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 cmH(2)O, tidal volume of 10-20 mL kg(-1) and respiratory rate adjusted to maintain normo-capnia. Constant PEEP of 5 cmH(2)O was continued (control group; n - 6) or titrated (PEEP group; n = 7) by increasing and decreasing PEEP from 5 to 20 cmH(2)O 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 cmH(2)O(-1)) was significantly higher and the shunt fraction ((Q) over dot(s)/(Q) over dot(t) 7-20%) and the alveolar-arterial oxygen difference [P(A-a) O-2 95-325 mmHg] were significantly lower than in the control group [Cst of 246-290 mL cmH(2)O (1), (Q) over dot(s)/(Q) over dot(t) of 16-19%, P(A-a)O-2 of 253-310 mmHg; p < 0.05]. CO (mean +/- SEM) was lower in the PEEP group (23 +/- 2 L minute(-1)) at 20 cmH(2)O 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.