Positive end-expiratory pressure titration in acute respiratory distress syndrome patients: impact on right ventricular outflow impedance evaluated by pulmonary artery Doppler flow velocity measurements.

OBJECTIVE: Positive end-expiratory pressure (PEEP) titration in acute respiratory distress syndrome patients remains debatable. We used two mechanical approaches, calculation of the compliance of the respiratory system and determination of the lower inflexion point of the pressure-volume curve of the respiratory system, to identify specific PEEPs (PEEPS and PEEPA) whose impact on right ventricular (RV) outflow was compared with Doppler analysis of pulmonary artery flow velocity.
DESIGN: Prospective, open, clinical study.
SETTING: Medical intensive care unit of a university hospital. PATIENTS: Sixteen consecutive ventilator-dependent acute respiratory distress syndrome patients.
INTERVENTIONS: Two PEEPs were determined: PEEPS was the highest PEEP associated with the highest value of respiratory compliance, and PEEPA was the coordinate of the lower inflexion point of the inspiratory pressure-volume curve on the pressure axis plus 2 cm H2O.
MEASUREMENTS AND MAIN RESULTS: We observed a large difference between the two PEEPs, with PEEPA (13 + 4 cm H2O) > PEEPS (6 + 3 cm H2O). Changes in RV outflow impedance produced by tidal ventilation with zero end-expiratory pressure (ZEEP) and after application of these two PEEPs were assessed by Doppler study of pulmonary artery flow velocity obtained by a transesophageal approach, with particular reference to the end-expiratory and end-inspiratory pulmonary artery velocity-time integral, as reflecting RV stroke output, and mean acceleration as reflecting RV outflow impedance during an unchanged flow period. A significant inspiratory reduction in pulmonary artery velocity-time integral (from 11.8 + 0.3 to 10.0 + 0.3 cm) and mean acceleration (from 11.9 + 0.9 to 8.0 + 0.9 m/sec2) was observed with ZEEP, showing a reduction in RV stroke index (from 29.0 + 0.9 to 26.0 + 0.6 cm3/m2) by a sudden increase in outflow impedance during tidal ventilation. Application of PEEPA, which improved Pao2 (102 + 40 vs. 65 + 18 torr with ZEEP), worsened the inspiratory drop in RV stroke index (21.6 + 0.8 cm3/m2), resulting in a significant reduction in cardiac index compared with ZEEP (from 3.0 + 1.0 to 2.7 + 1.1). Application of PEEPS, which also significantly improved Pao2 (81 + 21 torr), was associated with a lesser impact on RV outflow impedance (inspiratory mean acceleration: 9.5 + 1 m/sec2) and cardiac index (3.2 + 1.0) than PEEPA.
CONCLUSION: RV outflow impedance evaluated by the Doppler technique appeared sensitive to PEEP titration. Application of PEEPA worsened RV systolic function impairment produced by tidal ventilation. Conversely, application of PEEPS reduced RV systolic function impairment, suggesting an association with a lower pulmonary vascular resistance.