Introduction
Echocardiography in its current form, has becomean invaluable tool in a modern cardiac intensive care unit environment. Coupled with a clinical examination and monitoring techniques, echocardiography can provide real-time rapid and reliable diagnostic answers that are invaluable to patient care. This noninvasive test can be used to reliably evaluate cardiac anatomy of both normal hearts and those with congenital heart disease and has replaced cardiac angiography for the preoperative diagnosis of the majority of congenital heart lesions. In congenital or acquired cardiac disease, echocardiography may be further used to estimate intracardiac pressures and gradients across stenotic valves and vessels, determine the directionality of blood flow and pressure gradient across a defect, and examine the coronary arteries. Within the realm of critical care, echocardiography is useful to quantitative cardiac systolic and diastolic function, detect the presence of vegetations from endocarditis, and examine the cardiac structure for the presence of pericardial fluid and chamber thrombi. As with all tools, however, a thorough understanding of its uses and limitations are necessary before relying upon the information it provides.
Echocardiography in its current form, has becomean invaluable tool in a modern cardiac intensive care unit environment. Coupled with a clinical examination and monitoring techniques, echocardiography can provide real-time rapid and reliable diagnostic answers that are invaluable to patient care. This noninvasive test can be used to reliably evaluate cardiac anatomy of both normal hearts and those with congenital heart disease and has replaced cardiac angiography for the preoperative diagnosis of the majority of congenital heart lesions. In congenital or acquired cardiac disease, echocardiography may be further used to estimate intracardiac pressures and gradients across stenotic valves and vessels, determine the directionality of blood flow and pressure gradient across a defect, and examine the coronary arteries. Within the realm of critical care, echocardiography is useful to quantitative cardiac systolic and diastolic function, detect the presence of vegetations from endocarditis, and examine the cardiac structure for the presence of pericardial fluid and chamber thrombi. As with all tools, however, a thorough understanding of its uses and limitations are necessary before relying upon the information it provides.
Principles of Echocardiography
Echocardiography uses ultrasound technology to image the heart and associated vascular structures. Ultrasound is defined as sound frequencies above the audible range of 20,000 cycles per second. The primary components of an ultrasound machine include a transducer and a central processor. The transducer converts electrical to mechanical (sound) energy and vice versa. Electrical energy is applied to piezoelectric crystals within the transducer resulting in the generation of mechanical energy in the form of a series of sinusoidal cycles of alternating compression and rarefaction. The energy produced travels as a directable beam which may be aimed at the heart. The sound beam travels in a straight line until it encounters a boundary between structures with different acoustical impedance, such as between blood and tissue. At such surfaces, a portion of the energy is reflected back to the same crystals within the transducer, and the remaining attenuated signal is transmitted distally. Within the ultrasound, machine is circuitry capable of measuring the transit time for the beam to travel from the transducer to a given structure and back again then calculate the distance traveled. A cardiac image is constructed from the reflected energy, or so called ultrasound echoes.
Echocardiography uses ultrasound technology to image the heart and associated vascular structures. Ultrasound is defined as sound frequencies above the audible range of 20,000 cycles per second. The primary components of an ultrasound machine include a transducer and a central processor. The transducer converts electrical to mechanical (sound) energy and vice versa. Electrical energy is applied to piezoelectric crystals within the transducer resulting in the generation of mechanical energy in the form of a series of sinusoidal cycles of alternating compression and rarefaction. The energy produced travels as a directable beam which may be aimed at the heart. The sound beam travels in a straight line until it encounters a boundary between structures with different acoustical impedance, such as between blood and tissue. At such surfaces, a portion of the energy is reflected back to the same crystals within the transducer, and the remaining attenuated signal is transmitted distally. Within the ultrasound, machine is circuitry capable of measuring the transit time for the beam to travel from the transducer to a given structure and back again then calculate the distance traveled. A cardiac image is constructed from the reflected energy, or so called ultrasound echoes.
Differing properties of tissues affect the portion of acoustic energy transmitted versus reflected. For example, air reflects the majority of the signal it receives and, therefore, prevents images from being obtained through windows where it is present. Anything hindering or augmenting the reflection of this acoustic signal, such as air, bone, dressings, an open chest, or lines, tubes, or other foreign bodies, will diminish the overall quality of the examination. Therefore, in the intensive care unit, an ultrasound study may be limited by difficulty in finding a good acoustic window to allow for accurate analysis.
The Anatomical Echocardiographic Examination
In order to obtain the best imaging windows, whenever possible, patients are placed in a left lateral decubitus position during a transthoracic echocardiogram. During two-dimensional (2D) echocardiography, all planes are described in reference to the heart and not the heart’s position within the body. For a complete pediatric study, standard views (see Fig.1–5) are obtained from the high left chest just lateral to the sternum (parasternal window), the left lateral chest just inferior and lateral to the nipple (apical window), sub-xyphoid area (subcostal window), and the suprasternal notch (suprasternal window). In patients with more complex anatomy, additional windows, such as the high right parasternal border, may be used to obtain additional information.
In order to obtain the best imaging windows, whenever possible, patients are placed in a left lateral decubitus position during a transthoracic echocardiogram. During two-dimensional (2D) echocardiography, all planes are described in reference to the heart and not the heart’s position within the body. For a complete pediatric study, standard views (see Fig.1–5) are obtained from the high left chest just lateral to the sternum (parasternal window), the left lateral chest just inferior and lateral to the nipple (apical window), sub-xyphoid area (subcostal window), and the suprasternal notch (suprasternal window). In patients with more complex anatomy, additional windows, such as the high right parasternal border, may be used to obtain additional information.
