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2D Mode Hemodynamics
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Education>Expert TTE>Quantitative Assessment and Hemodynamics>2D Mode Hemodynamics>1
 
  
Introduction
 
There is a large variablility of how measurements are imaged, measeaured, recorded and interpreted. The guidelines attempt to standardize how images are performed, where measurements are made, what part of the cycle measurements are performed, and to provide cutoffs for mild, moderate, and severe disease so reports between labs can become standardized and useful in following patients.
 
2D Echocardiographic evaluation of left ventricular function starts with the anatomical makeup of the left ventricle. Two factors, mass and volume, allows characterization of the left ventricular anatomy which indicates the pathological state of the left ventricle. In order to calculate mass and volume true short and long axis measurements must be obtained. Foreshortening of the left ventricle will yield incorrect values and mislead the therapy. Two models, the prolate ellipse model and the hemiellipse model are used, along with other models, to calculate the left ventricular mass and volume. Once global function has be defined, 2D echocardiography also allows for the assessment of regional function of the left ventricle.
  
Foreshortening
 
Foreshortening is the incorrect measurement of the left ventricular axis, usually the long axis, due to improper sector scan of the left ventricle. The sector scan, instead of cutting across the true long axis of the left ventricle, cuts an acute angled sector scan that will yield a shorter axis than the true axis length. The apex of the left ventricle should remain stationary throughout the cardiac cycle in the TTE (apical) views. If the apex of the left ventricle appears to move towards the base of the heart then the sector scan is foreshortened. The apex of the left ventricle is relatively immobile throughout the cardiac cycle. If a foreshortened view is obtained, change the angle and/or location of the probe to obtain a non-foreshortened view.
 
script
script
Normal View Foreshortened View
  
Left Ventricular Contraction
 
The left ventricle wall is separated into the subendocardial, midwall, and subepicardial muscle fibers. Each section of fibers is arranged in different directions, such that, during contraction, each set of muscle fibers contributes to different aspects of the contraction process. When the left ventricle contracts it shortens the left ventricle's long axis, short axis, and lastly, a "wringing effect" whereby the base and the apex of the ventricle rotate around the long axis of the left ventricle.
Circumferential (SAX) Contraction Longitudinal (LAX) Contraction Axial (Wringing) Contraction
Long and Short Axis of the Left Ventricle
Image Axis of LV
The midwall fibers are run circumferentially around the left ventricle's short axis. During the contraction process the midwall fibers contribute to the shortening of the short axis of the left ventricle. The major portion of the stroke volume comes from the shortening of the short axis of the left ventricle.

The subendocardial and subepicardial fibers are oriented longitudinally. The longitudinally oriented fibers cause the shortening of the apex-to-base or long axis of the left ventricle. Note that it is the base, not the apex, of the heart that is displaced towards the apex of the heart.  The apex is relatively immobile during the contraction of the heart. 

The third contribution to the stroke volume is the apical-to-base twisting or wringing effect. The apex of the left ventricle will turn counter clockwise with respect to the base of the heart. The twisting or wringing effect is most due to the contractile state of the myocardium and the counter opposed spiral arrangement of the cardiac musculature. 
 
Electrocardiogram and Mechanical End-Systole and End-Diastole Events
 
When measuring end-diastolic and end-systolic parameters, use the ECG.  The onset of systole occurs at the peak of the QRS complex and the end of systole occurs at the end of the T wave (when the ventricle is repolarized).  End-diastole occurs just prior to the initiation of the QRS complex.   It is very difficult to be consistent in measuring cardiac timing events by looping thru the mechanical movements. Using the electrical events of the cardiac cycle will make the measurements more consistent.
  
