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Definition

Left ventricular function measurements are used to quantify how well the left ventricle is able to pump blood through the body with each heartbeat. Left ventricular function (LVF) is an extremely important parameter in echocardiography as it can alter in several diseases. LVF correlates with numerous clinical symptoms such as the severity of dyspnea you can encounter in your patients. LVF is also a key prognostic factor in acute myocardial infarction. LV global systolic function is generally assessed by measuring the difference between the end-diastolic and end-systolic value divided by the end-diastolic value. This can be applied for either a one-dimensional 2D image or in 3D. There are numerous ways to quantify and measure left ventricular function.

Parameters for quantification of left ventricular function

”Eyeballing” of LV function

The so called „eyeballing“ is a visual assessment of left ventricular function. It is based on observation of the regional myocardial function in other words the wall thickening and endocardial motion of several myocardial segments.
Regional deformation such as thickening and shortening or displacement should be the center of this observation, taking into consideration that wall motion abnormalities can be associated with reduced LVF.
Each segment should be assessed in multiple views. According to the guidelines of the American Society of Echocardiography a 17-segment model is used.
Following scoring system for the visual assessment of wall motion abnormalities is recommended:

1) normal or hyperkinetic
2) hypokinetic (reduced thickening)
3) akinetic (absent or negligible thickening)
4) dyskinetic (systolic thinning or stretching)

Following the assessment of these possible wall motion abnormalities and myocardial contractility LVF can be estimated visually.

Concerning wall motion abnormalities stress echocardiography is an important tool to reveal significant coronary artery stenosis. In Echocardiography, depending on the assessment, LVF and ischemia can be overestimated or underestimated. It therefore may prove useful to compare baseline and stress-echocardiographic images of your patients for a better result.

Fractional shortening

Fractional shortening (FS) is a 2D M-Mode method. Using the M-Mode the parameters left ventricular end-systolic diameter (LVESD) and the left ventricular end-diastolic diameter (LVEDD) can be derived. These parameters refer to the size of the ventricle (captured with the M-Mode) at the end of systole and diastole.
By using the formula: (LVEDD - LVESD / LVEDD) x 100 we get the percentage of size differences of the left ventricle as a parameter of how well the left ventricle is contracting itself and therefore reduces the size during systole. Values > 28% are considered to be normal.
It was very popular although there are some important pitfalls. Measuring the global left ventricular function from a linear image when wall motion abnormalities are present can lead to wrong measurements with overestimation or underestimation of the LVF.
Volume measurements based on linear measurements are considered to be inaccurate and according to latest guidelines not recommended anymore. The Teichholz formula (Vol = 7D3/ (2.4+D), where the ventricular diameter D is measured during M-Mode for example, was used over many years to quantify LVF. This formula, as well as the FS, is not mentioned in the current guidelines.

Ejection fraction (EF)

Ejection fraction is derived from the End Diastolic Volume and End Systolic Volume estimates. It is the relation between the amount of blood expelled during each cardiac cycle relative to the size of the ventricle. It can be performed in a 2D or 3D image. Currently the Simpson method, a biplane method, derived from a 2D image is recommended to assess the LV EF.

The following formula is applied for LVF: EF = (EDV-ESV)/EDV x 100

EF= Ejection fraction
EDV= End-Diastolic Volume
ESV= End-Systolic Volume

Left ventricular Ejection Fraction of lower than 52% in men and lower than 54% in women are considered abnormal left ventricular systolic function.

Normal range cut-off values for 2D LVF EF in % according to current ASA Guidelines

Cardiac output/index/Stroke volume

Doppler Echocardiography and 2D imaging can be used to calculate several hemodynamic parameters such as stroke volume, cardiac output and cardiac index. These parameters are important for LVF. They can be derived from two measurements: The velocity time integral (VTI) and the cross-section of the Left ventricular outflow tract (LVOT). The VTI represents the total flow across the area of the sample volume in systole. Therefore a PW doppler is placed in the LVOT. The diameter of the LVOT = the cross section of the LVOT. Stroke volume is the result of VTI multiplied with diameter of LVOT.

Keep in mind: Stroke volume cannot be calculated in patients with a LVOT obstruction!

Some ultrasound devices also calculate Stroke volume on the basis of Simpson Method or M-Mode. A major limitation of this approach is that it cannot be applied when Mitral Regurgitation is present. Due to the limitations, it is rarely used in clinical practice.

Global longitudinal strain rate

Global longitudinal strain (GLS) is a change in the length of the left ventricle in a certain direction related to the baseline length. It is a variation of TDI and provides an evaluation of regional myocardial function and therefore directly reveals the contractile function of the heart. It is a sensitive and early predictor of regional LV dysfunction. This is particularly important as contractile dysfunction often occurs before Ejection fraction drops. GLS measures the shortening of the myocardium as a correlate to contractility in contrast to the Ejection Fraction, which measures volumes. Strain rate is defined as the change of two velocities divided by the distance of the measured points. The commonly used formula is:

GLS (%) = (MLs-MLd)/MLd

Where MLs refers to the myocardial length during systole and MLd to the myocardial length during diastole. So a % of changes in length is the result. As myocardial length during systole is smaller than during diastole, GLS values are negative. In other words, negative GLS indicate active contraction whereas positive values relate to relaxation. A disadvantage in GLS remains - as in TVI - the angle dependence. This can be circumvented by newer techniques such as the Strain rate assessed by speckle tracking (see following chapter).

