By Jan Masila
United States
What is the LV Function? This is a question that is heard often with cardiac imaging due to many important diagnostic, prognostic, and treatment decisions resting upon left ventricle (LV) morphology/function analysis. “Many echocardiographic departments perform “eyeball” analysis of global and regional LV function and provide visual estimates of ejection fraction because existing M Mode and 2D Echo quantification methods can be both time consuming and difficult to perform.”(1) Visual estimation of global and regional ventricular function are termed qualitative. Quantitative measures to evaluate ventricular function include things such as stroke volumes (SV) and ejection fraction (EF). Two-dimensional Echo and M Mode echocardiography provide the ability to quantitatively evaluate ventricular function through the use of endocardial border tracing and Doppler but geometric assumptions can limit the accuracy. “Advances in 3D echocardiography instrument technology and computer processing power have led to a potentially powerful clinically applicable technique to evaluate LV function. Three dimensional echocardiography has shown to be an accurate and reproducible method for LV quantitation, mainly, by avoiding the use of geometric assumptions which are inherent in 2D and M Mode.” (2) Currently, there are several tools and methods available to assist in providing the answer to this commonly asked question – “What is the LV function?”.
Qualitative evaluation is a fast answer that is subjective person to person. The parasternal long-axis, parasternal short axis, apical four chamber, two chamber and three chamber view offer views of the various walls. The parasternal long-axis shows the anterior septum and posterior wall; parasternal short-axis the inferior, medial, anterior septum and anterior, lateral, posterior and inferior wall; apical four chamber the lateral and septum; two chamber the anterior and inferior wall; the three chamber the posterior and antero-septal wall. Also, keep in mind that the shape is important. The normal shape of the LV is symmetrically conal shaped or “bullet shaped”. These views together help provide a more complete picture of the LV for an experienced eye to determine a qualitative estimate of the LV function. Emergency rooms are a good example of when a quick evaluation of LV function may be needed, either to eliminate heart dysfunction or initiate a more in depth quantitative investigation. In other words use a qualitative approach to determine if there is primary cardiac dysfunction versus non-primary cardiac dysfunction. Qualitative functions can be classified as normal, mildly impaired, moderately impaired and severely impaired. Quantitative evaluation of ventricular function utilizes linear dimension measurements, ventricular volumes, LV mass and wall stress as parameters of ventricular function.
Tissue Doppler imaging (TDI) provides the ability to quantitatively measure the velocity of the myocardium lengthening in systole and shortening in diastole. It is a useful echocardiographic tool for quantitative assessment of LV systolic and diastolic function. Tissue Doppler is a method that is independent of volume to measure diastolic function. Often myocardial velocities are recorded from the apical four chamber view with a PW sample at the basal portion and/or mitral annulus of the LV. Pulse wave samples are taken from the basal septal side and/or the lateral wall side. Relaxation abnormalities, restrictive physiology and pseudo-normalization can be determined via tissue Doppler imaging. TDI is able to determine the percent change in the length of cardiac muscle, which can be use in evaluation regional systolic function. Tissue Doppler imaging is a reproducible echocardiographic tool which provides a quantitative assessment of both global and regional function and timing of myocardial events.
What is the LV Function? This is a question that is heard often with cardiac imaging due to many important diagnostic, prognostic, and treatment decisions resting upon left ventricle (LV) morphology/function analysis. “Many echocardiographic departments perform “eyeball” analysis of global and regional LV function and provide visual estimates of ejection fraction because existing M Mode and 2D Echo quantification methods can be both time consuming and difficult to perform.”(1) Visual estimation of global and regional ventricular function are termed qualitative. Quantitative measures to evaluate ventricular function include things such as stroke volumes (SV) and ejection fraction (EF). Two-dimensional Echo and M Mode echocardiography provide the ability to quantitatively evaluate ventricular function through the use of endocardial border tracing and Doppler but geometric assumptions can limit the accuracy. “Advances in 3D echocardiography instrument technology and computer processing power have led to a potentially powerful clinically applicable technique to evaluate LV function. Three dimensional echocardiography has shown to be an accurate and reproducible method for LV quantitation, mainly, by avoiding the use of geometric assumptions which are inherent in 2D and M Mode.” (2) Currently, there are several tools and methods available to assist in providing the answer to this commonly asked question – “What is the LV function?”.