Fig.1 Standard echocardiographic image planes from the high left chest just lateral to the sternum (parasternal window (a) and (b)), the left lateral chest just inferior to the nipple (apical window (c)), sub-xyphoid area (subcostal window (d)), and the suprasternal notch (suprasternal window (e) and (f)). RA right atrium; RV right ventricle; LA left atrium; LV left ventricle; Ao aortic valve; CS coronary sinus; RVOT right ventricular outflow tract; SVC superior vena cava (drawing from Steven P. Goldberg, MD)
1. Parasternal Window
In the anatomically normal heart, the parasternal window allows visualization of the heart aligned along its long axis and short axis. In the long axis (Fig.1a), the left ventricular inflow and outflow tracts can be seen well. As a result, comments can be made from this view regarding the aorta, including its annulus, the sinuses of Valsalva, and the proximal portion of the ascending aorta, as well as its relationship to the mitral valve. Additionally, the ballet-slipper appearance of the left ventricle is featured as the inferoposterior wall and interventricular septum are visualized. The anterior and posterior leaflets of the mitral valve can be visualized. By angulating the transducer and performing a sweep, the right ventricle is brought into focus and an examination of both its inflow including the right atrium and tricuspid valve and its outflow tract, including the pulmonary valve can be performed.
In the anatomically normal heart, the parasternal window allows visualization of the heart aligned along its long axis and short axis. In the long axis (Fig.1a), the left ventricular inflow and outflow tracts can be seen well. As a result, comments can be made from this view regarding the aorta, including its annulus, the sinuses of Valsalva, and the proximal portion of the ascending aorta, as well as its relationship to the mitral valve. Additionally, the ballet-slipper appearance of the left ventricle is featured as the inferoposterior wall and interventricular septum are visualized. The anterior and posterior leaflets of the mitral valve can be visualized. By angulating the transducer and performing a sweep, the right ventricle is brought into focus and an examination of both its inflow including the right atrium and tricuspid valve and its outflow tract, including the pulmonary valve can be performed.
The transducer may be rotated 90° providing a series of short-axis views (Fig.1b) that assist in the evaluation of the chambers of the heart, the semilunar and atrioventricular valves, and the coronary arteries. Sweeping from the apex of the heart toward the base will allow a close cross-sectional examination of the ventricular chambers. The normal left ventricle has circular geometry with symmetric contraction, whether it is visualized at the level of the mitral valve, papillary muscles, or apex. In contrast, the normal right ventricle appears as a more trabeculated crescent-shaped structure when visualized at or below the level of the mitral valve. Sweeping farther toward the base of the heart, the mitral valve’s papillary muscles and the valve itself are viewed. Progressing to the base of the normal heart, the tri-leaflet aortic valve takes the center stage with the right ventricular outflow tract and pulmonary wrapping in an inverted “U” anteriorly and leftward. Additionally a portion of the atrial septum and the tricuspid valve may be profiled. Finally, continuing the sweep allows for the examination of the atrial appendages, ascending aorta in cross-section and branch pulmonary arteries.
Fig.1b (continued)
2. Apical Window
For those not trained in echocardiography, the images obtained with the transducer in the apical position (Fig.1c) are perhaps the most intuitive as it allows for visualization of all four chambers and valves in the heart with a simple left-to-right orientation. Imaging is begun in the four-chamber view, in which the anatomic right and left ventricles may be identified. Sweeps of the transducer from this position identify the posterior coronary sinus and may indicate abnormalities such as a left superior vena cava or unroofed coronary sinus. Proceeding more anteriorly to a five-chambered view, the atrial and ventricular septa may be visualized looking for defects and the left ventricular outflow tract and ascending aorta may be examined. The four chamber view allows for the examination of the anterior and posterior mitral valve leaflets and pulmonary veins as they enter the left atrium. By rotating the transducer to 90° from the four-chamber view, a two-chamber view of the left ventricle and left atrium can be obtained to evaluate the anterior and posterior left ventricular wall function.
For those not trained in echocardiography, the images obtained with the transducer in the apical position (Fig.1c) are perhaps the most intuitive as it allows for visualization of all four chambers and valves in the heart with a simple left-to-right orientation. Imaging is begun in the four-chamber view, in which the anatomic right and left ventricles may be identified. Sweeps of the transducer from this position identify the posterior coronary sinus and may indicate abnormalities such as a left superior vena cava or unroofed coronary sinus. Proceeding more anteriorly to a five-chambered view, the atrial and ventricular septa may be visualized looking for defects and the left ventricular outflow tract and ascending aorta may be examined. The four chamber view allows for the examination of the anterior and posterior mitral valve leaflets and pulmonary veins as they enter the left atrium. By rotating the transducer to 90° from the four-chamber view, a two-chamber view of the left ventricle and left atrium can be obtained to evaluate the anterior and posterior left ventricular wall function.
Fig.1c (continued)
3. Subcostal Window
For pediatric patients with complex cardiac anatomy, the subcostal position (Fig.1d and Fig.1.e) provides the most detailed information and is often thebest starting place. In order to obtain images in this position, patients are placed supine with the transducer in the subxiphoid position. In larger cooperative patients beyond the infancy period, image quality may be improved by having the patient participate in the examination with held inspiration that allows the heart to move downward toward the transducer. Initial views in this position should determine visceral situs as well as the relationship of the inferior vena cava and aorta. Subsequent views and sweeps will provide detailed analysis of the atrial septum as well as the images related to the ventricular septum, the atrioventricular valves, atrial and ventricular chambers, and drainage of systemic veins. With the rotation of the transducer both ventricular outflow tracts may be visualized. Additionally in some patients the branch pulmonary arteries and the entire aorta may be examined from this position.
