Theoretical and practical differences between the Gorlin formula and the continuity equation for calculating aortic and mitral valve areas

Abstract

Although the Gorlin formula and the continuity equation are both used to calculate valvular areas in the clinical situation, there have been few comparisons of the 2 methods. Mathematically, it can be shown that both formulas are derived from similar hydrodynamic principles which basically give a measure of the physiologic or effective area occupied by flow. However, the Gorlin formula contains errors in formulation and incorporates a constant that purports to give a measure of the anatomic rather than of the effective area of the valve. If both formulas are applied to the same hemodynamic data from aortic and mitral bioprostheses studied in a pulse duplicator system, the Gorlin formula constantly yields results 1 to 2% higher than the continuity equation for aortic valves and 12 to 13% higher for mitral valves. For any given type and size of prosthesis, the areas calculated by either formula increase linearly in relation to increasing pressure and flow (up to 20% for aortic valves and up to 35% for mitral valves). It is concluded that the Gorlin formula and the continuity equation are both pressure- and flow-dependent and are primarily related to the effective area occupied by flow rather than to the anatomic area of the valve. The 2 methods yield consistently different results due to differences in mathematical formulation. Such factors are important to consider when interpreting valve area calculations clinically.