Aortic stenosis gradient by Doppler echocardiogram
Inset shows apical five chamber view from which the aortic stenosis jet (AS Jet) is assessed using a continuous wave Doppler cursor. Aortic stenosis jet moves away from the transducer in this view and is displayed below the baseline. Velocity scale on the right side is in meters per second (m/s). Aortic regurgitation jet (AR Jet) is seen above the baseline as the regurgitant flow into the left ventricle is towards the transducer kept at the left ventricular apex. AV Vmax: maximum velocity of the aortic stenosis jet; AV V mean: mean velocity of the aortic stenosis jet; AV maxPG: maximum pressure gradient across the aortic valve; AV meanPG: mean pressure gradient across the aortic valve; AV VTI: velocity time integral across the aortic valve; AV Env.Ti: aortic valve envelope time (corresponds to the aortic ejection time); HR: heart rate. All gradients are measured in millimeters of mercury (mm Hg), velocities in meters per second (m/s) and heart rate in beats per minute (BPM). Envelope time is measured in milliseconds (ms) and velocity time integral in centimeters (cm).
Severity of aortic stenosis by Doppler gradient
Conventionally, aortic stenosis has been classified into mild (peak gradient up to 50 mm Hg), moderate (peak gradient between 50 – 75 mm Hg) and severe, with peak transvalvular pressure gradients above 75 mm Hg. But gradients depend on flow and low flow severe aortic stenosis with low gradients have been described. Low gradients can also occur in the presence of severe left ventricular dysfunction, which improves after treatment.
There is a good correlation between Doppler derived mean gradient and catheterization derived gradients in aortic stenosis. A peak Doppler velocity of of 4.5 meters per second or more and mean gradient by Doppler of 50 mm Hg or more has a high specificity (over ninety percent) for severe aortic stenosis with catheterization derived aortic valve area of 0.75 sq cm or less .But the sensitivity of these measures were low (less than fifty percent).
Difference between Doppler derived and catheter derived peak gradient in aortic stenosis
Doppler echocardiography estimates peak instantaneous gradient while cardiac catheterization estimates peak to peak pressure gradient between the left ventricle and aorta. Moreover, in critical aortic stenosis, Carabello effect  can induce errors in assessment of gradients because the presence of catheter itself increases the severity of the orifice narrowing.
Phenomenon of pressure recovery
Phenomenon of pressure recovery can also cause a difference between cardiac catheterization derived gradient and Doppler derived gradient in aortic stenosis .Pressure recovery is the increase of downstream pressure in aortic stenosis (or any other stenosis) due to reconversion of kinetic energy into potential energy. When pressure recovery is present, Doppler estimated gradients will be significantly higher than catheter derived gradients .
Oh JK, Taliercio CP, Holmes DR Jr, Reeder GS, Bailey KR, Seward JB, Tajik AJ. Prediction of the severity of aortic stenosis by Doppler aortic valve area determination: prospective Doppler-catheterization correlation in 100 patients. J Am Coll Cardiol. 1988 Jun;11(6):1227-34.
Niederberger J, Schima H, Maurer G, Baumgartner H; Importance of pressure recovery for the assessment of aortic stenosis by Doppler ultrasound. the role of aortic size, aortic valve area, and direction of the stenotic jet in vitro. Circulation. 94 1996:1934-1940.
Baumgartner H, Stefenelli T, Niederberger J, Schima H, Maurer G. “Overestimation” of catheter gradients by Doppler ultrasound in patients with aortic stenosis: a predictable manifestation of pressure recovery. J Am Coll Cardiol. 1999 May;33(6):1655-61.