For pediatric patients with complex cardiac anatomy, the subcostal position (Fig.1d and Fig.1.e) provides the most detailed information and is often thebest starting place. In order to obtain images in this position, patients are placed supine with the transducer in the subxiphoid position. In larger cooperative patients beyond the infancy period, image quality may be improved by having the patient participate in the examination with held inspiration that allows the heart to move downward toward the transducer. Initial views in this position should determine visceral situs as well as the relationship of the inferior vena cava and aorta. Subsequent views and sweeps will provide detailed analysis of the atrial septum as well as the images related to the ventricular septum, the atrioventricular valves, atrial and ventricular chambers, and drainage of systemic veins. With the rotation of the transducer both ventricular outflow tracts may be visualized. Additionally in some patients the branch pulmonary arteries and the entire aorta may be examined from this position.
Fig.1d (continued)
Fig.1e (continued)
4. Suprasternal Window
The views are obtained in this position by placing the transducer in the suprasternal notch (Fig.1.f) with the neck extended. The suprasternal longand short-axis views provide detailed information regarding arch sidedness, anomalies in the ascending and descending aorta and head and neck vessels, the size and branching of the pulmonary arteries, as well as anomalies of systemic and pulmonary venous systems.
The views are obtained in this position by placing the transducer in the suprasternal notch (Fig.1.f) with the neck extended. The suprasternal longand short-axis views provide detailed information regarding arch sidedness, anomalies in the ascending and descending aorta and head and neck vessels, the size and branching of the pulmonary arteries, as well as anomalies of systemic and pulmonary venous systems.
Fig.1f (continued)
M-Mode Imaging
One of the earliest applications of ultrasound technology that remains an important tool in the evaluation of cardiac function, dimension, and timing, the M-mode echo provides an “ice-pick” view of the heart. An M-mode echo is obtained with the ultrasonic transducer placed along the left sternal border and directed toward the part of the heart to be examined. A single line of interrogation is repeatedly produced and the resultant image is displayed with time along the x-axis and distance from the transducer along the y-axis (see Fig. 2). M-mode obtains an estimate of ventricular function by measuring the short axis shortening fraction and wall thickness.
One of the earliest applications of ultrasound technology that remains an important tool in the evaluation of cardiac function, dimension, and timing, the M-mode echo provides an “ice-pick” view of the heart. An M-mode echo is obtained with the ultrasonic transducer placed along the left sternal border and directed toward the part of the heart to be examined. A single line of interrogation is repeatedly produced and the resultant image is displayed with time along the x-axis and distance from the transducer along the y-axis (see Fig. 2). M-mode obtains an estimate of ventricular function by measuring the short axis shortening fraction and wall thickness.
Fig.2 M-mode echocardiography obtained in the parasternal short axis through the right and left ventricular chambers at the level of the papillary muscles. LVEDD left ventricular end-diastolic dimension; LVESD left ventricular end-systolic dimension
Doppler Evaluation
Frequently in an intensive care setting the clinician is concerned with new or residual flow disturbances from shunt lesions, an abnormal cardiac valve, or narrowing of a blood vessel. While 2D echocardiography determines anatomical relationships, additional information regarding movement of the blood or myocardium is provided by looking for Doppler shifts in the reflected ultrasound waves. The Doppler principle, first described by Johann Christian Doppler, states that for a stationary object, the frequency of ultrasound reflected is identical to the transmitted frequency. Inherently the heart and the blood it pumps do not fit this basic definition. Therefore, when performing a cardiac ultrasound, the moving objects alter the frequency of the reflected signal (the Doppler shift) according to the direction and velocity with which they are moving in relation to the fixed transducer. Additional insights to intracardiac and vascular hemodynamics may be obtained when velocity data is collected. Doppler data are typically displayed as velocity rather than the actual frequency shift. The velocities can then be translated into pressure data using the modified Bernoulli equation: P1 – P2= 4[(V2)2 – (V1)2]. If one assumes that the level of obstruction and therefore the velocity of V1 is negligible compared with the obstruction at V2 the formula becomes even simpler: DP = 4(Vmax)2. Although the modified Bernoulli equation can only be applied in appropriate situations, it does help predict the pressure drop across an abnormal valve or septal defect to give a general estimate of the severity of the lesion which can prove to be valuable information to help manage patients in the intensive care setting.
Frequently in an intensive care setting the clinician is concerned with new or residual flow disturbances from shunt lesions, an abnormal cardiac valve, or narrowing of a blood vessel. While 2D echocardiography determines anatomical relationships, additional information regarding movement of the blood or myocardium is provided by looking for Doppler shifts in the reflected ultrasound waves. The Doppler principle, first described by Johann Christian Doppler, states that for a stationary object, the frequency of ultrasound reflected is identical to the transmitted frequency. Inherently the heart and the blood it pumps do not fit this basic definition. Therefore, when performing a cardiac ultrasound, the moving objects alter the frequency of the reflected signal (the Doppler shift) according to the direction and velocity with which they are moving in relation to the fixed transducer. Additional insights to intracardiac and vascular hemodynamics may be obtained when velocity data is collected. Doppler data are typically displayed as velocity rather than the actual frequency shift. The velocities can then be translated into pressure data using the modified Bernoulli equation: P1 – P2= 4[(V2)2 – (V1)2]. If one assumes that the level of obstruction and therefore the velocity of V1 is negligible compared with the obstruction at V2 the formula becomes even simpler: DP = 4(Vmax)2. Although the modified Bernoulli equation can only be applied in appropriate situations, it does help predict the pressure drop across an abnormal valve or septal defect to give a general estimate of the severity of the lesion which can prove to be valuable information to help manage patients in the intensive care setting.
