From the Guidelines
The formula for calculating the aortic valve area (AVA) is AVA = (CSA LVOT × VLVOT) / VAV, where CSA LVOT is the cross-sectional area of the left ventricular outflow tract (LVOT), VLVOT is the velocity in the LVOT, and VAV is the velocity in the aortic valve 1.
Calculation Components
- CSA LVOT: can be calculated using the formula CSA LVOT = π × (LVOT diameter / 2)^2, where LVOT diameter is measured in a parasternal long-axis view from the inner edge to inner edge of the septal endocardium, and the anterior mitral leaflet in mid-systole 1.
- VLVOT: is recorded with pulsed Doppler from an apical approach, in either the anteriorly angulated four-chamber view (five-chamber view) or in the apical long-axis view, with the pulsed Doppler sample volume positioned just proximal to the aortic valve 1.
- VAV: is the velocity in the aortic valve, which can be measured using continuous wave Doppler.
Important Considerations
- The continuity equation is a well-validated method for calculating AVA, but it assumes a circular outflow tract shape, which may not always be the case 1.
- 3D echocardiography may provide more accurate assessment of LVOT area, as it can be directly planimetered and avoid the underestimation of AVA when using a circular assumption for LVOT 1.
- The simplified continuity equation is less well accepted due to concerns about variability in results when using velocities instead of velocity-time integrals (VTIs) in the equation 1.
Clinical Application
- The calculated AVA can be used to assess the severity of aortic stenosis, with an AVA of < 1.0 cm^2 consistent with severe aortic stenosis 1.
- The continuity equation has been reported as a valuable parameter for prediction of clinical outcome and for clinical decision-making 1.
From the Research
Aortic Valve Area Calculation
The aortic valve area (AVA) can be calculated using various formulas, including:
- Hakki's formula: area = cardiac output/square root gradient 2, 3
- Gorlin's formula: a more complex formula that requires the assessment of the systolic ejection time or the transvalvular flow 2, 4
- Doppler ultrasound method: AVA = CO/(SEP X mean velocity), where CO is cardiac output measured by thermodilution and SEP is the systolic ejection period derived from the Doppler tracings 5
- Continuity equation: a formula that uses transthoracic echocardiography to calculate AVA 6
Key Considerations
When calculating AVA, it's essential to consider the following:
- Heart rate: the simplified formula may be less accurate in patients with bradycardia or tachycardia 4
- Valve status: some formulas may not be suitable for certain valve conditions, such as aortic incompetence 2
- Measurement methods: different methods, such as MRI planimetry or hybrid approaches, can be used to calculate AVA 3, 6
Formula Comparison
Studies have compared the accuracy of different formulas, including:
- Hakki's formula vs. Gorlin's formula: good correlation between the two formulas 2
- Hakki's formula vs. planimetry: similar results, with Hakki's formula being easier to use and more reproducible 3
- Doppler ultrasound method vs. cardiac catheterization: excellent correlation between the two methods 5
- Hybrid approach vs. standard TTE: good agreement between the two methods, with the hybrid approach being a reasonable alternative in certain cases 6