Objective: We have investigated the role that improvement in arterial oxygenation has, consequent on positive end-expiratory pressure (PEEP), in the reduction of cardiac index (CI) determined by applying PEEP.
Design: 2 x 2 factorial trial.
Setting: Department of intensive care medicine at a university hospital.
Patients: 13 patients on mechanical ventilation for acute lung injury.
Interventions: Four experimental conditions, each one characterized by one level of PEEP and one level of PaO2: LOLP = Low PaO2 (approximately 50 mmHg) Low PEEP (approximately 1 cmH2O) LOHP = Low PaO2 (approximately 50 mm Hg) High PEEP (approximately 10 cmH2O) HOLP = High PaO2 (approximately 80 mmHg) Low PEEP (approximately 1 cmH2O) HOHP = High PaO2 (approximately 80 mmHg) High PEEP (approximately 10 cmH2O) MEASUREMENTS AND RESULTS: Hemodynamic and gas exchange data were collected for each experimental condition. CI showed a 13% decline from LOLP (7.0 +/- 1.71/min per m2) to HOHP (6.1 +/- 1.31/min per m2). Both the direct effect of PEEP on the CI (LOLP + HOLP vs LOHP + HOHP, p < 0.01) and the indirect effect related to the improvement in oxygenation (LOLP + LOHP vs HOLP + HOHP, p < 0.01) contributed to the reduction in CI.
Conclusions: In evaluating CI changes induced by PEEP we should take into account the indirect effect of arterial oxygenation upon CI. This should be considered, at least in part, as a physiological adjustment rather than as impaired cardiovascular performance.