Of note, during Doppler imaging it is clinically important to recognize the angle of interrogation of blood flow and its impact on the accuracy of our velocity measures. It is important when performing Doppler studies that the line of beam interrogation should be directly in the line of flow, resulting in as little distortion of data as possible. The more off-angle the approach is, the increasingly more severe the underestimation of the true velocity will be. For practical purposes, an angle of interrogation less than 20° is essential to ensure clinically accurate information.
Two commonly used techniques are pulsed and continuous wave Doppler. Pulse wave Doppler allows determination of direction and velocity at a precise point within the imaged cardiac field. However, it is limited in its maximum detectable velocity by the Nyquist limit making it unusable for quantification of high-velocity flow (e.g., as seen with severe obstruction). In contrast, continuous wave Doppler interrogates all points along a given beam. Continuous wave Doppler imaging is not constrained by velocity limits and can hence record velocities exceeding those of pulsed Doppler imaging. The drawback is that while the line of interrogation is identifiable, knowledge of anatomy must already be obtained to identify the precise location of the maximum velocity. Clinically these two techniques are commonly used sequentially to identify the area of interest and then to obtain the maximum velocity.
1. Color Flow Doppler
Color flow Doppler is powerful technique for obtaining additional hemodynamic and anatomic data for patients undergoing echocardiography in the intensive care unit. Color flow Doppler allows velocity information to be overlaid on a 2D anatomic image therefore providing data regarding intracardiac and extracardiac shunts, valvar insufficiency or stenosis, and vessel obstruction. By convention, shades of red are used in identifying blood flowing toward the transducer and blue to indicate blood flowing away from the transducer. Therefore, color flow Doppler defines the presence and direction of shunts and is used to grade the severity of valvar insufficiency.
Color flow Doppler is powerful technique for obtaining additional hemodynamic and anatomic data for patients undergoing echocardiography in the intensive care unit. Color flow Doppler allows velocity information to be overlaid on a 2D anatomic image therefore providing data regarding intracardiac and extracardiac shunts, valvar insufficiency or stenosis, and vessel obstruction. By convention, shades of red are used in identifying blood flowing toward the transducer and blue to indicate blood flowing away from the transducer. Therefore, color flow Doppler defines the presence and direction of shunts and is used to grade the severity of valvar insufficiency.
Current Clinical Applications
Clinical applications of echocardiography within the intensive care unit may be divided into the following major areas:
1. The diagnosis and post-intervention evaluation of anatomic lesions.
2. Evaluation of cardiac function.
3. Diagnosis of intracardiac masses and extracardiac effusions.
4. Guidance of intervention within the intensive care unit
Clinical applications of echocardiography within the intensive care unit may be divided into the following major areas:
1. The diagnosis and post-intervention evaluation of anatomic lesions.
2. Evaluation of cardiac function.
3. Diagnosis of intracardiac masses and extracardiac effusions.
4. Guidance of intervention within the intensive care unit
Anatomic Lesions Pre and Post Intervention
Advances in technology have enabled most congenital heart defects to be diagnosed by echocardiography avoiding the risks, time, and cost of invasive cardiac catheterization. In addition, for infants and pediatric patients admitted to an intensive care unit due to being succumbed to shock, echocardiography may be useful for differentiating anatomic causes of shock from functional causes. Patients with obstruction to outflow on the left side of the heart who go undiagnosed at birth frequently present with signs of diminished cardiac output (CO) or frank shock. These lesions including aortic valve stenosis, coarctation of the aorta, and variations of hypoplastic left heart syndrome may be identified and defined by echocardiogram alone.
Advances in technology have enabled most congenital heart defects to be diagnosed by echocardiography avoiding the risks, time, and cost of invasive cardiac catheterization. In addition, for infants and pediatric patients admitted to an intensive care unit due to being succumbed to shock, echocardiography may be useful for differentiating anatomic causes of shock from functional causes. Patients with obstruction to outflow on the left side of the heart who go undiagnosed at birth frequently present with signs of diminished cardiac output (CO) or frank shock. These lesions including aortic valve stenosis, coarctation of the aorta, and variations of hypoplastic left heart syndrome may be identified and defined by echocardiogram alone.
Following surgical or catheter-based intervention patients convalesce in the intensive care unit. Most patients undergo a postprocedural echo before getting discharged home to document adequacy of the repair and lack of significant complications. In postoperative patients this assessment may prove more complicated as access to the patient and the correct windows may be severely compromised by dressings, intracardiac lines, and chest tubes. Occasionally postoperative patients in the intensive care unit may be found to have unexpected residual lesions (see Fig.3). For example, following repair of septal defects, echocardiography may be useful to screen for the presence of residual shunts which may be less well tolerated secondary to myocardial changes following cardiopulmonary bypass. Often, the presence of a residual lesion is known in the operating room through transesophageal echocardiography or direct discussion with the surgeon. An important role of echocardiography is to distinguish those lesions with hemodynamic consequences from those whose presence has no impact on postoperative care. Transthoracic echocardiography may be used to diagnose and assess the hemodynamic sequelae of shunt lesions, residual stenosis, and function. More complicated is the assessment of coronary flow, right ventricular dynamics, and distal obstruction following intervention. In patients who are experiencing arrhythmias postoperatively, special attention should be paid to the flow within the coronary arteries to ensure that it has not been compromised or that a line or mass in the heart is not causing ectopy.
