Effects of positive end-expiratory pressure titration on gas exchange, respiratory mechanics and hemodynamics in anesthetized horses.

OBJECTIVE: To assess if positive end-expiratory pressure (PEEP) titration improves gas exchange and respiratory mechanics, without hemodynamic impairment in horses during anesthesia. DESIGN: Prospective, randomized study. ANIMALS: Thirteen isoflurane-anesthetized healthy horses. METHODS: After 60 minutes of anesthesia with spontaneous breathing, mechanical ventilation was initiated with an inspiratory-expiratory ratio of 1:2, PEEP of 5 cmH2O, tidal volume of 10-20 mL kg(-1) and respiratory rate adjusted to maintain normocapnia. Constant PEEP of 5 cmH2O was continued (control group; n = 6) or titrated (PEEP group; n = 7) by increasing and decreasing PEEP from 5 to 20 cmH2O at 15-minute intervals. The horses were instrumented with an arterial catheter to measure blood pressure and allow collection of blood for pH and blood gas analysis and a Swan-Ganz catheter for measurement of cardiac output (CO) using thermodilution. Cardiopulmonary assessment was recorded before PEEP titration and after 15 minutes at each PEEP value. RESULTS: In the PEEP group, static compliance (range) (Cst 278-463 mL cmH2O(-1)) was significantly higher and the shunt fraction (Q.s/Q.t 7-20%) and the alveolar-arterial oxygen difference [P(A-a)O2 95-325 mmHg] were significantly lower than in the control group [Cst of 246-290 mL cmH2O(-1), Q.s/Q.t of 16-19%, P(A-a)O2 of 253-310 mmHg; p < 0.05]. CO (mean +/- SEM) was lower in the PEEP group (23 +/- 2 L minute(-1)) at 20 cmH2O PEEP than in the control group (26 +/- 4 L minute(-1), p < 0.05), with no significant changes in heart rate, blood pressure or central venous pressure. CONCLUSIONS: PEEP titration significantly improved gas exchange and lung compliance, with a small decrease in CO at the highest PEEP level. CLINICAL RELEVANCE: Gas exchange and respiratory mechanics impairment during inhalation anesthesia can be treated using PEEP titration from 5 to 20 cmH2O, without clinically important hemodynamic effects in healthy horses