Pressure recovery across the aortic valve.

The Bernoulli equation relates the pressure exerted on a fluid to its flow velocity and its density, in addition to its flow acceleration and its viscous friction loss. When flow velocity increases at a narrowing, the local pressure decreases proportionally. It has been wrongfully assumed that pressure lost distal to a stenosis can never recover. It is, however, the energy content of the fluid, equal to the kinetic plus the potential energy, which can not increase. When flow slows distal to a narrowing and little energy is lost to friction, pressure does actually increase. Pressure recovery has been well demonstrated to exist in a variety of pathophysiological states. Bicuspid aortic valve prostheses such as the St. Jude valves can produce quite remarkable pressure recovery. This causes a great discrepancy between pressure drop calculations based on continuous wave doppler on the one hand and true pressure drop across the prosthesis on the other. Reliance on doppler measurements only might wrongfully lead one to conclude that the prosthesis was malfunctioning. Less extreme pressure recovery is possible across a stenotic native aortic valve, but interpretation of the flow velocity across the valve might make the difference between recommendations to replace or to retain the valve. When interpreting doppler signals across narrowings the phenomenon of pressure recovery should be kept in mind.