Essentials of echocardiography (Proceedings)

Echocardiography has emerged as the most valuable non-invasive tool for evaluation of cardiac structure, function, blood flow patterns, and has greatly diminished the need for diagnostic cardiac catheterizations and angiocardiography in many cases. Echocardiography is one tool for evaluation of the cardiac patient, but should be used in conjunction with other diagnostic tests including thoracic radiography and electrocardiography for a global assessment of the patient.

There have been rapid and significant advances in the technology of ultrasound machines. Rapid frame rates are essential to image the dynamic changes of the heart during the cardiac cycle. Higher MHz transducers have higher frame rates, greater image resolution, but less tissue penetration. Lower MHz probes offer greater tissue penetration and stronger Doppler properties with higher maximal velocity measurements. Cats and small dogs are most often imaged with > 7 MHz transducer, medium dogs with a 5 MHz probe, and large dogs with 3-4 MHz transducer. Newer machines offer digital acquisition of still frames and real-time loops, and can be stored on PACS servers. Depth controls are adjusted until the cardiac image fills the field. Time gain compensation (TGC) controls the gain at specific depths, and often the near field is adjusted to have less gain than the far field. Compression adjusts the dynamic range of gray scale, so increased compression allows for more gray scale from weaker echoes, and less compression results in a higher contrast image. Often echocardiographic images are adjusted for less compression for better delineation of the cardiac structures from the blood pool. Persistence should be adjusted to zero or minimal since averaging several frames results in a blurred real time effect. Sector width can be adjusted, and the narrower the sector, the higher frame rate and greater the resolution. Frame rates of 15-30 frames per second are adequate for the appearance of real-time motion in most small animals.

Due to the reflection of sound waves from lungs, there are limited windows for adequate acoustic penetration. It is recommended to image from beneath the animal while it is in lateral recumbency, since there is a larger and better quality acoustic window. Specialized ultrasound tables are commercially available or can be constructed with a cut-out to image from beneath the animal. Shaving the hair at the left and right precordial transducer locations may improve image quality but is usually not necessary. The hair should be wet and parted to expose skin at the transducer placement and liberal and repeat applications of ultrasonic gel to the transducer is necessary.

An ordered approach to echocardiography is important. Two dimensional (2D) echocardiography is the first modality used to examine the structure and function of the heart. There are standard echocardiographic views that are obtained from the right and left thorax to evaluate cardiac structure and function and are necessary for standardization of cardiac measurements. The right parasternal window is the first location to image, and is located between the fourth and sixth intercostal spaces. Palpation of the area of the strongest apical beat typically identifies the most optimal position to image from. The probe is positioned at the level of the costo-chondral junction or slightly closer to the sternum. The right parasternal long axis 4-chamber view is the first to be obtained (Figure 1). The probe is aligned parallel to the long axis of the left ventricle, with the transducer mark pointed towards the cervical vertebrae (towards the head). The left and right ventricles, right and left atria, and atrioventricular valves are examined. Assessment of overall left and right heart size and morphology of the atrioventricular valves and chordae tendinae structure can be made. Mitral valve prolapse or flail may be identified using this view (Figure 2). The right parasternal long axis left ventricular outflow tract view is then obtained by slightly rotating the probe in the cranial direction (Figure 3). This view is essential for evaluation of the aorta, aortic cusps, interventricular septum, and the anterior mitral valve movement in systole. Subaortic stenosis, systolic anterior motion of the mitral valve, ventricular septal defects, aortic valve abnormalities, and heart base tumors are well visualized in this view.

