Aortic valve area calculation
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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.[1][2]
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.
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[edit] 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.
[edit] 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 in 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. Using echocardiography, the peak velocity at the level of the aortic valve and in the LV outflow tract can be measured, and the area of the LV outflow tract can be measured. From these, it is easy to calculate the area of the aortic valve.
Example: An individual undergoes trans thoracic 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, peak velocity in the LVOT of 1 m/s, and a peak velocity at the level of the aortic valve of 2.9 m/s. What is the aortic valve area? | ||
Answer: An LVOT diameter of 2 cm gives a LVOT area of π. This gives |
The weakest aspect of this calculation is the variability in measurement of LVOT area, because it involves squaring the LVOT dimension. Some doctors prefer to read just the velocity ratio, to eliminate this.
[edit] 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 (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).
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 (ie: 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.
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 liters/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 mmHg. What is the valve area as measured by the Gorlin equation? | ||
Answer: |
[edit] The Hakki equation
The Hakki equation[3] is a simplification of the Gorlin equation, relying on the observation that in most cases, the numerical value of . The resulting simplified formula is:
Example: An individual undergoes left and right cardiac catheterization for the evaluation of aortic stenosis. Measurements includes an aortic pressure of 120/60, LV pressure of 170/15, cardiac output of 3.5 liters/minute. What is the aortic valve area? | ||
Answer: The peak gradient between the LV and aorta is 50 mmHg. This gives |
[edit] See also
[edit] References
- ^ Charlson E, Legedza A, Hamel M (2006). "Decision-making and outcomes in severe symptomatic aortic stenosis.". J Heart Valve Dis 15 (3): 312-21. PMID 16784066.
- ^ "Survival in elderly patients with severe aortic stenosis is dramatically improved by aortic valve replacement: results from a cohort of 277 patients aged >/=80 years.". Eur J Cardiothorac Surg. PMID 16950629.
- ^ Hakki A, Iskandrian A, Bemis C, Kimbiris D, Mintz G, Segal B, Brice C (1981). "A simplified valve formula for the calculation of stenotic cardiac valve areas.". Circulation 63 (5): 1050-5. PMID 7471364.