Relief of hypoxamia 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 × 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 (∼ 50 mmHg) Low PEEP (∼ 1 cmH₂O) LOHP = Low PaO₂ (∼ 50 mmHg) High PEEP (∼ 10 cmH₂O) HOLP = High PaO₂ (∼ 80 mmHg) Low PEEP (∼ 1 cmH₂O) HOHP = High PaO₂ (∼ 80 mmHg) High PEEP (∼ 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.