The Gorlin equation is used to calculate mitral or aortic valve area during invasive hemodynamic assessment via cardiac catheterization. While initially developed to measure mitral valve area in patients with rheumatic mitral valve stenosis, the Gorlin equation is now most commonly used to calculate aortic valve area in patients with aortic valve stenosis.
The cardiac output and pressure gradient are directly measured. Cardiac output is measured using the Fick method, or thermodilution, while the pressure gradient is found simply by using the catheter to measure the pressure in the aorta, then advancing it into the LV and taking another pressure reading (vs. using a dual lumen catheter). The difference between these two pressures is the pressure gradient. The mean transaortic valve pressure gradient is used in the Gorlin equation to calculate the aortic valve area, not the peak gradient.
It is important to note that the Gorlin equation was originally derived using patients with mitral stenosis and not aortic stenosis, and thus may not relate perfectly to calculate the aortic valve area. The Gorlin equation is also flow dependant, so if the patient has a significantly decreased ejection fraction, the aortic valve area may be underestimated.