# Aortic valve area calculation

 Aortic Stenosis Microchapters

Editor-In-Chief: C. Michael Gibson, M.S., M.D. ; Associate Editor(s)-In-Chief: Cafer Zorkun, M.D., Ph.D. ; Lakshmi Gopalakrishnan, M.B.B.S. ; Usama Talib, BSc, MD 

## 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)

 Example: An individual undergoes left and right heart cardiac catheterization as part of the evaluation of aortic stenosis. The following hemodynamic parameters were measured. With a heart rate of 80 beats/minute and a systolic ejection period of 0.33 seconds, the cardiac output was 5 liter/minute. During simultaneous measurement of pressures in the left ventricle and aorta (with the use of one catheter in the left ventricle and a second in the ascending aorta), the mean systolic pressure gradient was measured at 50 mm Hg. What is the valve area as measured by the Gorlin equation? Answer: Aortic Valve Area (cms2) = (5000 mL/minute) ÷ (44.3 x 80 bpm x 0.33 secs x sq rt of 50 mm Hg) = 0.6046 cms2

### 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))

 Example: An individual undergoes left and right cardiac catheterization for the evaluation of aortic stenosis. Measurements includes an aortic pressure of 120/60, left ventricular pressure of 170/15, cardiac output of 3.5 liters/minute. What is the aortic valve area? Answer: The peak gradient between the left ventricle and aorta is 50 mm Hg → Aortic Valve Area = (3.5) ÷ (Sq root of 50) = 0.496 cms2

## 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.
• 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) }

 Example: An individual undergoes transthoracic echocardiography for the evaluation of a systolic ejection murmur with delayed carotid upstroke noted on physical examination. During echocardiography, the following measurements were made: LVOT diameter of 2 cm, LVOT TVI of 24 cm, and an Aortic Valve TVI of 50 cm. What is the aortic valve area? Answer: An LVOT diameter of 2 cm gives a LVOT cross-sectional area of, 2 x 2 x 0.78540 = 3.14 cm2. To calculate stroke volume: Cross-sectional area x LVOT TVI = 3.14 x 24 = 75.40 cc. Aortic valve area = Stroke volume ÷ Aortic valve TVI = 75.40 ÷ 50 = 1.51 cm2
• 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.