Fig.3 Parasternal short axis image in a patient with pulmonary atresia/VSD who acutely decompensated. White arrows demonstrate the large residual VSD than resulted when a patch dehisced. RA right atrium; RV right ventricle; AV aortic valve
Fig.4 (a) and (b): Four chambered view demonstrating color Doppler of tricuspid regurgitation and the corresponding spectral Doppler pattern. The velocity obtained by spectral Doppler may be utilized to estimate pulmonary artery pressures in the absence of downstream obstruction. A complete envelope by pulse wave or continuous wave Doppler provides the velocity of the regurgitant jet which may be translated into pressure data using the equation: DP = 4(Vmax)2. RA right atrium; RV right ventricle; LA left atrium; LV left ventricle.
Unanticipated pulmonary arterial hypertension may slow the progress of a patient in the intensive care unit. In the absence of a Swan Ganz catheter or a direct pulmonary arterial monitoring, echocardiography may be used to estimate the pulmonary artery pressures. There are several methods that may be used to determine the pulmonary artery pressures. In a patient with
tricuspid regurgitation, the velocity of the jet estimates the difference in pressure in the right atrium and the right ventricle (see Fig.4). If there is no stenosis of the pulmonary arteries, pulmonary valve, or right ventricular outflow tract, the difference in pressure between the right atrium and right ventricle plus the right atrial pressure (CVP) provides an estimate of the pulmonary arterial pressures. In the absence of tricuspid valve insufficiency, interventricular septal geometry may be used to help quantify the degree of pulmonary hypertension.
tricuspid regurgitation, the velocity of the jet estimates the difference in pressure in the right atrium and the right ventricle (see Fig.4). If there is no stenosis of the pulmonary arteries, pulmonary valve, or right ventricular outflow tract, the difference in pressure between the right atrium and right ventricle plus the right atrial pressure (CVP) provides an estimate of the pulmonary arterial pressures. In the absence of tricuspid valve insufficiency, interventricular septal geometry may be used to help quantify the degree of pulmonary hypertension.
Analysis of Ventricular Function
One of the most frequent uses of echocardiography in the ICU is related to the evaluation of ventricular performance. Improvements in technology allow assessment of both systolic and diastolic function with increasing accuracy.
One of the most frequent uses of echocardiography in the ICU is related to the evaluation of ventricular performance. Improvements in technology allow assessment of both systolic and diastolic function with increasing accuracy.
1. Systolic Function
Accurate and timely assessment of systolic function should be an integral part of the medical management of the hemodynamically unstable critically ill patient. Global assessment of LV contractility includes the determination of ejection fraction (EF), circumferential fiber shortening, and cardiac output (CO). There are several methods that may be used to garner this information. Each has its limitations and assumptions which are paramount to understand prior to clinically applying the information gathered. For assessment of left ventricular function, perhaps the simplest quantitative approach is to use M-mode echocardiography (see Fig.3) in either the parasternal short axis at the level of the papillary muscles or in the parasternal long axis at the tips of the mitral valve leaflets to measure the left ventricular end-diastolic dimension (LVEDD) and left ventricular end-systolic dimension (LVESD) for the determination of the fractional shortening (FS) percentage.
Accurate and timely assessment of systolic function should be an integral part of the medical management of the hemodynamically unstable critically ill patient. Global assessment of LV contractility includes the determination of ejection fraction (EF), circumferential fiber shortening, and cardiac output (CO). There are several methods that may be used to garner this information. Each has its limitations and assumptions which are paramount to understand prior to clinically applying the information gathered. For assessment of left ventricular function, perhaps the simplest quantitative approach is to use M-mode echocardiography (see Fig.3) in either the parasternal short axis at the level of the papillary muscles or in the parasternal long axis at the tips of the mitral valve leaflets to measure the left ventricular end-diastolic dimension (LVEDD) and left ventricular end-systolic dimension (LVESD) for the determination of the fractional shortening (FS) percentage.
Fractional shortening is derived by the following:
Normal values for fractional shortening in children and infants vary slightly with age, falling typically between 28 and 44%.
Fractional shortening, therefore, provides a method of assessing circumferential change but has several obvious drawbacks. This method assumes that the ventricle being examined has a circular shape in the axis in which it is examined. As a result, changes in diameter may be mathematically related to circumferential fiber-shortening providing an estimate of ventricular function. Therefore anything that alters the circular shape of the left ventricle (anatomic abnormalities intrinsic to congenital heart disease, pre and afterload changes, or ventricular–ventricular interactions) may affect the assessment of fractional shortening by altering the movement of the septum and causing an under or over estimation of the either end-systolic or diastolic dimension.
A second method of assessing ventricular function is via ejection fraction. Ejection fraction is a volumetric appraisal of ventricular fiber shortening. Echocardiographically the most common method of calculating ejection fraction is the biplane estimation of volumes from the apical four-and two-chamber views. One of the more commonly used mathematical algorithms is the Simpson method in which the left ventricle is traced manually at the end diastole and end systole along the endocardium. Using the method of disks the left ventricle is divided into a series of parallel planes and the resultant disks are individually summed to create each volume. Ejection fraction is calculated using the following equation:
Unfortunately, the determination of an accurate ejection fraction is also subject to ventricular shape with the left ventricle assumed to be its normal prolate elliptical shape. Variations from this shape, which occur frequently in pediatrics, significantly alter the relationship between fiber shortening and volume dependence upon when this equation is applied. In addition, patients in the intensive care environment frequently have suboptimal imaging windows making the endocardium difficult to distinguish and trace.
