The Effective Orifice Area (EOA) is calculated using a simplified continuity equation based on echocardiographic measurements.
Here's the breakdown of the calculation:
The formula used to calculate EOA is:
EOA = (LVOT Area × LVOT VTI) / Aortic VTI
Where:
- EOA is the Effective Orifice Area (cm²) - this is the valve area.
- LVOT Area is the cross-sectional area of the Left Ventricular Outflow Tract (cm²).
- LVOT VTI is the Velocity Time Integral of the LVOT flow (cm). This is essentially the stroke distance through the LVOT.
- Aortic VTI is the Velocity Time Integral of the Aortic Valve flow (cm). This is the stroke distance through the aortic valve.
Explanation of the components:
- Left Ventricular Outflow Tract (LVOT) Area: This is determined by measuring the LVOT diameter just proximal to the aortic valve. The diameter is then used to calculate the area, assuming a circular shape: Area = π * (diameter/2)². Accurate measurement of the LVOT diameter is crucial for accurate EOA calculation.
- Velocity Time Integral (VTI): VTI is measured using pulsed-wave Doppler (for LVOT) and continuous-wave Doppler (for aortic valve) during echocardiography. The Doppler signal traces the velocity of blood flow over time. The area under this curve represents the VTI, which reflects the distance the blood travels during each heartbeat.
Steps to Calculate EOA:
- Measure LVOT Diameter: Obtain a high-quality parasternal long-axis view of the heart. Measure the LVOT diameter just proximal to the aortic valve annulus during mid-systole.
- Calculate LVOT Area: Using the measured LVOT diameter, calculate the cross-sectional area: LVOT Area = π * (LVOT diameter/2)²
- Measure LVOT VTI: Using pulsed-wave Doppler, place the sample volume in the LVOT, just proximal to the aortic valve. Obtain the LVOT VTI from the spectral Doppler tracing.
- Measure Aortic VTI: Using continuous-wave Doppler, align the Doppler beam through the aortic valve to obtain the highest velocity signal. Obtain the aortic VTI from the spectral Doppler tracing.
- Calculate EOA: Plug the measured values into the EOA formula: EOA = (LVOT Area × LVOT VTI) / Aortic VTI
Importance of Accurate Measurements:
EOA calculation relies heavily on accurate measurements obtained during echocardiography. Errors in measuring LVOT diameter or VTIs can significantly impact the calculated EOA value. It is essential to adhere to standardized techniques and ensure optimal image quality during the echocardiographic examination.
Example:
Let's say we have the following measurements:
- LVOT Diameter = 2.0 cm
- LVOT VTI = 20 cm
- Aortic VTI = 40 cm
- Calculate LVOT Area: LVOT Area = π (2.0 cm / 2)² = π (1 cm)² = 3.14 cm²
- Calculate EOA: EOA = (3.14 cm² * 20 cm) / 40 cm = 1.57 cm²
Therefore, the Effective Orifice Area in this example is 1.57 cm². This would be interpreted in the context of other clinical findings to determine the severity of aortic stenosis.
