Patients with chronic obstructive pulmonary disease (COPD) are difficult to assess by conventional transthoracic echocardiography (TTE) because of emphysematous lungs or mediastinal deviation. We hypothesized that superior vena cava (SVC) flow is related to pulmonary circulation and may be useful for the detection of pulmonary hypertension (PH) in patients with COPD that cannot been assessed by direct evaluation using the tricuspid regurgitant Doppler velocity. SVC Doppler flow velocities were examined in 46 patients with COPD and the pressure gradient between the right ventricular and right atrial pressure (RV-RADeltaP) was calculated by tricuspid regurgitant Doppler velocities. The patients were divided into 2 groups: 11 patients with PH (RV-RADeltaP > 25 mm Hg) were compared with 35 without PH. There was no significant difference in the maximal SVC peak systolic forward flow velocity during inspiration (INS) between these 2 groups. However, the minimal SVC peak systolic forward flow velocity during expiration (EXP) in the group with PH was significantly higher than that in the group without PH (37.4 +/- 20.0 cm/s vs 26.4 +/- 8.5 cm/s, P =.01). Linear regression analysis revealed a significant correlation between RV-RADeltaP and the EXP/INS ratio (r = 0.61, P <.001). In COPD patients with PH, the increased expiratory SVC systolic flow supplemented the preload for the impaired right ventricular filling flow caused by PH, thereby maintaining the transtricuspid driving pressure. Our observation suggests that respiratory variation in SVC systolic forward flow may be a sensitive Doppler flow index for evaluating severity of PH in patients with COPD that cannot been assessed by conventional TTE.