Katharyn J. Mitchell1 and Colin C. Schwarzwald2 1 College of Veterinary Medicine, Cornell University, Ithaca, USA Over the last four decades, echocardiography has become a standard diagnostic tool in equine cardiology. Development of M-mode, two-dimensional (2-D) real-time echocardiography, and introduction of various Doppler modalities, including continuous-wave (CW) Doppler, pulsed-wave (PW) Doppler, and color flow Doppler, provided the basis for comprehensive evaluation of internal cardiac structures, chamber dimensions, blood flow characteristics, and mechanical function of the equine heart [1–3]. Newer echocardiographic modalities, including tissue Doppler imaging (TDI) and 2-D speckle tracking (2DST) have become valuable advanced tools for assessment of subtle changes to cardiac function in horses [4–11]. The practical application of three-dimensional real-time echocardiography is currently being explored. This modality potentially will provide the clinician with additional information about the valvular structures, better understanding of complex congenital malformations, and theoretically, a more accurate assessment of cardiac chamber size, particularly the right ventricle (RV), which is currently difficult to accurately evaluate due to it complex geometric shape. Nonetheless, accurate and reliable assessment of chamber dimensions and mechanical function of the heart remains challenging and is limited by a variety of technical, anatomical, and physiological issues that must be considered when performing echocardiographic examinations [3,12,13]. Knowledge of the technical principles of ultrasonography, strict adherence to a routine protocol in order to obtain high-quality standard sonographic images, and comprehensive understanding of normal cardiac anatomy and abnormal findings in horses with heart disease, are prerequisites to the successful use of echocardiography. Also, despite the availability of quantitative echocardiographic methods, subjective assessment of recordings remains a cornerstone of echocardiography and must not be neglected. Finally, independent of the methods used, the information obtained during echocardiography should be critically assessed in the light of medical history and clinical findings. The ultrasonographic equipment should include a phased-array sector transducer working at frequencies between 1.5 and 3.5 MHz. Tissue harmonic imaging often improves the image quality, particularly in the far field and in large horses, by providing a higher signal-to-noise ratio, better contrast, and higher spatial resolution. Frame rates of at least 15 Hz (15 images/second) are required for real-time 2-D imaging of the moving structures of the heart, but frame rates of 25 Hz or higher are preferable. Newer applications such as anatomical M-mode [14] or 2-D speckle tracking [6–8] demand a frame rate between 40 and 90 Hz to achieve sufficient temporal resolution. Color TDI requires frame rates of > 120 Hz [5,9]. The depth of penetration should reach at least 25 to 30 cm to scan the heart of an adult horse. Simultaneous recording of a surface ECG is required and allows exact timing of flow events, wall motion, and echocardiographic measurements. Modern echocardiography systems offer digital raw data storage of still frames and cine loop recordings. This is extremely useful, since it reduces the contact time with the patient and allows post processing and off-line analysis of the stored data. Capturing various measurements at the correct timing within cardiac cycle is critical and therefore, cine loop recordings provide the observer with opportunities to capture several measurements throughout the cardiac cycle (particularly useful for measurements of area or volume). The echocardiographic examination should take place in a location where the horse can be safely restrained. If possible, the patient should not be sedated prior to the examination, since cardiac dimensions, indices of cardiac function, and color Doppler signals of regurgitant flow or shunt flow might be altered because of drug effects on preload, afterload, contractility, heart rate, and rhythm [15–19]. However, some horses do not tolerate the echocardiographic procedure and require sedation to allow safe examination with sufficient quality and at normal heart rates. Clipping of the hair coat may be required to improve image quality, particularly in horses with thicker coats. A systematic approach using standardized image planes is important for comparison of studies over time and for comparison of studies obtained by different examiners (Figures 21.1, 21.2, 21.3, 21.4, and 21.5). By convention, the structures nearest to the transducer are displayed at the top of the screen. The dorsal (in the long-axis