Not infrequently in active pediatric intensive care units, a patient’s heart and/or lung function must be supported for a period of time. Two such modalities of support are extracorporeal membranous oxygenation and ventricular assist devices. Often the pediatric echocardiographer is asked to assist in the management of these patients by providing insight into the recoverability of cardiac function. This request can be one of the more challenging uses of echo in an intensive care setting. As discussed above, many of the techniques commonly used to determine ventricular systolic function and CO are dependent on the loading conditions of the heart as well as contractility. As a result, both of these support systems which unload the heart in an effort to allow recovery time severely limit echo’s utility as a prognostic indicator. Several newer methods of determining myocardial function including Tissue Doppler Imaging (TDI), strain and strain rate, color m-mode, calcium gating, and three-dimensional (3D) echocardiography are entering the realm of echo in the intensive care unit. These newer modalities may prove to be more efficacious than current standard echocardiography is at present.
Diastolic Function
Accurate assessment of diastolic function by echocardiography is an evolving field that has made great strides in the past few years. Diastolic heart failure and its impact on postoperative management also deserve consideration. Spectral Doppler evaluation is a relatively easy and useful method for evaluating diastolic function noninvasively at the bedside. A prominent pulmonary vein atrial reversal wave (a wave) is a marker of diastolic dysfunction. This finding represents marked flow reversal into the pulmonary veins during atrial systole in response to a noncompliant ventricular chamber. The mitral inflow Doppler pattern can also be a useful marker for diastolic dysfunction. Mitral inflow is composed of 2 waves – an E wave representing early passive ventricular filling (preload dependent) and the A wave representing active filling as a result of atrial systole. The E:A ratio, velocity of E wave deceleration and duration of the A wave can be altered in patients with diastolic dysfunction.
Accurate assessment of diastolic function by echocardiography is an evolving field that has made great strides in the past few years. Diastolic heart failure and its impact on postoperative management also deserve consideration. Spectral Doppler evaluation is a relatively easy and useful method for evaluating diastolic function noninvasively at the bedside. A prominent pulmonary vein atrial reversal wave (a wave) is a marker of diastolic dysfunction. This finding represents marked flow reversal into the pulmonary veins during atrial systole in response to a noncompliant ventricular chamber. The mitral inflow Doppler pattern can also be a useful marker for diastolic dysfunction. Mitral inflow is composed of 2 waves – an E wave representing early passive ventricular filling (preload dependent) and the A wave representing active filling as a result of atrial systole. The E:A ratio, velocity of E wave deceleration and duration of the A wave can be altered in patients with diastolic dysfunction.
Tissue Doppler imaging (TDI) is a newer technique for assessing diastolic ventricular function. TDI allows recording of the low Doppler velocities generated by the ventricular wall motion and directly measures myocardial velocities. In spectral TDI, pulsed Doppler is placed along the myocardial wall (mitral, septal, or tricuspid annulus) recording the peak myocardial velocities. Three waveforms are obtained: a peak systolic wave (Sa), an early diastolic wave (Ea), and an end-diastolic wave (Aa) produced by atrial contraction. The tissue Doppler systolic mitral annular velocity has been shown to correlate with global LV myocardial function [14]. TDI has also been used to estimate diastolic function, and is relatively independent of preload condition. The pulsed Doppler peak early mitral inflow velocity (E) divided by the TD early diastolic mitral annular velocity (Ea) results in a ratio that correlates with the pulmonary capillary wedge pressure. The E/Ea ratio is also useful in estimating mean LV filling pressure. At this time, TDI represents one of the most accurate techniques to assess diastolic function and is therefore of particular interest in the critical care population in whom abrupt changes in preload and afterload are common, making Doppler evaluation of diastolic function less reliable.
Detection of Intracardiac Masses and Extracardiac Effusions
An abnormal area of dense reflectance that is well localized within an echo may represent a mass, thrombus, or calcification. In the postoperative or critical care patient with multiple lines in place, especially in the setting of low flow, care must be taken to evaluate these areas for thrombus formation. Echo is the imaging modality of choice for elucidating and evaluating cardiac mass lesions. Differentiating an area of concern from artifact, can be challenging. Areas that move appropriately throughout the cardiac cycle and the presence of an abnormality in more than a single view, suggest a mass rather than an artifact (see Figs. 5a–d). These findings must in turn be distinguished from such anatomical variations as a prominent Eustacian valve or Chiari network.
An abnormal area of dense reflectance that is well localized within an echo may represent a mass, thrombus, or calcification. In the postoperative or critical care patient with multiple lines in place, especially in the setting of low flow, care must be taken to evaluate these areas for thrombus formation. Echo is the imaging modality of choice for elucidating and evaluating cardiac mass lesions. Differentiating an area of concern from artifact, can be challenging. Areas that move appropriately throughout the cardiac cycle and the presence of an abnormality in more than a single view, suggest a mass rather than an artifact (see Figs. 5a–d). These findings must in turn be distinguished from such anatomical variations as a prominent Eustacian valve or Chiari network.
Fig.5 Demonstrate a thrombus in the right ventricle seen in parasternal short axis (a) and modified four-chamber (b) views. RV right ventricle; LV left ventricle. (c) and (d): Demonstrate a thrombus in the left atrial appendage in both parasternal short axis and a modified four chamber views. RA right atrium; RV right ventricle; AV aortic valve; AO ascending aorta; LV left ventricle.
Major factors that predispose a patient to the development of intracardiac thrombi are the presence of intracardiac lines, diminished CO, and localized stasis in addition to changes within the clotting cascade from sepsis, bypass, intrinsic clotting disorders, or heparin use. Echocardiographic evaluation of patients within the intensive care setting must include an awareness of the increased incidence of thrombus formation and a careful evaluation of areas predisposed to become a nidus for thrombus.