Mass
  
Cylinder-hemiellipsoid Method
 
If area and length are measured then the mass of the left ventricle can be calculated from the cylinder-hemiellipsoid method. Using the formula:

V = 5/6 * Area * Length


where Area is the area of the left ventricle in a LVSAX view (just below the mitral valve leaflet tips or high papillary view) and length is the base-to-apex length (Long Axis (LAX)) in a non-foreshortened LVLAX view, usually the 4 chamber view. The measurements are made in end-diastole.  The cavitary volume is calculated using the endocardial border.  The heart volume is calculated using the epicardial border.  The difference between the heart volume and cavitary volume is the myocardial volume . To calculate the myocardial mass, the myocardial volume is multiplied by the myocardial specific gravity, 1.04 g/ml.
  
Area
  
Fractional Area Change (FAC)
 
2D Echocardiography can measure areas, circumferences and lengths from the caliper and tracing function on the echocardiographic machine. Various formulas can be used to calculate areas, volume, ejection fraction, fractional shortening, etc. Area can be measured using the echocardiographic tracing function. The tracing function will yield a circumference and an area. The area and circumference can be measured in systole and diastole to give an indication of the function of the heart. The fractional area of change (FAC) of the heart is calculated by dividing the difference between the diastolic and systolic areas by the diastolic area.  Normally, the EDA is approximately 14 cm2 and the ESA is approximately 6cm2 for a FAC of 60%.  The FAC is load dependent.  Also the SAX view may be truncated if it is not at perpendicular to the long axis.  Remote disease not viewable in the SAX may cause you to overestimate the FAC of the LV. 
  Fractional Area Change (FAC)
  
Circumferential Shortening
 
Instead of area, circumference can be used to indicate the function of the heart.  Circumferential shortening, which uses circumference, can be calculated by the formula:
 
Circumferential Shortening (CS)
 
CS is Circumferential shortening, EDC is end-diastolic circumference, and ESC is end-systolic circumference.  The circumference is measured along the endocardial border.  While area and circumference can indicate the function of the left ventricle, area and circumference only measures ejection fraction in one plane.  Also, wall thickening, an indication of ventricular function, is not measured. 
  
Fractional Shortening (FS)
 
Fractional shortening or change is the amount of change of the SAX of the left ventricle. Fractional shortening is usually measured utilizing M Mode. Fractional shortening is calculated using the left ventricular short axis in systole and diastole:
 
Fractional Shortening Image
 
Normal values for fractional shortening are 30 percent. Fractional shortening is affected by preload, afterload, and contractility, similar to ejection fraction. If focal regional wall motion abnormalities are present that are not recognized by M Mode evaluation, then the Fractional shortening measurement will overestimate the myocardial function.
 
Fractional shortening of the midwall, instead of the endocardium can be calculated using M mode echocardiography. Fractional shortening of the midwall, FSmw is calculated using a modified two-shell cylindrical model. Constant LV mass during the cardiac cycle is assumed. The two-shell cylindrical model does not require that the inner and out wall thickening fractions be equal. FSmw is not dependent upon relative wall thickness, and, therefore, is useful in left ventricular hypertrophy.
  
Volume
 
Volume measurements look at multiple planes of ventricular function.  By viewing multiple planes, the volume measurements can be more accurate than FAC or FS.  The volume of a cardiac chamber can be measured by two methods:
  • Method of Discs (MOD)
  • Area-Length Method
From the measurement of volumes in systole and diastole, the ejection volume, ejection fraction, end-diastolic volume, end-systolic volume and others can be calculated.
  
Method of Discs (MOD)
 
The method of discs (MOD) can be applied by using a Biplane method or a single plane method. The method of discs divides up the volume being measured into multiple discs and calculates the volume of each disc then adds the volumes together to yield a volume. This is more accurate than utilizing the assumptions of shape used by the M-Mode method.
 
Biplane Method
 
In the biplane method, two orthogonal (90 degrees from each other) planes of the left ventricle are used, usually the 2 chamber view and the 4 chamber view.  The longest axis of the two planes is used for the axis length.  The left ventricular cavity is divided into usually 20 discs.  The area of each of the discs are calculated then an integral of the disc area to calculate the volume of the left ventricle is performed.  The ventricle is divided into discs and the volume calculation is performed.  By using the biplane method, significant anatomical variations of the two planes will be included in the calculation, resulting in a more accurate value for left ventricular volume.
 