To begin with, the “function” or “job” of the LV should be understood. The hearts basic function is that of a pump. The LV is the largest pumping chamber of the heart and is muscular and thick as compared to the right ventricle. Literally the LV’s job is to pump oxygenated blood systemically throughout the body and as mentioned previously, this function is integral in diagnosing and making treatment decisions. The question of, “What is the LV function?” that is often asked, refers to how well the LV is doing it’s job. A normal left ventricle should squeeze or contract (systole) forcibly enough to eject blood to open the aorta and pump blood systemically. Typically, a normal, healthy adult hearts pump volume is 5 ml/min resting. A normal heart should also relax quickly after each systolic contraction so that it can fill properly (diastole) with blood from the pulmonary veins. “Left ventricular function and pump performance depend on contractility (the basic ability of the myocardium to contract), preload (initial ventricular volume or pressure), afterload (aortic resistance or end systolic wall stress) and ventricular geometry.”(3)
Qualitative evaluation is a fast answer that is subjective person to person. The parasternal long-axis, parasternal short axis, apical four chamber, two chamber and three chamber view offer views of the various walls. The parasternal long-axis shows the anterior septum and posterior wall; parasternal short-axis the inferior, medial, anterior septum and anterior, lateral, posterior and inferior wall; apical four chamber the lateral and septum; two chamber the anterior and inferior wall; the three chamber the posterior and antero-septal wall. Also, keep in mind that the shape is important. The normal shape of the LV is symmetrically conal shaped or “bullet shaped”. These views together help provide a more complete picture of the LV for an experienced eye to determine a qualitative estimate of the LV function. Emergency rooms are a good example of when a quick evaluation of LV function may be needed, either to eliminate heart dysfunction or initiate a more in depth quantitative investigation. In other words use a qualitative approach to determine if there is primary cardiac dysfunction versus non-primary cardiac dysfunction. Qualitative functions can be classified as normal, mildly impaired, moderately impaired and severely impaired. Quantitative evaluation of ventricular function utilizes linear dimension measurements, ventricular volumes, LV mass and wall stress as parameters of ventricular function.
LV internal dimensions and wall thickness are measured routinely in 2D echocardiography. Most often this is done in the TTE parasternal long-axis view, at the level of the mitral valve leaflet tips. These measurements can also be made in M-Mode. “M-mode is most helpful and used for timing of rapid cardiac motions, precise measurement of cardiac dimensions, or further evaluation of structures seen on 2D images (such as suspected vegetations) to aid in their identification.”(3) Transesophageal Echocardiography (TEE) measurement of LV internal dimensions are made in a transgastric two-chamber view a the junction between the basal third and apical third of the ventricle. TEE Wall thickness is measured in the transgastric short-axis.(3) The TEE process involves guiding a prove down the throat into the esophagus which provides a clearer image due to the closer proximity to the heart. Wall thickness is measured using leading-edge to leading-edge technique. Chamber size is measured in end-diastole and end-systole. These measurements provide an evaluation of contractility of the myocardium.
