How is the aortic valve area calculated?
The normal valve area is 2.5–3.5 cm2 and a valve area less than 0.75 cm2 is considered to be severe AS. In the cardiac catheterization laboratory, the aortic valve area is calculated using the Gorlin formula. The simplified version states that the valve area is proportional to the flow across the valve divided by the square root of the mean pressure gradient.
Knowing the pressure gradient in the absence of the car-diac output (flow) is not a reliable indicator of the severity of aortic disease. For example, a patient with extremely severe AS but with a very low cardiac output, would have a small measured transvalvular gradient because of the diminished flow across the valve. However, in most patients one can assume that a mean pressure gradient >50 mmHg or a peak pressure gradient >80 mmHg implies severe stenosis.