Elevated Positive End-Expiratory Pressure Decreases Cardiac Index in a Rhesus Monkey Model

Rationale: Clinicians are often concerned that higher positive end-expiratory pressure (PEEP) will decrease cardiac index (CI). PEEP affects CI through multiple inter-related mechanisms. The adult Rhesus monkey is an excellent model to study cardiopulmonary interaction due to similar pulmonary and chest wall compliances to human infants. Objective: Our goal was to examine the impact of increasing PEEP on CI in Rhesus monkeys as a model for critically ill children. Methods: Prospective, experimental animal study. Nine healthy anesthetized, intubated Rhesus monkeys were allowed to breathe spontaneously at a PEEP of 0, 5, 10, and 15 cm H(2)O while CI was measured with an ultrasonic Doppler (USCOM). Measurements and main results: Cardiac index decreased between PEEP levels of 5 and 15 cm H(2)O. The mean decrease in CI for the entire cohort of monkeys was 18% (p < 0.01) with a range of −11 to 49%. Stroke volume and oxygen delivery also decreased between PEEP levels of 5 and 15 cm H(2)O (p < 0.01). Conclusion: Between PEEP levels of 5 and 15 cm H(2)O, there was a decrease in CI, stroke volume, and oxygen delivery in intubated Rhesus monkeys. A plausible mechanism is that over-distention of normally compliant lungs at increased PEEP resulted in decreased preload to the right ventricle, outweighing the potentially beneficial decrease in left ventricular afterload or pulmonary vascular resistance. Further investigation is warranted, particularly in children with lung injury, who have historically benefited from increased PEEP levels without over-distention.