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 respi ratory distress syndrome patients remains debatable. We used two mechanical approaches, calculation of the compliance of the respiratory system and de termination of the lower inflexion point of the pressure-volume curve of th e respiratory system, to identify specific PEEPs (PEEP, and PEEPA) whose im pact on right ventricular (RV) outflow was compared with Doppler analysis o f 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 distre ss syndrome patients. Interventions: Two PEEPS were determined: PEEPS was the highest PEEP associ ated with the highest value of respiratory compliance, and PEEPS was the co ordinate of the lower inflexion point of the inspiratory pressure-volume cu rve on the pressure axis plus 2 cm H2O. Measurements and Main Results: We observed a large difference between the t wo 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 pr essure (ZEEP) and after application of these two PEEPs were assessed by Dop pler study of pulmonary artery flow velocity obtained by a transesophageal approach, with particular reference to the end-expiratory and end-inspirato ry pulmonary artery velocity-time integral, as reflecting RV stroke output, and mean acceleration as reflecting RV outflow impedance during an unchang ed flow period. A significant inspiratory reduction in pulmonary artery vel ocity-time integral (from 11.8 + 0.3 to 10.0 + 0.3 cm) and mean acceleratio n (from 11.9 + 0.9 to 8.0 + 0.9 m/sec(2)) was observed with ZEEP, showing a reduction in RV stroke index (from 29.0 + 0.9 to 26.0 + 0.6 cm(3)/m(2)) by a sudden increase in outflow impedance during tidal ventilation. Applicati on of PEEP,, which improved Pao(2) (102 + 40 vs. 65 + 18 torr with ZEEP), w orsened the inspiratory drop in RV stroke index (21.6 + 0.8 cm(3)/m(2), res ulting 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 im proved Pao(2) (81 + 21 torr), was associated with a lesser impact on RV out flow impedance (inspiratory mean acceleration: 9.5 + 1 m/sec(2)) and cardia c index (3.2 + 1.0) than PEEPS. Conclusion: RV outflow impedance evaluated by the Doppler technique appeare d sensitive to PEEP titration. Application of PEEPA worsened RV systolic fu nction impairment produced by tidal ventilation. Conversely, application of PEEPS reduced RV systolic function impairment, suggesting an association w ith a lower pulmonary vascular resistance.