The right parasternal short-axis views are obtained by rotating the transducer 90 degrees from the long axis, with the transducer mark oriented cranially. The following levels of the heart are systematically evaluated from the right parasternal short-axis view starting from the apex and moving to the most basilar aspect of the heart: the apex, the left ventricle and papillary muscles, the left ventricle at the level of the chordae tendinae (Figure 4), the mitral valve, the left atrium and aorta (Figures 5 and 6), the right ventricular outflow tract and the pulmonic valve (Figure 7), and lastly the pulmonary artery branches, the right auricle and caudal vena cava. M-mode measurements of the left ventricular size during systole and diastole are made at the level of the chordae tendinae in dogs (Figures 8 and 9). 2-D measurements of left ventricular size in cats is recommended since there may be assymetrical hypertrophy that may not be within the M-mode cursor. Left atrial and aortic diameters are most accurately measured in 2-D at the level of the aortic cusps, during diastole when they are closed. LA:Ao ratio is calculated, and > 1.5 suggests left atrial dilation (Figure 6). Allometric scaling of left ventricular size normalized to body length has been validated by several investigators and provides the most accurate reference values (Table 1).Sight hounds often have larger sized left ventricles than according to body size. When the left ventricular end diastolic diameter is increased, the term is eccentric hypertrophy, and signifies a volume overload to the left ventricle. When the left ventricular systolic diameter is increased, this signifies systolic myocardial failure, which may be primary or secondary. Reduced fractional shortening (EDD-ESD/EDD) is indicative of myocardial failure (Figure 9). E-point to septal thickness (EPSS) is measured by M-mode at the level of the mitral valve in the right parasternal short-axis view (Figure 10), and increases in EPSS are indicative of systolic myocardial failure (Figure 11). Increased wall thickness is termed concentric hypertrophy, and occurs secondary to pressure overload or is a primary abnormality due to hypertrophic cardiomyopathy (Figure 12). Left ventricular concentric hypertrophy in cats is defined as wall thickness ≥ 6 mm.

Table 1

The second thoracic acoustic window is the left apical (caudal) parasternal window, which is located between the left 5th and 7th intercostal spaces adjacent to the sternum. The transducer is aligned parallel to the long axis of the heart with transducer marker directed cranially. The left apical 4-chamber view is obtained, which depicts the heart "upside-down" with the apex closest to the transducer (Figure 13). The mitral and tricuspid valves can be carefully inspected, and this position allows excellent alignment for mitral inflow Doppler studies as well as color flow and Doppler assessment of atrioventricular valve insufficiencies. By slightly rotating the transducer, the left apical 5-chamber view is then obtained, which visualizes the left ventricular outflow tract and aorta in addition to the left and right chambers of the heart (Figure 14). This view is essential for color flow and Doppler evaluation of the left ventricular outflow tract and aorta since the beam is aligned parallel to blood being ejected out the aorta.

The third thoracic acoustic window is the left cranial parasternal location. The transducer is positioned between the left third and fourth intercostal spaces between the sternum and costo-chondral junction, with the probe aligned parallel to the long axis of the body and heart. Left cranial long axis views are obtained. The initial long axis view includes the left ventricular outflow tract and aorta. This view allows excellent visualization of heart base tumors and aortic valve abnormalities. By directing the beam to the left side of the thorax (almost parallel to the surface of the body), the pulmonary artery can be visualized. This is often the most effective view to image a patent ductus arteriosus and to measure minimal and maximal ductal diameters. It also often allows the best alignment for continuous wave Doppler measurement of PDA flow or pulmonic artery blood flow in pulmonic stenosis. By directing the beam towards the right side of the thorax, there is visualization of the right atrium, right auricle, tricuspid valve, and caudal vena cava. This view is essential when evaluating for possible right atrial/auricular masses (Figure 15).

Color flow Doppler is used to evaluate the direction, velocity, and flow characteristics (laminar versus turbulent) of blood. The color is encoded red for blood flow away from the probe, and blue for blood flow towards the probe. Aliasing occurs with laminar blood flow that just exceeds the Nyquist limit, and appears as a series of organized, layered color changes. Variance is the assignment of another color such as green, to indicate turbulent blood flow, and gives a "sparkling" appearance within the background of red and blue shades. Color flow Doppler is essential for rapid detection of valvular insufficiency, turbulent blood flow in outflow tract obstructions or valvular stenosis, and allows for spatial orientation of flow patterns with respects to the anatomic structures. Color flow also assists in alignment of the spectral Doppler beam for measurement of blood flow velocity.

