Objectives: To examine the effect of positive end-expiratory pressure (PEEP) on right ventricular stroke volume variation (SW), with possible implications for the number and timing of pulmonary artery catheter thermodilution measurements. Design: Prospective, clinical pilot study. Setting: Academic medical center. Participants: Patients who underwent volume-controlled mechanical ventilation and had a pulmonary artery catheter. Intervention: PEEP was increased from 5-to-10 cmH(2)O and from 10-to-15 cmH(2)O with 10-minute intervals, with similar decreases in PEEP, from 15-to-10 cmH(2)O and 10-to-5 cmH(2)O. Measurements and Main Results: In 15 patients, right ventricular parameters were measured using thermodilution at 10% intervals of the ventilatory cycle at each PEEP level with a rapid-response thermistor. Mean right ventricular stroke volume and end-diastolic volume declined during incremental PEEP and normalized on return to 5 cmH(2)O PEEP (p = 0.01 and p = 0.001, respectively). Right ventricular SW remained unaltered by changes in PEEP (p = 0.26), regardless of incremental PEEP (p = 0.15) or decreased PEEP (p = 0.12). The coefficients of variation in the ventilatory cycle of all other thermodilution-derived right ventricular parameters also were unaffected by changes in PEEP. Conclusions: This study showed that increases in PEEP did not affect right ventricular SW in critically ill patients undergoing mechanical ventilation despite reductions in mean right ventricular stroke volume and end-diastolic volume. This could be explained by cyclic counteracting changes in right ventricular preloading and afterloading during the ventilatory cycle, independent of PEEP. Changes in PEEP did not affect the number and timing of pulmonary artery catheter thermodilution measurements. (C) 2016 Elsevier Inc. All rights reserved.