TWENTY-TWO: Ultrasonography of the Heart

Ultrasonography of the Heart

Colin C. Schwarzwald

University of Zurich, Zurich, Switzerland

Over the last three decades, echocardiography has become a standard diagnostic tool in equine cardiology. Development of M-mode and two-dimensional (2D) 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 [1,2,3]. Newer echocardiographic modalities, including tissue Doppler imaging (TDI) and 2D speckle tracking (2DST), might become valuable tools for assessment of cardiac function in horses in the near future [4,5,6,7,8]. Nonetheless, accurate and reliable assessment of chamber dimensions and mechanical function of the heart will remain challenging and is limited by a variety of technical, anatomic, and physiologic issues that must be considered when performing echocardiographic examinations [3,9,10]. Knowledge of the technical principles of ultrasonography, strict adherence to a routine protocol in order to obtain high-quality standard sonographic images, and comprehensive knowledge 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 will remain a cornerstone of echocardiography and must not be neglected. Finally, independent of the methods used, the findings obtained during echocardiography should be critically assessed in the light of medical history and clinical findings.

Technical Considerations

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 2D 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 [11] or 2D speckle tracking [6,7,8] demand a frame rate between 40 and 90 Hz to achieve sufficient temporal resolution. The depth of penetration should reach at least 25–30 cm to scan the heart of an adult horse. Simultaneous recording of a surface electrocardiogram (ECG) is required and allows exact timing of flow events 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.

Ultrasonographic Examination of the Normal Equine Heart

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 [12,13,14,15]. However, some horses do not tolerate the echocardiographic procedure and require sedation to allow safe examination with sufficient quality and at normal heart rates.

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 22.1, 22.2, 22.3, 22.4, 22.5). 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.

c22-fig-0001 — Figure 22.1 Standard echocardiographic views for assessment of left ventricular size and function. (A) Right-parasternal long-axis four-chamber view centered on the left ventricle (LV). The transducer is positioned in the right fourth 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 anatomic 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° clockwise from a four-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) 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 peripheral wall (LVPW) are displayed over time. The right ventricle (RV) and the right ventricular free wall are displayed in the near field. A small amount of spontaneous echo contrast is frequently seen in the ventricular lumen (arrowheads). Measurement of LV wall thickness and internal dimensions, evaluation of septal motion, and calculation of LV fractional shortening allow assessment of LV size and systolic function. (D) Anatomic M-mode (AMM) image of the LV (bottom), reconstructed from a digitally stored 2D 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 2D image, independent of the sector apex, to bisect the IVS, the LV cavity, and the LVPW into two equal parts throughout the cardiac cycle.

c22-fig-0002 — Figure 22.2 Standard echocardiographic views for assessment of left atrial size and function. (A,B) Right-parasternal four-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° clockwise from a right-parasternal left-ventricular outflow tract view (Figure 22.3A). 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 fifth 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,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. (Source: Part C – Bonagura, J.D., Reef, V.B., & Schwarzwald, C.C. (2010) [2]. Reproduced with permission of Elsevier.)

c22-fig-0003 — Figure 22.3 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 four-chamber view (Figure 22.1A), 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. (B) Right-parasternal long-axis view of the right ventricular outflow tract (RVOT). Starting from a 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 fifth or fourth 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.

c22-fig-0004 — Figure 22.4 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, seen 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 two-dimensional methods (Figures 22.2, 22.3). Ao: aorta; LAA: left atrial appendage; RV: right ventricle; TV: tricuspid valve.

c22-fig-0005 — Figure 22.5 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 towards 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.

Indications and Clinical Use of Echocardiography in Horses

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 22.6, 22.7, 22.8, 22.9, 22.10, 22.11, 22.12, 22.13, 22.14, 22.15, 22.16, 22.17, 22.18, 22.19, 22.20, 22.21, 22.22, 22.23, 22.24, 22.25, 22.26, 22.27, 22.28). Echocardiography is indicated in horses with heart murmurs to differentiate physiologic flow murmurs from pathologic 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 in foals; and to determine the cause of heart failure and monitor progression and response to treatment in horses with signs of congestive heart failure.