views) and cranial (in the short-axis views) structures of the heart are displayed to the right side of the screen. Figure 21.1 Standard echocardiographic views for assessment of left ventricular size and function. (A) Right-parasternal long-axis 4-chamber view centered on the left ventricle (LV). The transducer is positioned in the right 4th intercostal space at a level slightly above the olecranon, angled caudally, and rotated clockwise to the 1-o’clock position. Slight changes in transducer placement may be necessary to optimize the image plane. This view is best suited to assess the structures, dimensions, and mechanical function of the LV. Since this image is centered on the LV, the left atrium (LA) may not be imaged in its entirety throughout the cardiac cycle. Assessment of the right heart is limited due to its anatomical position, its complex geometry, and its visualization in the narrow near field of the imaging sector. (B) Right-parasternal short-axis view of the LV at the level of the chordae tendineae (arrowheads). This view is obtained by rotating the transducer 90 degrees clockwise from a 4-chamber view. It is commonly used for measurement of the diameter and the area, respectively, of the LV and evaluation of LV systolic function. (C) Same image plane as in (A), showing tracings (green dotted lines) of the LV internal area and longitudinal axis at end-diastole (top left) and peak-systole (top right). The bottom image shows the end-diastolic area tracing superimposed to the peak-systolic area tracing (green dotted lines). Applying a geometric model such as the modified (single-plane) Simpson’s model of discs allows calculation of the estimated LV volume at end-diastole (LVIVd) and peak-systole (LVIVs), ejection fraction [%EF = (LVIVd – LVIVs)/LVIVd × 100], stroke volume [SV = LVIVd – LVIVs], and cardiac output [CO = SV × HR]. HR, heart rate; LVIVd (S) (500), LVIVd estimated by Simpson’s model and allometrically scaled to a body weight of 500 kg. (D) Right-parasternal short-axis view (top) and corresponding M-mode recording (bottom) of the LV at the chordal level. The motion of the interventricular septum (IVS) and the left-ventricular free wall (LVFW) are displayed over time. Care must be taken that the LV is bisected by the cursor line into two symmetrical parts throughout the cardiac cycle. The right ventricle (RV) and the right-ventricular free wall (RVFW) are displayed in the near field. A small amount of spontaneous echo contrast is commonly seen in the ventricular lumen (arrowheads). Measurement of LV wall thickness and internal dimensions, evaluation of septal motion, and calculation of LV fractional shortening (%FS) allow assessment of LV size and systolic function. The blue dotted lines indicate measurement of the IVS, the left-ventricular internal diameter (LVID), and the LVFW at end-diastole (defined as the onset of the electrocardiographic QRS complex) and at peak systole (defined as the time at which the LV internal lumen is narrowest). These measurements allow calculation of the LV fractional shortening [%FS = (LVIDd – LVIDs)/LVIDd × 100], mean wall thickness at end-diastole [MWTd = (IVSd + LVFWd)/2], and relative wall thickness at end-diastole [RWTd = (IVSd + LVFWd)/LVIDd]. LVIDd(500), left-ventricular internal diameter allometrically scaled to a body weight of 500 kg; LA Diam (max)/LVIDd, ratio of maximum LA diameter to LVIDd; d: measurements at end-diastole; s: measurements at peak systole. (E) Anatomical M-mode (AMM) image of the LV (bottom), reconstructed from a digitally stored 2-D cineloop recording obtained from a right-parasternal short-axis view at the chordal level (top). Notice that the AMM cursor (green line) can be freely positioned on the 2-D image, independent of the sector apex, to bisect the IVS, the LV cavity, and the left-ventricular posterior wall (LVPW; also called left ventricular free wall, LVFW) into two equal parts throughout the cardiac cycle. A small amount of spontaneous echo contrast is commonly seen in the ventricular lumen (arrowheads). (Source: C and D: with permission from Schwarzwald, C.C. (2019) Equine echocardiography. Veterinary Clinics: Equine Practice 35(1): 43–64). Figure 21.2 Standard echocardiographic views for assessment of left atrial size and function. (A) and (B) Right-parasternal 4-chamber view centered on the left atrium (LA), to image the LA in its entirety throughout the cardiac cycle. At an imaging depth of 30 cm, this view is best suited to assess the mitral valve (MV) apparatus, LA dimensions, and LA mechanical function. Only in rare cases (e.g., giant draft breeds, horses with severe cardiomegaly), can the LA not be displayed in its entirety from this window. (A) Image recorded at the end of