Various assumptions about LV shape have been used to derive formulas for calculating ventricular volumes, linear dimensions, cross-sectional areas, (2D) or 3D volumes. Some of the common formulas used are listed below:
• LV Imaging: SV = EDV(end diastolic volume) – ESV (end systolic volume) (3)
• Doppler: SV = CSA * VTI (3)
• Cardiac Output (Q) = SV * heart rate (3)
• Cardiac Index (CI) = Q / Body Surface Area (BSA) = SV × HR/BS
• EF = {(EDV-ESV) / EDV] * 100 (4) or EF = (SV/EDV) * 100% (3)
Stroke volume is a measure of the amount of blood pumped from a ventricle of the heart in one beat. Cardiac output is the volume of blood pumped by the heart per minute.(3) EF is a measurement of the fraction of blood leaving your heart each time it contracts. Specifically, when referring to the left ventricular ejection fraction it is termed LVEF. The normal for the above quantifications are listed below:
• Stroke Volume : 75-100 ml
• Cardiac Output: 4-8 L/min
• Cardiac Index: 2.4 – 4.2L/min/m2
• Ejection Fraction: ≥ 55% (4)
“One method for determination of ventricular volumes (EDV and ESV) is based on endocardial border tracing at end diastole and end systole in one or more tomographic planes on TTE or TEE images.” (3) In order to accomplish this, the image needs to include the apex and show adequate and accurate visualization of the endocardial borders. The best accuracy is obtained from multiple view tracing and with the fewest geometric assumptions. The Biplane Simpson’s is an example of a method used in acquiring LV volumes. Tracing is done during end diastole and end systole in the apical 4 and 2 chamber view. “The greatest accuracy would be expected with 3D reconstructions that use data from multiple tomographic images of known orientation and make no geometric assumptions. Real time data sets provide the promise of rapid measurement of LV volumes, without geometric assumptions.” (3) “The latest generation of advanced quantification tools available with the iE33 offer the possibility to use voxel data instead of interpolated slice data to generate a full surface mesh for a more assumption free echocardiograph analysis of global volume and ejection fraction.” (2) The Live 3D image is a pyramid-shaped volume of information, allowing you to visualize anatomy in three dimensions: lateral, elevation, depth.“Other useful parameters to evaluate LV function include wall stress and LV mass.”(3) “LV mass is the total weight of the myocardium, derived by multiplying the volume of myocardium by the specific density of cardiac muscle.” (3) LV mass = 1.05(total volume-chamber volume). Endocardial border tracing is used for this method but has limitations in that epicardial definition is often not adequate. Via 3D echocardiography, LV mass can potentially be better identified through the use of analysis software to calculate an epicardial cast of the ventricle. The volume of this cast can be subtracted from an endocardial cast to give the volume of the LV myocardium. By multiplying this by the specific gravity of myocardium, LV mass is derived. 3D echo can provide direct and assumption-free analysis of LV mass and volume with the possibility to accurately assess volumes in other cardiac chambers.
Afterload can be thought of as the "load" that the heart must eject blood against. In simple terms, the afterload is closely related to the aortic pressure. “More precisely, afterload is related to ventricular wall stress (σ), where σ = P (pressure) * R (radius)/2Th (wall thickness). LV wall stress is the force per unit area exerted on the myocardium.” (5) In other words wall stress is wall tension divided by wall thickness. This relationship is similar to the Law of LaPlace, which states that wall tension is proportionate to the pressure times radius for thin-walled spheres or cylinders. Over stressed walls lead to hypertrophy, which can eventually lead to dilation of the ventricular chamber.
Tissue Doppler imaging (TDI) provides the ability to quantitatively measure the velocity of the myocardium lengthening in systole and shortening in diastole. It is a useful echocardiographic tool for quantitative assessment of LV systolic and diastolic function. Tissue Doppler is a method that is independent of volume to measure diastolic function. Often myocardial velocities are recorded from the apical four chamber view with a PW sample at the basal portion and/or mitral annulus of the LV. Pulse wave samples are taken from the basal septal side and/or the lateral wall side. Relaxation abnormalities, restrictive physiology and pseudo-normalization can be determined via tissue Doppler imaging. TDI is able to determine the percent change in the length of cardiac muscle, which can be use in evaluation regional systolic function. Tissue Doppler imaging is a reproducible echocardiographic tool which provides a quantitative assessment of both global and regional function and timing of myocardial events.
