The Hakki formula (with subsequent correction by Angel) relies on the ratio of peak-to-peak instantaneous gradients and thus may be non-physiologic. In mild-moderate stenosis the peak-to-peak gradient, does not reflect mean gradient however it is often close to mean gradient for severe stenosis. In general it has excellent correlation with the Gorlin equation.

The normal aortic valve area is 3-4 cm². Symptoms tend to not be apparent until the AS is severe (<1cm²). If a patient presents with symptoms and the AVA is measured in the mild-moderate range alternative causes should be explored before attributing them to AS alone.

References
ACC/AHA 2006 Guidelines for the Management of Patients With Valvular Heart Disease: A Report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines (Writing Committee to Revise the 1998 Guidelines for the Management of Patients With Valvular Heart Disease): Developed in Collaboration With the Society for Cardiovascular Angiography and Interventions: Endorsed by the Society of Cardiovascular Anesthesiologists and the Society of Thoracic Surgeons. Circulation 2006;114;84-231
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Oh J, Seward JB, Tajik AJ. The Echo Manual 3rd edition. Lippincott, Williams, Wilkins. 2007

Kern MJ. The Cardiac Catheterization Handbook 4th Edition. Mosby 2003

Hakki AH, Iskandrian AS, Bemis CE, Kimbiris D, Mintz GS, Segal BL, Brice C: A simplified valve formula for the calculation of stenotic cardiac valve areas. Circulation 1981; 63; 150-1055
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Angel J, Soler-Soler J, Anivarro I, Domingo E: Hemodynamic evaluation of stenotic cardiac valves :II. Modification of the simplified formula for mitral and aortic valve area calculation. Cathet Cardiovasc Diagn 1985; 11; 127
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