Respiratory variation of > or = 25% in mitral E velocity is a characteristic Doppler echocardiographic feature in constrictive pericarditis. However, a subset of patients with constriction do not exhibit the typical respiratory change, most likely because of marked increase in the left atrial pressure, and preload reduction may unmask the respiratory variation. In 12 patients with surgically confirmed constrictive pericarditis who had < 25% respiratory variation in mitral E velocity during an initial preoperative examination, the Doppler study was repeated after an attempt to decrease left ventricular filling pressure. At baseline, mean mitral E velocity was similar after inspiration and expiration (0.81 +/- 0.24 and 0.84 +/- 0.21 m/s, respectively). On repeat Doppler examination, with the patient in a head-up tilt or sitting position the decrease in mitral E velocity with inspiration (0.61 +/- 0.13 m/s) was significant (P < .004), whereas it did not change significantly with expiration. The mean percent respiratory change in E velocity was 5 +/- 7% at baseline and 32 +/- 28% with preload reduction. Eight (75%) of the 12 patients developed respiratory variation of > or = 25%. When the respiratory variation in Doppler mitral E velocity is blunted or absent during the evaluation of suspected constrictive pericarditis, repeat Doppler recording of mitral flow velocities after maneuvers to decrease preload is recommended to unmask the characteristic respiratory variation in mitral E velocity.