Positive-pressure ventilation has often been advocated to increase oxygen delivery. This ventilation mode itself, however, can impair right ventricular ejection and, thus, diminish cardiac output. In this study, alterations of right ventricular outflow impedance were evaluated after stepwise increases of positive end-expiratory pressure (PEEP). Different pulmonary artery flow characteristics were evaluated with transesophageal echocardiography in mechanically ventilated postoperative coronary artery bypass surgery patients without pulmonary hypertension. A progressive decrease of pulmonary artery flow velocity and time velocity integrals was found with increasing PEEP levels. No changes in acceleration time or pre-ejection period were observed. In order to decrease the influence of heart rate, the ratios of the different pulmonary artery flow characteristics were calculated. At end-inspiration, both the ratio of acceleration time to right ventricular ejection period and the ratio of pre-ejection period to right ventricular ejection period showed progressive increases above 10 cmH2O positive end-expiratory pressure (13.3% at the level of 15 cmH2O and 8.5% at the level of 20 cmH2O). In this study, acceleration time appears not to be of importance in ventilated patients. These data strongly support the hypothesis that intermittent squeezing of the pulmonary arterial tree during inspiration, rather than positive end-expiratory pressure, creates an increase of right ventricular outflow impedance.