To calculate the Doppler Velocity Index (DVI), divide the subvalvular velocity by the maximum velocity across the prosthetic valve.
Here's a breakdown:
The Doppler Velocity Index (DVI) is a valuable echocardiographic measurement primarily used to assess the function of aortic prosthetic valves and to screen for valve obstruction (stenosis). It's a simple ratio that provides insight into the pressure drop across the valve.
Formula:
DVI = (Subvalvular Velocity) / (Maximum Velocity Across the Prosthetic Valve)
Where:
- Subvalvular Velocity: This is the peak velocity of blood flow measured just before it enters the prosthetic valve. It's typically obtained using Pulsed-Wave (PW) Doppler. This reflects the inflow velocity.
- Maximum Velocity Across the Prosthetic Valve: This is the highest velocity of blood flow measured as it passes through the prosthetic valve. It's usually obtained using Continuous-Wave (CW) Doppler. This reflects the velocity at the vena contracta (narrowest point of the jet).
Steps for Calculation:
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Obtain Subvalvular Velocity (PW Doppler): Place the PW Doppler sample volume just proximal to the aortic prosthetic valve in the left ventricular outflow tract (LVOT). Measure the peak velocity of the blood flow. Ensure proper alignment of the Doppler beam to avoid underestimation of velocity.
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Obtain Maximum Velocity Across the Prosthetic Valve (CW Doppler): Use CW Doppler to trace the entire velocity profile through the aortic valve. The highest velocity recorded represents the maximum velocity across the prosthetic valve. Angle correction is crucial for accurate assessment.
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Calculate the DVI: Divide the subvalvular velocity (from PW Doppler) by the maximum velocity (from CW Doppler).
Interpretation:
- A normal DVI typically ranges from 0.25 to 0.5, but this can vary depending on the type and size of the prosthetic valve.
- A low DVI (e.g., <0.25) suggests a higher pressure gradient across the valve, which may indicate stenosis or obstruction. Consult published reference ranges for specific valve types and sizes.
Example:
Let's say:
- Subvalvular Velocity (PW Doppler) = 1.0 m/s
- Maximum Velocity Across the Prosthetic Valve (CW Doppler) = 4.0 m/s
Then, DVI = 1.0 m/s / 4.0 m/s = 0.25
Clinical Significance:
The DVI helps in:
- Assessing Prosthetic Valve Function: Identifying potential stenosis or dysfunction.
- Monitoring Valve Performance Over Time: Tracking changes that may indicate valve deterioration.
- Differentiating Between Normal and Abnormal Valves: Helping to distinguish normally functioning prosthetic valves from those with problems.
Limitations:
- The DVI is dependent on accurate Doppler measurements. Improper technique or poor image quality can lead to inaccurate results.
- Values can vary based on heart rate and cardiac output.
- It's best to use DVI in conjunction with other echocardiographic parameters, such as mean pressure gradient and valve area, for a comprehensive assessment.
