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| Editor's Note: |
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| The quantitative chapter contains some material that will be repeated in later chapters. Since the quantiative chapter covers a wide range of topics with the common thread that something is being measured or calculated, each measurement or calculation will be repeated in the appropriate chapter. The quantitative chapter provides you with a quick and easy method to look up calculations quickly and provides some general overview on measurements and calculations. |
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| Measurement Accuracy |
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Small errors in
measurements can lead to large errors in calculated
results. Measurement accuracy is dependent upon
the resolution of the image and following standard
measurement techniques. The transducer that
will provide the best resolution for the measurement
should be selected. Use the highest frequency
possible that will allow visualization of the structure
to be measured. The focal point should be located
at the structure to be measured. The display
should be maximized (magnified using zoom, depth,
width) so only the area of interest is visualized.
Timing measurements should be done under higher sweep
speeds. Consistent settings and probes should
be used for similar measurements between patients
and between studies of the same patients. Do
not change settings or change probes and expect that
the measurements will be equally accurate.
Measurements
of length or area of a cardiac structure should be
made on the leading edge (inside edge) to leading
edge technique. When measuring slopes, use two
points as far apart as possible. Use the ECG
for locating mechanical end-diastolic and end-systolic
phases.
Some measurements can be made by M-Mode, 2D mode, or 3D mode. M-Mode measures along a single line of sight and may miss abnormalities that are not present in it's line of sight. 2D provides a better view than M-Mode when making measurements. However, on some measurements in 2D mode, some assumptions may need to be used that may make the measurement inaccurate. 3D mode will not include the assumptions and may provide more accurate measurements than 2D in some cases. Therefore, as a general rule, the measurment accuracy is 3D > 2D > M-Mode.
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Accuracy
- Resolution
- Focal Point
- Display Size
- Sweep Speed
- Consistency
- Leading Edge Technique
- Slope Points
- ECG Utilization
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| Sources of Error |
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Interoperator
and intraoperator variability is a source of error.
Different sonographers will image and measure the
same structure differently and the same operator will
measure the same structure differently as they gain
experience and knowledge of measurement techniques.
Training and consistency are key methods of decreasing
interoperator and intraoperator errors.
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Error Sources
- Interoperator Error
- Intraoperator Error
- Speed of Sound Variance
- Doppler Angle
- Formula Assumptions
- Pixel Variance/Interpolation
- Patient Values
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| The
speed of sound will vary from the assumed 1540 m/sec
used in calculations. The speed of sound can
vary as much as 5% if adipose tissue is present between
the transducer and the interrogated structures, but
it typically has a variance of 2%. |
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The Doppler
angle may be off more than 20 degrees which will induce
an error greater than 5%. While the Doppler
in one plane may appear to be correct, in another
plane it can be off more than 20 degrees.
Formulas
may include assumptions that increase the error.
Volume formulas assume a certain shape of the structure
to be measured. Therefore, the volume calculated
may not be the actual volume of the structure being
measured if it doesn't match the formula's shape assumption.
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The pixels that are displayed may not match the actual
structure. Pixel values can be the actual value
returned by the ultrasound signal or it can be a calculated
value. Since resolution is poorer in the far
field, many of these pixel values are calculated.
Interpolation of pixel values when the lines of pixels
must be added in order to display the image in an
analog system also can induce errors in pixel values.
Patient measurements, such as height, weight, and
body surface area that are used in some formulas may
be inaccurate for obvious reasons.
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Left
Ventricular Volume, Mass and Geometry
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| Introduction
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In response to valvular disorders, hypertension,
heart failure the left ventricle undergoes remodeling.
The mass, volume, and/or geometry of the left ventricle
may change. Left ventricular measurements that characterize
the remodeling are relative wall thickness (RWT), short
axis (SAX), long axis (LAX), volume (V), mass (M), and
the volume/mass (V/M) ratio. The measurements of the left
ventricle can be predictive in certain disease states.
For example, patients with idiopathic dilating cardiomyopathy
have a poorer survival rate if their ventricle remodels
to a more spherical model where the LAX and the SAX are
similar.