ventricular systole, one frame prior to opening of the MV, when the LA is at its maximum dimensions. Chordae tendineae are seen in the left ventricular (LV) cavity (arrowheads). (B) Image recorded at the end of diastole, immediately after closure of the MV, when the LA is at its minimum dimensions. Note the obvious change in LA dimensions within a single cardiac cycle (i.e., between (A) and (B)), indicating that timing of measurements of LA dimensions is critical. Generally, LA dimensions should be assessed at the end of ventricular systole (A). (C) Right-parasternal short-axis view at the level of the aortic valve. This view is obtained by rotating the transducer 90 degrees clockwise from a right-parasternal left-ventricular outflow tract view (Figure 21.3A). In this view, the aortic valve (AV) with its three cusps is visible in the center of the image. The surrounding structures include right atrium (RA), tricuspid valve, right ventricle (RV), right-ventricular outflow tract (RVOT), LA, and left atrial appendage (LAA). The apparent triangular separation (at the 12-o’clock position) evident between the non coronary cusp (NCC) and the right coronary cusp (RCC) is a normal finding and does not represent an anomaly. In this view, the end-systolic size of the LA and LAA can be measured and compared to the aortic area. (D) Left-parasternal long-axis view of the LA, MV, and LV. The transducer is positioned in the 5th intercostal space slightly above the olecranon, oriented perpendicular to the chest wall and angled dorsally. This view has traditionally been used for assessment of LA dimensions. However, as opposed to the right-parasternal views (A) and (B), this view often does not allow imaging of the LA in its entirety due to interference with the ventral lung border. Nonetheless, imaging LA and MV from a left thoracic window provides additional information and should complement the right-parasternal views. LCC: left coronary cusp of the aortic valve; PV: pulmonary vein; TV: tricuspid valve. Figure 21.3 Standard echocardiographic views for assessment of the great vessels. (A) Right-parasternal long-axis view of the left ventricular outflow tract (LVOT). Starting from a 4-chamber view (Figure 21.1A), the transducer is angled more cranially, tilted dorsally, and rotated to the 2-o’clock position to obtain this view. It is best suited to assess the structures, dimensions, and function of the aortic valve, the sinus of valsalva, and the ascending aorta (Ao) and the pulmonary artery (PA). The diameter of the PA should be smaller than the diameter of the aorta. Chordae tendineae are seen in the left ventricular (LV) cavity (arrowheads). (B) Right-parasternal long-axis view of the right ventricular outflow tract (RVOT). Starting from an LVOT view (A), the transducer is angled more cranially to obtain this view; occasionally the transducer will have to be moved one intercostal space cranially. The right atrium (RA), the tricuspid valve, the right ventricle (RV), the RVOT, the pulmonic valve, the pulmonary artery (PA), and a cross-section through the right coronary artery (arrow) can be visualized. (C) Left-parasternal long-axis view of the LVOT. The transducer is positioned in the 5th or 4th intercostal space at a level slightly above the olecranon and angled slightly cranially. (D) Left-parasternal long-axis view of the RVOT. Starting from the LVOT view (C), the transducer is moved one intercostal space cranially to obtain this image. LA: left atrium; LV: left ventricle. Figure 21.4 Mitral and aortic valve motion. (A) M-mode recording of mitral valve motion obtained from a right-parasternal short-axis view at the level of the mitral valve. The M-mode cursor (dotted line) is placed across the MV leaflets. The motion of the septal mitral leaflet and the time of mitral valve closure (MVC) and opening (MVO) can be identified on the M-mode tracing. The E wave (E) represents early-diastolic opening of the mitral valve during passive inflow of blood from the left atrium into the ventricle. The A wave (A) represents late-diastolic opening of the mitral valve during atrial contraction. The E point-to-septal separation (EPSS) can be increased with aortic regurgitation (regurgitant jet impinging on the valve), LV dilation with left ventricular failure (reduced transmitral flow), or mitral stenosis/tethering (rare defects or acquired from endocarditis). (B) M-mode recording of aortic valve motion obtained from a right-parasternal short-axis view of the aorta. The M-mode cursor (dotted line) is placed across the aortic valve leaflets. Motion of the left-coronary cusp and the time of aortic valve opening (AVO) and aortic valve closure (AVC) can be identified. This view has traditionally been used for assessment