Velocity distributions in the left ventricular outflow tract in patients with valvular aortic stenosis. Effect on the measurement of aortic valve area by using the continuity equation.

Abstract

The cross-sectional velocity distribution in the left ventricular outflow tract was studied in 40 patients with valvular aortic stenosis. Doppler colour flow mapping and a time-interpolation method were used to construct the cross-sectional velocity and time-velocity integral (TVI) profiles at different levels. By using pulsed Doppler, the subaortic flow velocity was sampled from the anterior, middle and posterior regions along the diameter of the left ventricular outflow tract (at 0.5 to 1.0 cm proximal to the aortic anulus) in the apical long axis view. Thus, for each patient, three aortic valve areas were calculated by using the continuity equation. Each patient was assigned to one of three subgroups according to the left ventricular ejection fraction (EF): subgroup I with EF < or = 25% (n = 10), subgroup II with 25% < EF < or = 50% (n = 17) and subgroup III with EF > 50% (n = 13). Velocity distributions in the three subgroups were compared to each other. (1) The velocity distribution in the left ventricular outflow tract was skewed with the highest velocities and TVIs along the anterior wall and septum. The skewness of the velocity distribution was more pronounced in the apical long axis view than in the four chamber view (P < 0.05). The extent of skewness of the TVI profile was positively correlated to the left ventricular EF both in the long axis view (r = 0.63; P < 0.001) and in the four chamber view (r = 0.57; P < 0.001). (2) Pulsed Doppler sampling from different regions along the diameter produced different TVIs, and therefore yielded significantly different calculated aortic valve areas, especially in subgroup III. Due to the skewness of the velocity distribution in the left ventricular outflow tract, location of the pulsed Doppler sample volume significantly affects the accuracy of aortic valve area calculation by using the continuity equation, especially in patients with relatively high left ventricular EF. In patients with low EF, selection of pulsed Doppler sampling site is less important.