Principles of Hemodynamic Assessment

One of the most fundamental applications of Doppler echocardiography is the determination of transvalvular pressure gradient using Bernoulli equation (Fig. 4.1). This theorem states that the pressure drop across a discrete stenosis in the heart or vasculature occurs because of energy loss due to three processes (1) acceleration of blood through the orifice (convective acceleration), (2) inertial forces (flow acceleration), and (3) resistance to flow at the interfaces between blood and the orifice (viscous friction).1 Therefore, the pressure drop across any orifice can be calculated as the sum of these three variables. As one can realize, the original Bernoulli equation is complex; however, most of its components (flow acceleration and viscous friction) are clinically insignificant and can be ignored. Furthermore, the flow velocity proximal to a fixed orifice (V 1) is usually much lower than the peak velocity through the orifice (V 2) and can also be ignored (Fig. 4.2)