Does spontaneous ventilation with IMV protect from PEEP-induced cardiac output depression?

Controlled mechanical ventilation (CMV) with positive end-expiratory pressure (PEEP) has been shown to be effective in improving oxygenation and diminishing intrapulmonary shunt in patients with severe pulmonary failure [l, 10, 161. However, cardiac output depression may occur at levels of PEEP above 10 cm Hz0 and limit its usefulness [lo, 141. Civetta et al. [2] and Kirby et al. [8] have successfully administered PEEP in excess of 30 cm Hz0 to spontaneously breathing patients using continuous positive airway pressure (CPAP) and intermittent mandatory ventilation (IMV). Kirby et al. [9] were able to maintain the cardiac output in monkeys ventilated with IMV and 34 cm H,O PEEP by massive fluid infusion. Cardiac output depression can be avoided when large volumes of fluid are administered to baboons on CMV at 20 cm H,O PEEP [ 181. In order to determine whether CPAP and IMV respiration offer significant protection against PEEP-induced circulatory impairment when compared to controlled ventilation, each mode of administering PEEP was separately studied in the same group of baboons without volume expansion.