Relief of hypoxemia contributes to a reduction in cardiac index related to the use of positive end-expiratory pressure.

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.