of aortic size and left atrial dimensions, but has largely been replaced by 2-D methods (Figures 21.2 and 21.3). Ao: aorta; LAA: left atrial appendage; RV right ventricle; TV: tricupsid valve. Figure 21.5 Color Doppler findings in apparently healthy horses. Color Doppler recordings obtained from an 8-year-old healthy Hanoverian Warmblood gelding in athletic condition (eventing). Blood flow toward the transducer is coded in red to yellow and flow away from the transducer is coded in shades of blue to cyan. Flow that is turbulent according to the system algorithm is colored in green. Traces of valvular regurgitation are frequently found on echocardiograms of healthy horses with normal exercise capacity, often even in absence of an audible murmur. Many of these small regurgitant jets are evident only in distinct imaging planes and most are not considered clinically relevant. (A) Right-parasternal left ventricular outflow tract view. The region of interest (ROI) is positioned over the tricuspid valve. Trivial tricuspid regurgitation is evident in the right atrium (RA) during systole, color-coded in blue, cyan, and green. (B) Left-parasternal long-axis view with the ROI positioned over the mitral valve. Mild mitral regurgitation is evident in the left atrium (LA) during systole. Two distinct regurgitant jets (color-coded in blue, cyan, and green) are seen: a small jet originating from the parietal leaflet and a larger jet stemming from the septal leaflet. (C) Right-parasternal right ventricular outflow tract (RVOT) view. The ROI is positioned over the pulmonic valve. Trivial, low-velocity pulmonic insufficiency (color-coded in red) is evident in the RVOT during diastole. (D) Right-parasternal left ventricular outflow tract view. The ROI is positioned over the aortic valve. A trace of aortic insufficiency (color-coded in green) can be seen in the left ventricle (LV) at end-diastole, immediately before opening of the aortic valve. Ao: aorta; PA: pulmonary artery; RV: right ventricle. Echocardiography can be used to identify cardiac disorders, assess hemodynamic and structural consequences of disease, and monitor response to treatment and progression of disease (Figures 21.6– 21.28). Echocardiography is indicated in horses with heart murmurs to differentiate physiological flow murmurs from pathological murmurs and assess their clinical relevance; to detect underlying cardiac disease in horses with cardiac dysrhythmias; to diagnose or rule out cardiac disease in horses with exercise intolerance, poor performance, collapse, or episodic weakness; to screen for endocarditis or pericarditis in horses with fever of unknown origin; to diagnose or rule out pericardial disease in horses with muffled heart sounds; to diagnose the cause of persistent tachycardia (e.g., because of severe myocardial disease); to detect pulmonary hypertension in horses with severe respiratory disease; to diagnose suspected congenital cardiac defects; and to determine the cause of heart failure and monitor progression and response to treatment in horses with signs of congestive heart failure. Figure 21.6 Restrictive ventricular septal defect. (A) Right-parasternal long-axis view of the left ventricular outflow tract obtained from a 3-year-old Friesian filly with a restrictive paramembranous ventricular septal defect. The small defect (arrow) can easily be missed on the grayscale image (left). Color flow mapping facilitates identification of the defect and clearly shows transseptal blood flow (right). Notice the classic color aliasing pattern in which flow changes from dark red, to yellow, to blue, and finally to a green turbulent encoding. A small aortic regurgitant jet is seen as well (arrowhead). The size of the pulmonary artery (PA) relative to the aorta (Ao) appears normal and does not suggest significant pulmonary hypertension. (B) Long-axis echocardiogram and corresponding continuous-wave Doppler recording indicating left-to-right shunt flow through the ventricular septal defect. The peak systolic velocity (arrows) is 5.25 m/s, corresponding to a high (normal) left-to-right pressure gradient of 110 mmHg (modified Bernoulli equation: dp = 4 × vmax2). The diastolic flow component (arrowheads) corresponds to aortic regurgitant flow or diastolic shunt flow. LV: left ventricle; RA: right atrium; RV: right ventricle. Figure 21.7 Non-restrictive ventricular septal defect. (A) Right-parasternal long-axis view obtained from a 1-year old Paint filly with a non-restrictive paramembranous ventricular septal defect. Obvious malalignment occurs from the aortic root to the ventricular septum, characterized by over-riding or straddling of the root over the defect. The large defect, with an estimated size of 3.6 cm, is evident