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| Doppler Echocardiography |
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| Doppler echocardiography utilizes PWD
and CWD to calculate an areas, volume, or pressure measurements
of the heart. The basic concept is the flow profile, whether
it is a PWD or a CWD measurement. Doppler
echocardiography can only measure during flow states.
It cannot measure non-flow state values such as LVEDV.
To adequately calculate some of the values used in Doppler
echocardiography, some measurements will need to be made
in the 2D mode. |
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| Velocity Time Integral (VTI) |
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| Doppler echocardiography is used to
calculate velocity time integrals (VTI) of a valve. A
velocity time integral is the integral of all of the velocities
during the time of flow across a valve. The VTI results
in a measurement of length, cm. A VTI represents the length of flow or how far blood moves during a stroke. If the velocity of the blood is very fast, then the length of the VTI will be long. If the velocity of the blood is small, then the length of the VTI will be small. To obtain a velocity time integral, perform a pulse wave (PWD) or continuous wave Doppler (CWD) on a valve or a jet. On the Doppler scan time line, profiles of the velocities of the valve or jet are displayed. Trace the flow profile envelope on it's leading (outside) edge utilizing the tracing function and the trackball. Once the tracing is completed, the echocardiography machine will integrate to traced curve and display the velocity time integral of the traced curve. |
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| Small VTI |
Average VTI |
Large VTI |
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| Area or Cross Sectional Area (CSA) |
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| If the area of the valve is known then a stroke volume across the valve can be calculated. Area of a valve is given by the area of a circle (if the valve is a circle). The area of a circle is: |
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| Area of a Circle |
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Areacircle = pi * r2 |
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| In echocardiography the area of a circle
is called the cross-sectional area or CSA. The cross sectional
area can be further simplified by using diameter of the
valve, d, to: |
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CSA = pi * r2
CSA = 3.14 * (d/2)2
CSA = 3.14/4 * d2 CSA = 0.785 * d2 |
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CSA Formula |
CSA Image |
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| Volume |
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| To calculate a volume, an area of a valve or tube must be multiplied by the length of the stroke or VTI. Multiplying an area (cm2) by a length (cm) yields a volume (cm3 or mls). |
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| Multiplying the CSA (cm2)
by the VTI (cm) yields the stroke volume (SV) (ml). |
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| Stroke Volume Image |
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| The most important factor in the stroke
volume equation is the diameter. Small errors in diameter
result in large errors of stroke volume. Another potential
source of error is the Doppler angle to the jet being
interrogated. If the angle of incidence between the Doppler
scan line and the jet is > 20 degrees then an
error greater than 5% will occur. The Doppler scan line
must be parallel to the jet for correct measurements.
Since the geometry or geometric assumptions are not part
of the Doppler measurements, Doppler avoids the errors
produced by M mode or 2D echocardiography. Doppler measurements,
however, include only ejection phase indices (stroke volume
and peak flow velocity) which are affected by afterload. |
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