Following cardiac surgery it is not uncommon for patients to develop small collections of fluid in the pericardial space (see Fig.6). Typically, this is of little concern to the clinician; however, in a postoperative patient experiencing tachycardia and/or hypotension, the necessity of recognizing the potential for and screening for cardiac tamponade becomes paramount. In young infants and children, it is frequently difficult to rely on physical exam findings of increased jugular venous pressure or the late finding of pulsus paradoxus. In this instance, a directed and easily performed 2D and Doppler echocardiography can confirm the presence of an effusion and provide accurate assessment of its hemodynamic significance.
Fig.6 Subcostal image demonstrating a large circumferential pericardial effusion (green arrows)
The size and extension of a pericardial effusion may be diagnosed from parasternal, apical, or subcostal windows. The apical view is the easiest for obtaining information regarding the effusions hemodynamic significance. From the apical four chamber view both the mitral and tricuspid valve flow patterns are evaluated with the respiratory monitoring in place. Examining the changes in inflow hemodynamics with respiration allows for the evaluation of tamponade physiology. Greater than 25% variability in maximal e wave velocity of the mitral valve with inspiration or 50% of the e wave velocity of the tricuspid valve (see Figs.7a, b) is indicative of significant hemodynamic compromise resulting from the effusion. Additionally, collapse (differentiated from contraction) of the free wall of the right and left atrium (see Figs.8a, b) when the pericardial pressure exceeds the atrial pressure may be seen from this view in a patient with a significant effusion.
Fig.7 (a) and (b): Respiratory changes in the mitral and tricuspid valve e wave Doppler patterns consistent with tamponade physiology. The tricuspid valve inflow demonstrates more than 50% variability between inspiration and expiration (a). During mitral valve inflow Doppler, the peak E wave velocity alters more than 30% between inspiration and expiration (b).
Fig.8 (a) and (b): Four chambered views demonstrating right atrial and right ventricular collapse (green arrows) as a finding of tamponade physiology. RA right atrium; RV right ventricle; LA left atrium; LV left ventricle.
Echocardiography GuidedProcedures
1. Pericardiocentesis
Performing “blind” percutaneous pericardiocentesis as a treatment for significant pericardial effusion dates back to the early eighteenth century and it is historically fraught with complications. Improved techniques in the 1970s with the advent of 2D echo allowed more accurate localization of the fluid and the development of echo-guided pericardiocentesis. Echo-guided pericardiocentesis (see Fig.9) has been found to be a safe and effective procedure with insertion of a catheter for drainage used to reduce the rate of recurrence found to complicate simple needle drainage and is considered the primary and often the definitive therapy for patients with clinically significant effusions.
1. Pericardiocentesis
Performing “blind” percutaneous pericardiocentesis as a treatment for significant pericardial effusion dates back to the early eighteenth century and it is historically fraught with complications. Improved techniques in the 1970s with the advent of 2D echo allowed more accurate localization of the fluid and the development of echo-guided pericardiocentesis. Echo-guided pericardiocentesis (see Fig.9) has been found to be a safe and effective procedure with insertion of a catheter for drainage used to reduce the rate of recurrence found to complicate simple needle drainage and is considered the primary and often the definitive therapy for patients with clinically significant effusions.
Fig.9 Echoguided pericardiocentesis. Green arrow is in the pericardial space demonstrating the large fluid collection. Blue arrow is pointing to the needle that has been advanced into the pericardial space to drain the fluid collection. The large effusion allows the echocardiographer to direct the individual performing the pericardiocentesis away from areas that could lead to complications such as perforation of the myocardium.
2. Balloon Atrial Septostomy (BAS)
Part of any echocardiographic assessment of a patient with congenital heart disease should include evaluation of the atrial septum. Cardiac lesions such as transposition of the great arteries, hypoplastic left heart syndrome, and tricuspid atresia require an adequate atrial communication. In the setting of a restrictive atrial septal communication or intact septum, a BAS is required to improve mixing and CO. In the past, the procedure, originally described by William Rashkind was performed in the cardiac catheterization laboratory under fluoroscopic guidance. However, during the last decade BAS has been routinely performed at the bedside in the intensive care unit under echocardiographic guidance (see Figs.10a–d). Most commonly either a subcostal view that includes a focused look at the atrial septum, pulmonary vein, and mitral valve or an apical four-chamber view is used. For the echocardiographer, the primary role is to provide continued visualization of the catheters and communicate well with the interventionalist. Advantages of this technique are multifactorial; echocardiography is superior to fluoroscopy during BAS due to a lack of radiation, the ability to perform the procedure at bedside rather than transporting to a catheterization laboratory, and direct, continuous visualization of the atrial septum, pulmonary veins, and mitral valve. The disadvantages of this technique include the potential for interference with maneuverability for both echocardiographer and catheter operator around a small neonate and therefore the risk of contamination of the sterile field. Additionally there is the possibility of poor acoustic windows in an ill neonate who may be mechanically ventilated. However, with proper planning and communication, the limitations of transthoracic echocardiographic guidance of BAS may be minimized.
Part of any echocardiographic assessment of a patient with congenital heart disease should include evaluation of the atrial septum. Cardiac lesions such as transposition of the great arteries, hypoplastic left heart syndrome, and tricuspid atresia require an adequate atrial communication. In the setting of a restrictive atrial septal communication or intact septum, a BAS is required to improve mixing and CO. In the past, the procedure, originally described by William Rashkind was performed in the cardiac catheterization laboratory under fluoroscopic guidance. However, during the last decade BAS has been routinely performed at the bedside in the intensive care unit under echocardiographic guidance (see Figs.10a–d). Most commonly either a subcostal view that includes a focused look at the atrial septum, pulmonary vein, and mitral valve or an apical four-chamber view is used. For the echocardiographer, the primary role is to provide continued visualization of the catheters and communicate well with the interventionalist. Advantages of this technique are multifactorial; echocardiography is superior to fluoroscopy during BAS due to a lack of radiation, the ability to perform the procedure at bedside rather than transporting to a catheterization laboratory, and direct, continuous visualization of the atrial septum, pulmonary veins, and mitral valve. The disadvantages of this technique include the potential for interference with maneuverability for both echocardiographer and catheter operator around a small neonate and therefore the risk of contamination of the sterile field. Additionally there is the possibility of poor acoustic windows in an ill neonate who may be mechanically ventilated. However, with proper planning and communication, the limitations of transthoracic echocardiographic guidance of BAS may be minimized.
