A Barebones Guide To PISA
Proximal Isovelocity Surface Area (PISA)
is the name given to a region of flow that develops as a flowing
liquid flows through a hole. As the flowing liquid approaches
the hole, the relationship between pressure and the size of the
hole causes the liquid to accelerate. This leads to production
of a succession of laminar “shells” of velocity in the liquid.
Suppose that ping pong balls came in 4 sizes, and that you had
one of each size. Now suppose that you cut each ping pong ball
in half, and discarded one half of each ball. If you then
stacked the halves so that the smallest ball’s half was on the
bottom (face down) and the next smallest ball was placed face
down over it, and then the remaining two balls were placed in
the same fashion, you would have a PISA array.
Each of the
“half ping pong balls” in this PISA array would represent a flow
velocity shell. Of course, as the liquid gets closer to the
hole, it flows faster. A PISA shell is larger in more severe
mitral regurgitation and smaller in less severe mitral
regurgitation.
By using
ColorFlow Doppler ultrasound, it is possible to identify each of
these shells of flow on the ventricular side of a mitral valve
showing regurgitation. This identification process is carried
out by adjusting the ColorFlow scale factor. The lower the scale
factor (i.e., the slower the velocity), the larger the PISA.
Here’s the schematic:
This is the
first step. The next step is to determine the ColorFlow Doppler
characteristics of the mitral regurgitant jet. Once the jet is
imaged, its peak velocity is recorded then the jet is traced to
measure its Velocity Time Integral (VTI). Here’s the schematic
for this step:
At this point,
it is possible to calculate the Effective Regurgitant Orifice (ERO)
and the Regurgitant Volume (RV).
Here are two
sample cases:
Case 1. A
patient with severe mitral regurgitation.
Nyquist Velocity for the shell (Vr) = 38 cm/sec
Proximal convergence region radius (r ) = 1.56 cm
Mitral Regurgitation jet Vmax = 466 cm/sec
Mitral Regurgitation jet VTI = 123.2 cm
Calculated Effective Regurgitant Orifice (ERO) = 1.25 cm2
Calculated Regurgitant Volume (RV) = 154 cc
Case 2. A
patient with mild-moderate mitral regurgitation.
Nyquist Velocity for the shell (Vr) = 19 cm/sec
Proximal convergence region radius (r ) = 0.68 cm
Mitral Regurgitation jet Vmax = 475 cm/sec
Mitral Regurgitation jet VTI = 134.8 cm
Calculated Effective Regurgitant Orifice (ERO) = 0.12 cm2
Calculated Regurgitant Volume
(RV) = 15.8 cc
It must be remembered that PISA can vary
during different parts of ventricular systole, and this leads to
potential variability in the measurement.
Source: Chan KT et al: Comparison of Severity of Mitral
Regurgitaion by Angiography and PISA Method by Echocardiography.
J HK Coll Cardiol 4:2-7, 1996. |