DIFFERENTIAL ARTIFICIAL-VENTILATION IN ANESTHETIZED HORSES POSITIONEDIN LATERAL RECUMBENCY

Effects of differential ventilation on gas exchange were studied in 7 isoflurane-anesthetized, laterally recumbent horses, and were compared with effects of conventional ventilation using similar minute volume. A tracheal tube-in-tube intubation technique allowed each lung to be connected separately to an anesthetic circle system with a ventilator. Two distribution patterns of tidal volume were investigated; half the tidal volume was distributed to each lung and two-thirds the tidal vo lume was distributed to the dependent lung. Effects of the combination of these patterns with positive end-expiratory pressure (PEEP) of 10 and 20 cm of H2O to the dependent lung were investigated. Differential ventilation maintained Paco(2), but significantly increased Pao(2), f rom 180 to 270 mm of Hg (+44%) and decreased shunt perfusion from 22 t o 19% (-15%), regardless of the distribution pattern used. Mean airway pressure was lower than the value detected during conventional ventil ation. The combination of differential ventilation with selective PEEP was followed by a decrease in Paco(2) and further increase of Pao(2) and decrease of shunt, which were similar for bath distribution patter ns. Effects of PEEP of 29 cm of H2O were more pronounced than those of PEEP of 10 cm of H2O Owing to the combined effects of differential ve ntilation and selective PEEP, Pao(2) increased to 399 mm of Hg and shu nt decreased to 15%. This represents increase of 112% and decrease of 33% respectively, compared with values for conventional ventilation. M ean airway pressure increased maximally to 23 cm of H2O, which was 11 cm of H2O greater than the value for conventional ventilation. During differential ventilation, alveolar dead space in the dependent lung be came greater than that in the nondependent lung and maximum was 39%. T here were no significant changes in arterial blood pressure. Beneficia l effects on gas exchange can be explained by improved matching of ven tilation and perfusion, possibly attributable to reopening of previous ly closed units in the dependent lung.