immediately below the aortic valve (asterisk). The pulmonary artery (PA) appears enlarged compared to the aorta (Ao), indicating increased transpulmonary flow due to left-to-right shunting or pulmonary hypertension. (B) Color Doppler echocardiogram in a right-parasternal long-axis view indicates systolic flow through the defect. Notice the region of proximal flow convergence in the left ventricular outflow tract, characterized by acceleration of blood flow leading to aliasing (sudden color change from yellow to blue), and the turbulent flow in the right ventricle, color-coded in green. (C) Long-axis echocardiogram and corresponding continuous-wave Doppler recording indicating left-to-right shunt flow through the ventricular septal defect (arrows). The peak systolic velocity is 2.6 m/s, corresponding to a (very low) left-to-right pressure gradient of 27 mmHg. Flow is detected during systole and from mid-diastole to end-diastole. The lack of significant flow during early diastole is either due to translational motion of the heart, causing the defect to move out of the cursor line, transient equilibration of early-diastolic ventricular pressures, or cyclical obstruction or diversion of flow by the prolapsing tricuspid valve. (D) Short-axis echocardiogram and corresponding M-mode recording of the left ventricle (LV). The large end-diastolic LV diameter and the hyperkinetic motion of the interventricular septum (IVS) indicate left ventricular volume overload. The fractional shortening of the left ventricle is 39%, indicating that the systolic function of the left ventricle is preserved. The relative wall thickness is 0.29 (normal 0.4–0.6), indicating the ventricular walls are thinner than expected for the diameter of the ventricle. LA: left atrium; LVPW: left ventricular posterior wall; RV: right ventricle. Figure 21.8 Apical muscular ventricular septal defect. Right (A) and left (B) parasternal long-axis view of the left (LV) and right ventricle (RV) in a 4-month-old Thoroughbred colt presented with a right-sided grade 3/6 holosystolic apical murmur that had been detected as an incidental finding. The image plane is focused on the apical part of the interventricular septum. In B-mode (A and B: left), a hypoechoic area in the septum (arrows) suggests the presence of a muscular ventricular septal defect. Color Doppler imaging (A and B: right) allowed to confirm the presence of a defect. Notice the region of proximal flow convergence in the LV (A), characterized by acceleration of blood flow leading to aliasing (sudden color change from yellow to blue). Turbulent flow in the RV is color-coded in green. Post-mortem examination one year after the initial examination (for reasons other than the cardiac malformation) revealed severe chronic endocardial fibrosis and complete closure of the ventricular septal defect. Figure 21.9 Endocardial cushion defect. (A) Right-parasternal 4-chamber view obtained from a 4-month-old Morgan colt with a complete endocardial cushion defect. The primum atrial septal defect is evident in the ventral atrial septum (arrow) and an inlet ventricular septal defect component (arrowhead) is observed below the common atrioventricular valve. (B) Right-parasternal short-axis view at the level of the common atrioventricular valve. The ECG electrodes were displaced resulting in the artifact to the lower right. One of the main goals of echocardiography is detection of cardiac chamber under filling or dilation and grading of the enlargement, if present, as mild, moderate, or severe. Left atrial (LA) and left ventricular (LV) dimensions can be quantified using linear measurements, area measurements, or volumetric estimates that are based on two-dimensional echocardiography (2DE) and M-mode echocardiography (Figures 21.1 and 21.2) [3,7,13,14,20,21]. The relationship between LV chamber diameter and wall thickness can be evaluated using relative wall thickness (RWT), which is calculated as [IVSd + LVFWd]/LVIDd where IVSd is interventricular septal thickness in diastole, LVFWd is the LV free wall thickness in diastole (also termed LV posterior wall, LVPW), and LVIDd is the LV internal diameter in diastole when measured in short-axis. The RWT can indicate physiological remodeling or eccentric hypertrophy (e.g., as seen in athletic horses following training), pathological concentric hypertrophy (Figure 21.23
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Ultrasonography of the Heart
2 Clinic of Equine Internal Medicine, University of Zurich, Zurich, Switzerland
Technical Considerations
Ultrasonographic Anatomy of the Normal Equine Heart
Indications and Clinical Use of Echocardiography in Horses
Assessment of Chamber Dimensions
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