2. Assessment of Severity of AS : Quantitative assessment

Kathryn Statham

Quantitative Assessment of AS

• Peak AV jet velocity (m/sec)
• Mean AV gradient (mmHg)
• Valve area by continuity equation (cm2)
• Dimensionless severity index (dimensionless)

1. Pressure Gradients 

Stenosis results in a pressure gradient. The pressure gradient is high before an obstruction and low distal to the obstruction. The greater the stenosis the greater the pressure gradient. Doppler measurements in AS measure the gradient between the LV and the aorta.

It is possible to measure the Peak velocity (ie  the highest velocity that can be recorded) and the Mean Pressure gradient. This is done in the following way :

• Place a CW Doppler beam through the AV using the apical 5 chamber view
• The trace obtained will rely  on a parallel intercept angle through the aortic valve with the flow of blood. The accuracy of this measurement is highly dependent on the angle of intercept so the Doppler beam must be as parallel to the flow of blood as possible. Due to distortion of flow through stenotic valves, this measurement should be taken several times and an attempt every time to get as parallel to the beam
• To check whether the Doppler beam is well aligned, the LVOT trace should be visible at the base of AV trace.
• The highest velocity on the spectral CW Doppler trace is measured (Peak velocity) and by tracing the velocity curve throughout the  ejection cycle, the mean pressure gradient is calculated. This is done electronically. This is an average of all instantaneous pressure gradients throughout ejection. Mean gradients correlate often better than peak velocity with the degree of AS as the gradient is averaged over the entire systolic period rather than one instantaneous pressure peak

 Peak AV jet velocity

• Mild stenosis Vmax 2.6 -3.0 m/s
• Moderate stenosis Vmax 3.0m/s – 4.0 m/s
• Severe stenosis Vmax > 4.0 m/sec

 

Mean Pressure Gradient

• Mild : Mean pressure gradient  < 20 mmHg
• Moderate : Mean pressure gradient 20 -40 mmHg
• Severe : Mean Pressure gradient > 40 mmHg

Caveats

Both the peak and mean pressure gradients of the AV are dependent on

1. stroke volume and HR
2. correct alignment of the beam
3. there being no obstruction proximal to the AV valve which may falsely  be attributed to the aortic valve such as subvalvular obstructions or HOCM

High output states such as in AR, anaemia, fever, tachycardia, sepsis or pregnancy may  overestimate peak velocity & peak gradient.

Low output states will underestimate gradients and therefore AS

Low output states may occur in patients

• with small ventricles (secondary to LV hypertrophy for example) and normal EF
• in patients with coexisting significant MR and normal EF
• patients with reduced EF and normal sized ventricles.

In these clinical situations the Dimensionless Severity Index is more accurate in determining the severity of the AS

Measurements may also change from beat to beat in patients with irregular rhythms such as AF. In this case an average of 3 beats should be taken to determine correct pressure measuremets

2. Valve area using the continuity equation

The continuity equation states that the flow through the LVOT should be equal to the flow through the aortic valve area.

               

The CSA or cross sectional area can be measured by measuring the diameter of the LVOT. This is measured in systole from the blood tissue interface of the LVOT 0.5 – 1.0 cm out from the aortic valve

1. The aortic valve gradient is measured by tracing the CW doppler signal obtained from the apical 5 chamber view, through the aortic valve.

2. The LVOT signal is obtained using the PW doppler from an apical 5 chamber view, using a sample gate in the LVOT (0.5 cm from aortic valve).

1.  2.

AVA  (aortic valve area) grading of severity :

• Mild > 1.5 cm2
• Moderate 1.0 – 1.5 cm2
• Severe < 1 cm2

Caveats

The continuity equation is dependent on

• Correct measurement of the LVOT which may be difficult in patients with a poor PLAX view
• Correct Doppler alignement

3. Dimensionless Severity Assessments

• The dimensionless severity index eliminates errors in LVOT diameter measurement
• It is not flow dependent

DSI – VTI of LVOT (using PW)/  VTI of AV (using CW)

Example : LVOT VTI = 18 cm, AV VTI = 56 cm    DSI = 0.32 ( moderate AS)

• Mild > 0.5
• Moderate 0.25 – 0.5
• Severe < 0.25

Summary

 

References

1. Echocardiographic predictors of pulmonary hypertension in patients with severe aortic stenosis Nikhil Kapoor, Padmini Varadarajan, Ramdas G. Pai
   European Journal of Echocardiography, Volume 9, Issue 1, January 2008, Pages 31–33, https://doi.org/10.1016/j.euje.2007.01.005