Strain and Strain rate assessed by speckle tracking

2D Speckle tracking is a new technique. With 2D speckle tracking, similar as with TDI, myocardial velocities and deformation parameters such as strain and strain rate can be calculated. It therefore measures different components of myocardial contraction and delivers information on global contraction by not only assessing the longitudinal strain, but also the circumferential and radial strain. Speckle tracking is an offline method. The recorded digital loops are processed by a software where the myocardium is traced and tracked throughout the cardiac cycle. The software then calculates strain and other deformation parameters. The results are then visualised in various forms. The strain rate can be color encoded in a “bulls eye” display, or curves showing the change in strain over time in form of an anatomical M-mode.
An additional advantage of speckle tracking is the angle independence (in contrast to global strain rate). It has already become an established prognostic value in several clinical conditions. Strain rate is currently used for the quantification of regional myocardial function as it delivers exact information concerning wall motion. A strong disadvantage of speckle tracking remains the vendor dependence of the measurements.

Keep in mind: Because of intervendor and software variability between ultrasound devices, Strain rate assessed by speckle tracking should be performed using the same equipment and software.

3D Speckle Tracking Echocardiography (STE) can capture the motion of speckls independent from their direction. This carries an advantage in comparison to 2D STE. 3D STE measurements of LV volumes were comparable to MRT values. Furthermore a significant higher number of segments can be analysed with 3D STE. The greatest pitfall of 3D STE is the dependence on image quality. Normal values still vary among publications and depend on the equipment used. A consensus document of lower limits of normal range with Doppler described 18,5% - for longitudinal and 44,5% for radial strain as well as 1.00 and 2.45 sec ^-1 for longitudinal and radial SR.

Contractility (dp/dt)

Dp/dt is a parameter of myocardial isovolumetric contraction measuring the rate of pressure increase within the left ventricle during systole. In short words: The faster the left ventricle is able to build up pressure, the better its function is. This rise in pressure can be captured by the mitral regurgitation profile (CW Doppler over mitral valve - velocities represent pressure gradients applying the Bernoulli Equation). The velocity is measured at two different time points: 1m/s and 3 m/s. If we calculate the difference between these two timepoints the result represents the time it takes for a 32 mmHg change to occur within the left ventricle.

The formula therefore is: dp/dt = 32 mmHg/ time (seconds)

This method also carries several pitfalls. For example in several pathologies and diseases such as LBBB, RV pacing and WPW syndrome dp/dt may be reduced due to dyssynchrony and not because of reduced contractility. Besides a good Mitral regurgitation signal is needed for this calculation which can be difficult depending on the image quality. Only small changes in the CW spectrum lead to significant differences in dp/dt.

Tei index / Myocardial performance index

The Tei index or myocardial performance index (MPI) is a parameter for global ventricular performance. The Tei index consists of 3 variables which are derived from Doppler spectrum.

The formula is: MPI = (IVCT + IVRT) / ET

IVCT = Isovolumetric contraction time
IVRT = Isovolumetric relaxation time
ET= Ejection time

When systolic dysfunction is present in patients IVCT will increase and ET decrease. Applied in the formular this will lead to an increased MPI. Normal range is considered at 0,39+/-0,05. An MPI over 0,5 is considered abnormal.

TDI velocity of the myocardium

Tissue Doppler Velocity imaging (TDI) is a signal which correlates with myocardial motion. There is a color display over the anatomical 4 chamber view, a pulsed wave doppler signal is then placed on different myocardial regions. Different velocities within the region of interest can hereby be determined. Usually TDI is placed on the septal or lateral mitral annulus. The annular velocity in systole has shown a correlation with left ventricular ejection fraction. TDI is angle dependent so we can only measure velocities parallel to the ultrasound beam.
Disadvantages of TDI is that we asses the LVF only in a specific area depending on where we place the sample volume. TDI is a technique to detect impaired longitudinal systolic function. It is not present in the current guidelines for quantification of left ventricular systolic function.

MAPSE - Mitral Annular Plane Systolic Excursion

Mitral Annular Plane Systolic Excursion is another approach to quantify LVF. It is an M-Mode derived marker of longitudinal Left ventricular function. For the calculation the M-Mode is placed in an apical four-chamber view on the lateral mitral annulus, measuring the excursion of mitral valve during systole and diastole. A MAPSE > 1 cm is considered normal.
The MAPSE value can also be used in a formula to derive the Ejection Fraction.
One approach, tested in adult males with impaired LVF is: EF = 4.8 × MAPSE (mm) + 5.8.

Currently this formula is rarely used in clinical practice and in the current guidelines, as the recommended approaches to quantify LVF, MAPSE are not listed.

3D Echocardiography

Rapid technological developments during the last years have led to newer techniques. With the aid of matrix array transducers real time images of the beating heart can be created. This newer technique carries several advantages as it does not rely on geometrical assumptions, it is unaffected by foreshortening, it is more accurate and reproducible and thus reduces the limitations of 2D imaging.
Disadvantages of 3D Echocardiography are the dependence on image quality and yet there are less published data on normal values. Normal range cut-off values for 3D LVF EF vary among ethnic populations. Still there are no standard normal values.

According to the current guidelines 3D assessment of LVF should only be performed in laboratories with experience in 3D echocardiography and when image quality makes it feasible.

Summary

To sum up, there are numerous indications for measuring LVF. In various clinical situations Echocardiography and the assessment of the function are essential as they can give important information in STEMI patients, cardiac arrest, cardiogenic shock or hemodynamic instability, heart failure (with reduced, mid-range or preserved EF) to name a few. In all these situations echocardiography and LVF can give a good overview and provide important information for further treatment and therapeutic decisions in your patients.

For further insight in this complex topic take a look at following link:
https://www.123sonography.com/book/294

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