The simplified Bernoulli equation may be  applied to peak velocity measurements to make non-invasive estimates of pressure gradients. Where  Vl = peak velocity proximal to an obstruction

V2 = peak velocity distal to the obstruction.

This equation takes into consideration the velocity of flow on both  sides of an obstruction.

Example: Peak velocity measured  in  the ascending aorta is  3  dsec, peak velocity measured in the  left ventricular outflow  is 2  dsec.

Gradient across the aortic valve  = 4  (3²-2²) = 4 (9-4) = 20 mmHg.

If V  is close to  1 dsec, it can be neglected and then even simpler version of the Bernoulli equation may be used: 

P = 4V²