Method of Discs (MOD) Formula
Method of Discs Volume Calculation
Method of Discs Volmue Calculation
Method of Discs (MOD) Formula
Volume by Method of Discs (MOD)
Volume by Method of DIscs (MOD) 4 chamber view
 
To describe each disc or ellipse the left ventricular cavity long axis should be measured in two perpendicular views (2CV and 4CV). The diameters from each view can then be used to calculate the area of each disc (pi * a * b). Multiplying each disc by it's thickness yields a volume. The total volume of the discs is the left ventricular volume.
  
Acoustic Quantification (AQ)
 
Acoustic quantification is a special method to detect the endocardial border in an echocardiographic scan.  Once the border has been optimally detected, the volume of the left ventricle (or other chamber) can be calculated in real time.  The patient's ejection fraction, end-diastolic volume, end-systolic volume and cardiac output can be performed beat-to-beat.  This advanced method has many uses but it is technically difficult to do and has a significant error rate if not properly performed.
 
To perform AQ the LVLAX is imaged in 2D mode.  The endomyocardial borders will need to be visible by manipulating the LGC and TGC gain controls.  An region-of-interest (ROI) is drawn.  The area of interest must include the whole LVLAX view and the endocardial movement.  If the endocardial moves beyond the region-of-interest (ROI) then that portion of the LVLAX will not be included in the calculations.  Reproducibility determines the quality of the scan.  If multiple successive cardiac cycles yield values which are close to each other (accuracy) then the scan is considered to be of high quality.  However, if successive values are not accurate then the scan is considered to be of low quality.  The MOD calculation is used for AQ and carries all of the inherent errors in the MOD method.  AQ can calculate the following values:
  • FAC (Fractional Area Change)
  • dA/dt (Area Change versus Time Change)
  • EDA (End Diastolic Area)
  • EDV (End Diastolic Volume)
  • dV/dt (Volume Change versus Time Change)
  • EF (Ejection Fraction)
  • PEF (Peak Ejection Rate)
  • PRFR (Peak Rapid Filling Rate)
  • Atrial Filling Fraction (AFF)
 
 
script



AQ of the LVLAX
1X 2X 3X 4X
Poor Quality AQ
1X 2X 3X 4X
Good Quality AQ
  
SAX/LAX Method
 
Tortoledo et al developed a formula that uses the LAX and the SAX of the left ventricle to calculate the end-diastolic left ventricular volume.  The formula is:
EDV = (SAX * LAX * 3.42) - 6.44 LAX Measurement LAX Measurement
LV Volume by SAX & LAX LAX SAX
  
Area-Length Method
 
The area-length method uses the LAX of the left ventricle and the endocardial area of the left ventricle.  A formula, given below, has been developed to calculate the volume of the left ventricle. 
As above, Acoustic Quantification (AQ) can utilize the Area-Length Method.  Again, geometric errors can occur since the Area-Length method assumes a geometric ellipsoid and the left ventricle may not match that assumption.
Area-Length Formula
Volume by Area-Length Method
Area-Length Formula Area-Length Method
  
Ejection Fraction (EF)
 
Ejection Fraction (EF) is the fraction of left ventricular volume that is ejected during systole.  The difference between the end-diastolic volume (EDV) and the end-systolic volume (ESV) is the stroke volume (SV).  The stoke volume is divided by the end-diastolic volume.  Ejection Fraction is preload, afterload, and contractility dependent.  The formula for Ejection Fraction is:
 
 SV = EDV - ESV
 EF = SV / EDV
 
To calculate an ejection fraction, the volume of the left ventricle can be calculated utilizing the Teicholz or spherical formulas and measuring the left ventricular end-diastolic and end-systolic diameters.  The Teicholz formula is accurate for non dilated hearts.  For dilated, spherical hearts the spherical formula is more accurate.
 
