Aortic valve area calculation is an indirect method of determining the
area
Area is the quantity that expresses the extent of a region on the plane or on a curved surface. The area of a plane region or ''plane area'' refers to the area of a shape
A shape or figure is a graphics, graphical representation of an obje ...
of the
aortic valve
The aortic valve is a valve in the heart of humans and most other animals, located between the left ventricle and the aorta. It is one of the four valves of the heart and one of the two semilunar valves, the other being the pulmonary valve. The ...
(aortic valve area). The calculated aortic valve orifice area is currently one of the measures for evaluating the severity of
aortic stenosis
Aortic stenosis (AS or AoS) is the narrowing of the exit of the left ventricle of the heart (where the aorta begins), such that problems result. It may occur at the aortic valve as well as above and below this level. It typically gets worse ove ...
. A valve area of less than 1.0 cm
2 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
An echocardiography, echocardiogram, cardiac echo or simply an echo, is an ultrasound of the heart.
It is a type of medical imaging of the heart, using standard ultrasound or Doppler ultrasound.
Echocardiography has become routinely used in t ...
. For interpretation of these values, the area is generally divided by the
body surface area
In physiology and medicine, the body surface area (BSA) is the measured or calculated surface area of a human body. For many clinical purposes, BSA is a better indicator of metabolic mass than body weight because it is less affected by abnormal ad ...
, to arrive at the patient's optimal aortic valve orifice area.
Planimetry
Planimetry is the tracing out of the opening of the aortic valve in a still image obtained during echocardiographic acquisition during ventricular
systole
Systole ( ) is the part of the cardiac cycle during which some chambers of the heart contract after refilling with blood. The term originates, via New Latin, from Ancient Greek (''sustolē''), from (''sustéllein'' 'to contract'; from ''sun ...
, 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.
The continuity equation
The continuity equation states that the flow in one area must equal the flow in a second area if there are no shunts between the two areas. In practical terms, the flow from the left ventricular outflow tract (LVOT) is compared to the flow at the level of the aortic valve. In echocardiography the aortic valve area is calculated using the velocity time integral (VTI) which is the most accurate method and preferred. The flow through the LVOT, or LV stroke volume (in cm
3), can be calculated by measuring the LVOT diameter (in cm), squaring that value, multiplying the value by 0.78540 (which is π/4) giving a cross sectional area of the LVOT (in cm
2) and multiplying that value by the LVOT VTI (in cm), measured on the spectral Doppler display using pulsed-wave Doppler. From these, it is easy to calculate the area (in cm
2) of the aortic valve by simply dividing the LV stroke volume (in cm
3) by the AV VTI (in cm) measured on the spectral Doppler display using continuous-wave Doppler.
Stroke volume = 0.785(π/4) x Diameter
2 x VTI of LVOT
Cross sectional area of LVOT = 0.785(π/4) x LVOT Diameter
2
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.
The measurement using echocardiogram may be inaccurate in cases of Aortic subvalvular stenosis, because there is not a uniform diameter, as assumed during echocardiogram. The viewpoint of the echocardiogram may distort the true diameter of the LVOT in some patients. For verification purposes of the obtained valve area using echocardiogram and doppler measures, especially if the obtained valve area is in the range requiring surgery and cardiac output is low, the Gold standard of left heart catheterization for true hemodynamics should be obtained for validation using the Gorlin formula, so patient does not go for unneeded surgery.
The Gorlin equation
The Gorlin equation states that the aortic valve area is equal to the 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
In cardiac physiology, cardiac output (CO), also known as heart output and often denoted by the symbols Q, \dot Q, or \dot Q_ , edited by Catherine E. Williamson, Phillip Bennett is the volumetric flow rate of the heart's pumping output: t ...
(measured in liters per minute) and dividing it by the heart rate (to give output per
cardiac cycle
The cardiac cycle is the performance of the human heart from the beginning of one heartbeat to the beginning of the next. It consists of two periods: one during which the heart muscle relaxes and refills with blood, called diastole, following ...
) and then dividing it by the systolic ejection period measured in seconds per beat (to give flow per ventricular contraction).
The Gorlin equation is related to flow across the valve. Because of this, the valve area may be erroneously calculated as stenotic if the flow across the valve is low (i.e. if the cardiac output is low). The measurement of the true gradient is accomplished by temporarily increasing the cardiac output by the infusion of positive inotropic agents, such as
dobutamine
Dobutamine is a medication used in the treatment of cardiogenic shock (as a result of inadequate tissue perfusion) and severe heart failure. It may also be used in certain types of cardiac stress tests. It is given by IV only, as an injection ...
.
The Hakki equation
The Hakki equation
is a simplification of the Gorlin equation, relying on the observation that in most cases, the numerical value of
. The resulting simplified formula is:
The Agarwal-Okpara-Bao equation
The Agarwal-Okpara-Bao equation is a new form of AVA evaluation equation named after
Ramesh K. Agarwal,
Emmanuel c Okpara, and
Guangyu Bao.
It was derived from curve fitting of CFD simulation results and 80 clinical data obtained by Minners, Allgeier, Gohlke-Baerwolf, Kienzle, Neumann, and Jander
using a multi-objective genetic algorithm. The comparison of the results calculated from Gorlin Equation, Agarwal-Okpara-Bao Equation, and clinical data is shown in the figures on the right.
References
{{Cardiovascular physiology
Valvular heart disease