Fig.10 Subcostal images demonstrating echo-guided balloon atrial septostomy (BAS). (a): shows the initial small atrial communication in both 2 dimensional (2D) and color Doppler imaging. (b): reveals the deflated balloon that has been advanced across the atrial communication. It is important during this portion of the procedure for the echocardiographer to ensure that the balloon has not been advanced across the left atrioventricular valve. (c): demonstrates the inflated balloon within the left atrium. It is important to note the balloon’s position away from the mitral valve and pulmonary veins. (d): demonstrates the atrial communication following septostomy using both 2D and color Doppler imaging. RA right atrium; RV right ventricle; LA left atrium; LV left ventricle; Green arrows atrial communication.
Future Directions
There are several areas of advanced imaging that are becoming more commonplace in the practice of pediatric echocardiography. Primary assessment of cardiac mechanics by evaluating myocardial motion, strain, and strain rate has been validated in healthy children and provides additional information regarding myocardial performance. Three-dimensional real-time echocardiography has a growing role in evaluating anatomic defects, valves, and right and left ventricular function independently of geometric assumptions that constrained the previous methods.
There are several areas of advanced imaging that are becoming more commonplace in the practice of pediatric echocardiography. Primary assessment of cardiac mechanics by evaluating myocardial motion, strain, and strain rate has been validated in healthy children and provides additional information regarding myocardial performance. Three-dimensional real-time echocardiography has a growing role in evaluating anatomic defects, valves, and right and left ventricular function independently of geometric assumptions that constrained the previous methods.
1. Myocardial Mechanics
In the past several years, myocardial strain and strain rate have emerged as promising quantitative measures of myocardial function and contractility. Strain (e) is a dimensionless parameter defined as the deformation (L) of an object relative to its original length (Lo), and is expressed as a percentage. Strain rate (SR) is defined as the local rate of deformation or strain (e) per unit of time, and is expressed in 1/s. Strain and strain rate measurements can be obtained from data acquired by Doppler Tissue Imaging or 2D tissue tracking. Strain and strain rate should be of great help in the future in the evaluation of ventricular function, since conventional M-mode and 2D echocardiography have limitations due to complex morphology of the right ventricle and altered left ventricle morphology that occurs in complex congenital heart defects. Left and right ventricular values of strain and strain rate are available for healthy children.
In the past several years, myocardial strain and strain rate have emerged as promising quantitative measures of myocardial function and contractility. Strain (e) is a dimensionless parameter defined as the deformation (L) of an object relative to its original length (Lo), and is expressed as a percentage. Strain rate (SR) is defined as the local rate of deformation or strain (e) per unit of time, and is expressed in 1/s. Strain and strain rate measurements can be obtained from data acquired by Doppler Tissue Imaging or 2D tissue tracking. Strain and strain rate should be of great help in the future in the evaluation of ventricular function, since conventional M-mode and 2D echocardiography have limitations due to complex morphology of the right ventricle and altered left ventricle morphology that occurs in complex congenital heart defects. Left and right ventricular values of strain and strain rate are available for healthy children.
2. 3D Echocardiography
Off-line 3D reconstruction consists of acquisition of sequential 2D slices which are converted to a rectangular coordinate system for 3D reconstruction and provides accurate anatomic information suitable for quantitative analysis. Left ventricular volume, mass, and function can be accurately assessed using RT3D independently of geometric assumption, and ejection fraction can be calculated. The wideangle mode is often used to acquire the entire LV volume, from which further analysis allows determination of global and regional wall motion. Wall motion is evaluated from base to apex with multiple slices from different orientations. The advantage of 3D over 2D is the ability to manipulate the plane to align the true long axis and minor axis of the LV, thus avoiding foreshortening and oblique image planes. LV volume assessment by RT3D is rapid, accurate, reproducible and superior to conventional 2D methods and is comparable to MRI, which represents the gold standard. Three dimensional reconstruction of the tricuspid valve has been shown to be helpful for anatomical assessment of Ebstein’s malformation or after atrioventricular septal defect repair. 3D Echocardiography is a useful adjunct to standard 2D imaging and should be increasingly used in the future.
Off-line 3D reconstruction consists of acquisition of sequential 2D slices which are converted to a rectangular coordinate system for 3D reconstruction and provides accurate anatomic information suitable for quantitative analysis. Left ventricular volume, mass, and function can be accurately assessed using RT3D independently of geometric assumption, and ejection fraction can be calculated. The wideangle mode is often used to acquire the entire LV volume, from which further analysis allows determination of global and regional wall motion. Wall motion is evaluated from base to apex with multiple slices from different orientations. The advantage of 3D over 2D is the ability to manipulate the plane to align the true long axis and minor axis of the LV, thus avoiding foreshortening and oblique image planes. LV volume assessment by RT3D is rapid, accurate, reproducible and superior to conventional 2D methods and is comparable to MRI, which represents the gold standard. Three dimensional reconstruction of the tricuspid valve has been shown to be helpful for anatomical assessment of Ebstein’s malformation or after atrioventricular septal defect repair. 3D Echocardiography is a useful adjunct to standard 2D imaging and should be increasingly used in the future.
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