Teicholz Formula Spherical Formula
 
The ejection fraction does not equal fractional area change (FAC).  Ejection fraction is a volume change calculation whereas, fractional area change is an area change calculation.  The table below shows how ejection fraction and fractional area change relate in a normal heart.  Also, the three animations show the central area displacement for different fractional area changes.
 
FAC Ejection Fraction
60%
75%
50%
66%
40%
54%
30%
42%
20%
29%
10%
15%
60 % FAC
40% FAC
20% FAC
FAC and Ejection Fraction Relationship
60 % FAC
40 % FAC
20 % FAC
 
 
Left Ventricle
 
Measurements of the left ventricle should be made at end-diastole or end-systole so calulations of ventricular performance can be performed. Chamber sizes should be indexed to BSA to allow comparisons between patients of different sizes.
 
Linear measurements of the left ventricle should be done in the parasternal long axis so the walls are perpendicular to the ultrasound beam and be measured immediately below the mitral valve leaflet tips. M-Mode can be used but avoid an oblique angle of interrogation so 2D may be preferred for measurements.
 
Teicholz and Quinonnes methods for volume measurements rely on assumptions and are no longer used. Volume measurements that rely on linear measurements have assumptions that do not apply in many disease states. When useing the left ventricle long axis length, the longest length is used in imaged in two different views (apical two and four chamber views). A linear line at the base of the mitral valve represents the base of the heart. If the endocardial border is poorly visualized, contrast agents should be used, recognizing that contranst enhanced imaging results in a larger left ventricular cavity measurement than 2D measurements.   The recommended method of volume measurements by 2D is the biplane method of disks (MOD). If the apex is not visible then the area-length method is the next best calculation, recognizing that assumptions of shape are inherent in the calculation. 3D calculations do not have shape assumptions and are the most accurate when compared to CMR as long as endocardial borders are accurately detected.
 
Method Images Advantages Disadvantages
M-Mode Reproducible
High Temporal Resolution
Single Dimmension
Beam Orientation
2D Perpendicular measurement Single Dimmension Lower Frame Rate
Biplane MOD Less Geometric Assumptions Apex Foreshortened (frequently)
Endocardial Dropout
Miss out-of-plane shape distortion
Area Length More geometric Assumptions Apex Foreshortened (frequently)
Geometric Assumptions
Contrast Enhanced Good Endocardial Borders Basal Shadowing
Same geometric assumptions of method used
3D No geometric assumption
Most accurate
Low temporal resolution
Image quality dependent
Strain Angle Independent
Prognostic
Vendor Dependent
 
Normal Values for Left Ventricular Size and Function by Gender
Parameter Male (±2SD) Female (±2SD)
Dimmensions    
   LVIDs (mm) 50.2 ± 4.1 (42.0-58.4) 45.0 ± 3.6 (37.8-52.2)
   LVIDd (mm) 32.4 ± 3.7 (25.0-39.8) 28.2 ± 3.3 (21.6-34.8)
Biplane MOD    
   LVEDV (mls) 106 ± 22 (62-105) 76 ± 15 (46-106)
   LVESV (mls) 41 ± 10 (21-61) 28 ± 7 (14-42)
   LVSV (mls) 55 ± 12 48 ± 8
     
   LVEDV (mls/m2) 54 ± 10 (34-74) 45 ± 8 (29-61)
   LVESV (mls/m2) 21 ± 5 (11-31) 16 ± 4 (8-24)
   LVSV (mls/m2) 33 ± 5 29 ± 4
     
   LVEF(%) 62 ± 5 (52-72) 64 ± 5 (54-74)
*LVEF = (LVEDV - LVESV)/LVEDV
 
Conclusions
 
Left ventricular volume calculations, although fairly accurate, tend to underestimate the left ventricular volume when compared to angiographic data. Foreshortening of the left ventricle, LV LAX measurement differences, and contrast filling intermyocardial interstices account for the underestimation of echocardiographic left ventricular volume measurements.
 
 
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