Blood forced through the narrow orifice accelerates sharply. By conservation of energy, this surge of kinetic energy comes at the cost of pressure — pressure is lowest exactly where velocity peaks, at the vena contracta.
As the jet exits into the wider aorta it decelerates. Some kinetic energy is lost forever as heat and turbulence — an abrupt, wide expansion is turbulent and wasteful. That lost energy can never come back as pressure.
Whatever kinetic energy is not lost to turbulence is reconverted back into pressure as flow slows down. This rise in measured static pressure downstream is pressure recovery. The smoother and smaller the aorta, the more recovers.
This is why the two numbers differ. Continuous-wave Doppler samples the peak velocity right at the vena contracta and reports 4v² — the maximum pressure drop, before any recovery. A catheter pullback measures LV pressure minus the recovered aortic pressure — the net gradient. Neither is wrong; they are reading different points on the same pressure curve.
In most adults with a dilated ascending aorta, recovery is small and safely ignored. It becomes clinically important — and can lead to overestimation of stenosis severity by Doppler — in these settings:
The single most important determinant. A narrow aorta gives orderly, low-turbulence deceleration, so a larger share of kinetic energy is reconverted to pressure. This is the threshold the guideline flags.
Highlighted by the guideline as a setting where recovery is particularly relevant — often younger patients with a smaller, non-dilated aorta and a doming valve.
With a smaller body and aorta, moderate stenosis can look severe by gradient alone. Recovery-corrected indices matter most here to avoid over-calling severity.
Flow through the central slot of a bileaflet mechanical prosthesis produces localised high velocities and pronounced recovery, a classic cause of falsely elevated Doppler prosthetic gradients.
A quick, robust safeguard needs no aortic area at all: the velocity ratio (dimensionless index) = VTILVOT / VTIAV. Because it is a ratio of the same flow, it is inherently recovery-insensitive; a value < 0.25 indicates severe AS. Note that outcome-validated cut-offs for the recovery-corrected indices remain limited — the guideline treats them as adjuncts, not replacements.