.


CONCLUSIONS AND CLINICAL RELEVANCE
Use of PEEP during IPPV changes distribution of inspired gas. Increased in PaO2 can be attributed to improved ventilation-perfusion, especially in the dependent lung, in which previously collapsed lung units might have been reopened and participated again in gas exchange after redistribution of inspired gas. The most pronounced effects of IPPV and PEEP were associated with high airway pressures, which are likely to offset the beneficial effects of the increase of PaO2 on total oxygen availability to the tissues because of the expected negative effects on cardiac output.