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| M-Mode |
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| Mass |
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| Prolate Ellipsoid
Method |
| A prolate ellipsis is
an ellipsis with a LAX/SAX ratio of 2:1. The
normal left ventricle is a prolate ellipsoid.
To find the mass of the left ventricle, the
volume of the left ventricular muscle must be
calculated and then multiplied by its specific
gravity, 1.04. If the volume of the whole
left ventricle can be calculated as the outer
prolate ellipsis and the endocardial or blood
volume of the left ventricle can be calculated,
the difference is the left ventricular volume.
A correction factor, 0.8 is needed to calculate
the final value for left ventricular volume.
The formula for left ventricular mass by the
prolate ellipsoid method is: |
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LV mass = 0.8{1.04[(STd
+ LVIDd + PWTd)]3
- LVIDd3}}
+ 0.6g
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| Prolate Ellipsis |
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The measurements required
for the calculation of LV mass are the:
- Septal Thickness in Diastole (STd)
- Left Ventricular Internal Diameter in Diastole
(LVSIDd)
- Posterior Wall Thickness in Diastole (PWTd)
- Specific Gravity of the Myocardium (1.04).
Since the calculation will overestimate the LV
mass by 20%, a correction factor, 0.8, is used to
arrive at the true LV mass. The correct beam angle
for M Mode calculations can be obtained via the
parasternal acoustic window LVSAX view. The view
of the LVSAX is obtained just below the mitral valve
leaflet tips. The beam must be perpendicular to
the interventricular septum and the posterior wall.
Since wall thickness is being measured, the interventricular
septum and the posterior wall must not have regional
wall motion abnormalities. 2D echocardiography
is utilized to obtain and inspect the correct view
before switching to M Mode to obtain the measurements.
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M Mode
Left Ventricular
Measurements |
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| Volume |
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| If the LVID dimension is measured, the volume of the left ventricle can be calculated from the volume formula of a sphere: |
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V = D3 or V = LVID3 |
| Volume of a Sphere |
LV Volume (Spherical) |
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Overestimation of left ventricular volumes
occurs in disease states such as a dilated cardiomyopathy
or chronic valvular regurgitation. Teicholz et al published
a formula that compensates for the remodeling of the left
ventricle using only the LVSAX in a spherical heart. The
formula is:
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Vd = [7/{2.4+LVSAXd)]
* LVSAXd3
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This formula can be used in systole or
diastole to arrive at a stroke volume that is fairly accurate
when compared to other methods.
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| Relative Wall Thickness |
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While mass and volume measurements are
important predictors when the left ventricle has undergone
remodeling because of disease, relative wall thickness
indicates the type of geometric remodeling in a hypertrophic
process. Relative wall thickness can be an independent
predictor of outcome rather than mass and volume of the
left ventricle. Relative wall thickness is the ratio of
posterior wall thickness and the LVSAX.
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| LV Remodeling States |
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Relative wall thickness increases
in direct response to systolic pressure. In hypertensive
disease the patients can be classified or subdivided
based upon the response of the heart.
Concentric
hypertrophy is an increase in LV mass and relative
wall thickness.
Eccentric hypertrophy is an increase
in LV mass but normal relative wall thickness.
Concentric
remodeling occurs when normal LV mass is present
but increased relative wall thickness.
The last
state is a normal LV mass and normal relative wall
thickness.
Patients with concentric remodeling or
concentric hypertrophy have higher incidence of
cardiovascular events when compared to patients
with normal relative wall thickness (normal or
eccentric hypertrophy). A relative wall thickness
> 0.8 indicates an inappropriately hypertrophied
ventricle compared to the systolic pressure needed
to generate a stroke volume.
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| Image Relative Wall Thickness |
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| LV Remodeling State |
LV Mass |
RWT |
| Normal |
Normal |
Normal |
| Concentric
Remodeling |
Normal |
Increased |
| Concentric
Hypertrophy |
Increased |
Increased |
| Eccentric
Hypertrophy |
Increased |
Normal |
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