The determination of aortic valve area by the Gorlin formula: what the cardiac sonographer should know. The application of the Gorlin formula in the cardiac catheterization laboratory is the standard of reference for the determination of aortic valve area. The continuity equation now enables the cardiac sonographer to determine aortic valve area noninvasively in the echocardiography laboratory. The comparison of the results obtained by the two methods is inevitable. The cardiac sonographer should have a basic understanding of the theory and pitfalls of the Gorlin formula so that when conflicting results are obtained, the possible reasons why will be clear. 