Aortic valve area calculation

# Overview
Aortic valve area calculation is an indirect method of determining the area of the aortic valve.  The calculated aortic valve orifice area is currently one of the measures for evaluating the severity of aortic stenosis. A valve area of less than 0.8 cm2 is considered to be severe aortic stenosis.
There are many ways to calculate the valve area of aortic stenosis.  The most commonly used methods involve measurements taken during echocardiography. For interpretation of these values, the area is generally divided by the body surface area, to arrive at the patient's optimal aortic valve orifice area.

# Cardiac Catheterization

## Gorlin Equation
Grolin Equation can be used to calculate Aortic Valvular functional dynamics.
- In 1951, S.G. Gorlin and Dr. Richard Gorlin, were the first to develop a formula to calculate the cardiac valvular orifices using the flow and pressure-gradient data.
- The Gorlin equation states that the aortic valve area is equal to the blood flow through the aortic valve during ventricular systole divided by the systolic pressure gradient across the valve times a constant.
- The flow across the aortic valve is calculated by taking the cardiac output (measured in liters/minute) and dividing it by the heart rate (to give output per cardiac cycle) and then dividing it by the systolic ejection period measured in seconds per beat (to give flow per ventricular contraction).
Aortic Valve Area (cms2) = (Stroke volume (mL/beat)  Systolic ejection period (secs/beat))  ( 44.3 x square root of mean systolic pressure gradient between the left ventricle and aorta (mm Hg))
Simplified Equation: Aortic valve area = (Cardiac output/Heart rate)  (44.3 x Sq rt mean systolic pressure gradient x Systolic ejection period)  AVA = Cardiac output  (44.3 x Heart rate x Systolic ejection period x Sq rt mean systolic pressure gradient)
- The Gorlin equation is related to blood flow across the valve. Therefore, in cases of low cardiac output, the valve area may be erroneously calculated as stenotic. Therefore, the measurement of the true gradient may be accomplished by temporarily increasing the cardiac output by the infusion of positive inotropic agents, such as dobutamine.
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## Hakki Equation
- The Hakki equation is a simplification of the Gorlin equation, relying on the observation that in most cases, the numerical value of 44.3 x heart rate (bpm) x systolic ejection period (secs) is 1000.
- The resulting simplified formula is:
Aortic Valve Area (cms2) = (Cardiac output (liters/minute))  (Square root of mean systolic pressure gradient between the left ventricle and aorta (mmHg))

# Echocardiography

## Planimetry
Planimetry is the tracing out of the opening of the aortic valve in a still image obtained during echocardiographic acquisition during ventricular systole, when the valve is supposed to be open.  While this method directly measures the valve area, the image may be difficult to obtain due to artifacts during echocardiography, and the measurements are dependent on the technician who has to manually trace the perimeter of the open aortic valve. Because of these reasons, planimetry of aortic valve is not routinely performed.

## Continuity Equation
The role of continuity equation in diagnosing Aortic Stenosis can be understood with the following details. 
- The continuity equation states that the flow in one area must equal the flow in a second area if there are no shunts in between the two areas.
- In practical terms, the blood flow from the left ventricular outflow tract is compared to the blood flow at the level of the aortic valve.
- Using echocardiography, the aortic valve area calculated using the time velocity integral (TVI) which is the most accurate and preferred method.
- The blood flow through the LVOT (i.e., left ventricular stroke volume (cm3 or cc), can be calculated by measuring the LVOT diameter (cm), squaring that value, multiplying the value by 0.78540 giving cross sectional area of the LVOT (cm2)and multiplying that value by the LVOT TVI (cm), measured on the spectral Doppler display using pulsed-wave Doppler.
- From these, it is easy to calculate the aortic valve area (cm2) of the aortic valve by simply dividing the stroke volume (cm3) by the aortic valve time velocity integral (cm) measured on the spectral Doppler display using continuous-wave Doppler.
Aortic Valve Area (cms2) = { (LVOT Diameter2 x 0.78540 x LVOT Time Velocity Integral)  (Aortic Valve Time Velocity Integral) }
- The weakest aspect of this calculation is the variability in measurement of LVOT area, because it involves squaring the LVOT dimension. Therefore, it is crucial for the sonographer to take great care in measuring the LVOT diameter.