Q. How to investigate aortic stenosis by Echo Cardiography?
2D Echo cardiogram shows the number of cusps. But presence of raphae and distorted anatomy due to calcification may make if difficult to assess. Localization of stenosis to sub-valvular or supra valvular or valvular location can easily be done. Ascending aorta may be disproportionately dilated in some patients with bicuspid aortic valve with a tendency for aortic dissection.
Hemodynamic severity of aortic stenosis can be measured by Doppler echocardiography. Peak and mean trans valvular aortic gradients, aortic valve area and ratio of LVOT and aortic valve time velocity integral (VTI) are important measurements. Meticulous search using all transducer positions should be made to obtain maximal aortic velocity. In patients with normal LV function and cardiac output, aortic stenosis is considered severe when peak aortic velocity is more than or equal to 4.5 m/sec; mean pressure gradient equal to or more than 50 mmHg, aortic valve area less than or equal to 0.75 cm2 or LVOT/Aortic valve VTI ratio less than or equal to 0.25. It should be noted that in-patients with left ventricular dysfunction and/or low cardiac output, transvalvular pressure gradients will be under estimated while in situations with increased flow across aortic valve like aortic regurgitation and/or anemia transvalvular gradient will be over estimated. Since in both these situation LVOT and Aortic valve VTI move in the same direction and value area as measured by continuity equation makes use of ratio of these two, valve area measurements tends to be more accurate. Also valve resistance which takes into account valve area and flow across the valve is a more reliable index. Further while following a patient serially with echocardiography combined use of transvalvular gradients, left ventricular function and valve area calculation are more useful. Otto and his colleagues have shown that in asymptomatic patients, peak aortic jet velocities increase by 0.32 ± 0.34 m/sec per year, mean gradient by 7± 7 mmHg and valve area decrease by 0.12 ± 0.19 cm per year. Beyond a peak velocity of 4.0 m/sec most of the patients become symptomatic within 2 years. Aortic valve area is d" 0.75 cm2 when peak aortic velocity is > 4.5 m/sec and mean gradient is e" 50 mm Hg. However, half of patients with aortic valve area d" 0.75 cm2 have lower peak velocities and mean gradients due to lower cardiac output.
Ratio of LVOT/Aortic valve VTI is another good index in measuring aortic valve area. A ratio of LVOT/AV VTI is < 0.25 correlates well with a aortic valve area of < 0.75 m2.