The left ventricle global and segmental systolic function can be evaluated with stress echocardiography. There are several contraindication for performing a stress echo such as pregnancy, acute myocardial infarction within two days, uncontrolled cardiac arrhythmias, etc. Exercise and pharmacologic stress testing are the two common methods. The response to exercise is an increase in left ventricular contractility, which can highlight myocardial ischemia. “This shows as a decrease/cessation of contractility in the myocardial region supplied by the stenosed vessel.” (4) The process of stress echo is to take resting images and then post exercise images after achievement of a predicted heart rate. A sonographer has only 60 second to take the post exercise images. With 3D Echo, it is possible to acquire volumetric images of the LV from the apical approach, with display of simultaneous apical four, two and long axis views. This approach may be particularly useful for evaluation for regional LV function during stress echocardiography. Stress echo is another option for a more in depth look into the LV function for patients who are suspected of having LV dysfunction but resting evaluations are not conclusive. This is another tool to add to our LV function evaluation toolbox.
There are several methods other than echocardiography used to evaluate LV function. “Cardiac magnetic resonance (CMR) also known as MRI, provides precise and accurate measure of LV volumes, mass, EF and cardiac output.”(3) CMR is noninvasive and utilizes a powerful magnetic field, radio frequency pulses and a computer to produce detailed pictures. MRI doesn't use ionizing radiation or carry any risk of causing cancer. It should be noted that a 3-D CMR also is becoming available. An MRI can be considered the gold standard but cost/benefits are usually taken into consideration. The cost of an MRI itself, time and equipment trends higher than the standard echocardiography with reasonably comparable results. Angiography is also used and is a imaging technique that is done by injecting a radiopaque contrast agent into the blood vessel and imaging using x-ray based techniques such as fluouroscopy. Radionuclide ventriculography is also available but it has largely been replaced by echocardiography because it is less expensive and does not require radiation. The choice of which technique to use is steered by cost/benefit, availability, and other clinical questions that may be present.
There are numerous methods available to address the evaluation of LV function, both globally and regionally. As a summarization, global function is typically described as normal, depressed, or hyperdynamic. Depressed systolic function may be characterized both in severity (mild, moderate, severe) and focality (global or regional). Systolic function in general can be a major prognostic factor in acute cardiac disease. Hyperdynamic function is frequently associated with high-output syndromes (e.g. anemia, thyrotoxicosis), hypertrophic cardiomyopathies, or in response to exercise or pharmacologic stress (e.g. dobutamine). Regional function refers to the specific regions of the LV wall. It can be used to narrow down a specific area of wall motion abnormality. The LV is divided into segments, which correlate to coronary artery territories, which aids in determining which artery may have a problem. It is important to know which coronary arteries supply blood to which wall segments. Qualitative evaluations tend to be global in nature and regional evaluation more quantitative.
In conclusion, assessment of left ventricular function is one of the most valuable assessments in cardiac evaluation which is why the question, “What is the LV function?” is so commonly requested. There are two ways to answer the question; qualitatively and quantitatively. Both are common practice and can go hand in hand. Qualitative answers are subjective and can vary from person to person and/or from echo to echo. The LV function is that first look in the investigation in evaluation of the hearts function. It points the way and provides guidance to diagnosing and determining treatment plans. There are different paths and technology that are available to put together a complete picture of the LV function. With existing and advancing technology, the picture is on the way to becoming more clear, dynamic and precise for both qualitative and quantitative answers for LV function.
References
(1) “Role of Real Time 3D Echocardiography in Evaluating the Left Ventricle”, Mark J. Monaghan, BMJ Publishing Group and British Cardiac Society, Department of Cardiology, King's College Hospital, Denmark Hill, London, © 2006,
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1861009/
(2) “Why 3D Echo”
(3) “Left and Right Ventricular Systolic Function”, p126-154, Clinical Echocardiography 4th Edition, Catherine M. Otto, MD, ©2004, ISBN 978-1-4160-5559-4
(4) “Evaluation of Left Ventricular Systolic and Diastolic Function”, p176 – 190, The Echocardiographer’s Pocket Reference 3rd Edition, Terry Reynolds, BS RDCS, ©2010
(5) “Cardiac Afterload”, Cardiovascular Physiology Concepts, Richard E. Klabunde, Ph.D., Ohio University Athens, Revised 08/07/07, http://www.cvphysiology.com
