Prior clinical and animal studies have shown a markedly different relationship between left atrial pressure and the systolic fraction of pulmonary venous flow but have not discussed possible reasons for this discrepancy. To examine the possibility that these disparate results are due to differences in baseline mitral and pulmonary venous flow velocities, we recorded both velocities with left atrial and left ventricular pressure under different loading conditions in eight lightly sedated normal dogs. With constant atrial pacing at 85 beats/min, mean left atrial pressure was increased from 5.3 +/- 1.1 mm Hg at baseline to 16.1 +/- 1.7 mm Hg with volume and methoxamine infusion (p < 0.05). As left atrial pressure increased, the operating compliance of the left atrium decreased, whereas left atrial volumes and ejection fraction increased. Baseline pulmonary venous diastolic flow velocity was larger than systolic velocity (66 +/- 9 versus 36 +/- 11 cm/sec), with the systolic fraction of pulmonary venous flow 31% +/- 8%. With increasing left atrial pressure, pulmonary venous diastolic velocity did not change, but peak systolic velocity (57 +/- 16 cm/sec) and the systolic fraction (48% +/- 9%) both increased (p < 0.05). Changes in pulmonary venous diastolic flow velocity closely followed changes in early diastolic mitral flow velocity (r = 0.85, p < 0.05). Mean left atrial pressure, or change in mean left atrial pressure, was related to the ratio of pulmonary venous systolic to diastolic velocity time integral (r = 0.59 to 0.62, p < 0.01) and the pulmonary venous systolic fraction (r = 0.58 to 0.60; p < 0.01). When expressed as change from baseline, these variables showed even stronger correlations with left atrial pressure (r = 0.72 to 0.76, p < 0.001). These results are consistent with previous animal and clinical results that indicate pulmonary venous diastolic flow is closely related to early mitral flow velocity, whereas systolic flow is determined primarily by left atrial systolic function. The markedly different relationships observed between left atrial pressure and pulmonary venous systolic flow in animal and clinical studies are most likely due to different baseline flow velocity patterns and differences in left atrial systolic reserve. Future studies investigating these relationships should include data on mitral and pulmonary venous flow velocities as well as left atrial size and systolic function.