Spectral Doppler consists of pulsed wave and continuous wave Doppler, and recordings are displayed with time on the x-axis and velocity on the y-axis. Blood flow toward the transducer is plotted above baseline, and away from the transducer is below baseline. Pulsed wave (PW) Doppler is used to assess flow characteristics (laminar versus turbulent) and measure low velocity blood flow within a small region of the heart. The gate of the cursor is positioned at the particular region of interest, and blood flow velocity at that particular region is measured. The Nyquist limit is the upper limit of velocity measured by PW Doppler, and is determined by the sample volume depth and the transmitted Doppler frequency. The lower the frequency of transducer, the higher the Nyquist limit. Laminar flow consists of red blood cells accelerating uniformly to a peak rate, and is depicted as a sharp narrow line (envelope) with a hollow core (Figure 16). Turbulent flow consists of blood moving at variable velocities, with a higher peak velocity. This causes the envelop to be filled in, termed "spectral broadening". PW Doppler enables anatomic localization of abnormal blood flow. Continuous wave (CW) Doppler is used to measure peak blood flow velocity, and is not constrained by low Nyquist limits (Figure 17). Signals are continuously transmitted and received along the length of the cursor, so anatomic localization of abnormal flow is impossible. Using the modified Bernouilli's equation, pressure gradient = 4x velocity2 , non-invasive estimates of pressure differences between 2 chambers is possible. This allows quantification of severity of stenotic lesions, and assessment of pressure gradients in valvular insufficiency or shunts. Stenotic lesions (subaortic stenosis or pulmonic stenosis) are characterized as mild when pressure gradients are < 50 mmHg, moderate when pressure gradients are 50-80 mmHg, and severe when gradients are > 80 mmHg (Figure 17). Pulmonary hypertension is diagnosed by an increased tricuspid regurgitation velocity (> 3.2 m/s; RV:RA pressure gradient > 40 mmHg) in the absence of an increased pulmonary artery systolic blood flow velocity. An increased pulmonic insufficiency velocity also indicates pulmonary hypertension. Increased mitral regurgitation blood flow velocity (> 6.3 m/s; LV:LA pressure gradient > 160 mmHg) in the face of normal aortic blood flow velocity indicates systemic hypertension.

Echocardiographic assessment of diastolic function can be difficult and time consuming. The most straight forward approach is assessment of atrial size. Left atrial dilation in the absence of other cardiac diseases such as mitral regurgitation, left to right shunting congenital heart defects, myocardial failure, or high output cardiac disease suggests significant diastolic dysfunction. More advanced methods of assessment of diastolic function include PW Doppler measurement of mitral or pulmonary venous inflow. Mitral inflow normally consists of a dominant early diastolic wave (E) and a smaller A wave during atrial systole (E:A 1-1.9). During high heart rates (> 150 bpm), there may be fusion of E and A waves. There are three patterns that occur during progressive left ventricular diastolic dysfunction. The initial pattern is termed delayed relaxation, and consists of prolonged isovolumic relaxation time (IVRT), reduced E wave, E:A reversal, and prolonged E deceleration time (Figure 18). This pattern is caused by impaired early diastolic filling, but left atrial pressure is usually normal. As diastolic function worsens, left atrial pressure increases, and a pseudonormal pattern occurs, where there is normal E:A ratio. This can be difficult to distinguish from a normal mitral inflow pattern, and measurement of pulmonary venous inflow may be helpful by identifying a retrograde Ar wave during atrial systole. In the most extreme form of diastolic dysfunction, left atrial pressure and left ventricular stiffness are markedly increased, resulting in a restrictive flow pattern with shortened IVRT, increased E wave, shortened E deceleration time, and increased E:A ratio > 2. Aside from E-A summation, other problems with mitral inflow velocity for assessment of diastolic function is the influence of preload. Increased preload may mask delayed relaxation pattern, whereas decreased preload may result in a delayed relaxation pattern.

Tissue Doppler imaging (TDI) echocardiography has emerged as a relatively preload independent measure of regional and global systolic and diastolic function. TDI selectively measures low frequency, high amplitude velocity of the myocardial movement. There is a Sm wave during systole (above baseline), an Em wave during early diastole and an Am wave during atrial systole (below baseline). TDI may be performed on the left ventricular free wall imaged in the right parasternal short-axis view, or may be performed on the lateral mitral annulus using the left apical 4-chamber view (Figure 19). TDI can be performed using PW Doppler peak velocity measurements or by color M-mode and off-line analysis of mean myocardial velocity and velocity gradients between the endocardium and epicardium. Decreased Em wave amplitude denotes diastolic dysfunction (Figure 19). Decreased S wave amplitude denotes systolic dysfunction. Left atrial pressure may be non-invasively estimated by E/Em, and has been validated in dogs. Left atrial pressure > 20 mmHg is highly probable when E/Em is > 9.1 in dogs with iatrogenic mitral regurgitation. TDI may allow early detection of systolic and diastolic dysfunction prior to detection of abnormalities on 2-D echo.

References

Thomas, W. P., Gaber, C. E., Jacobs, G. J., Kaplan, P. M., Lombard, C. W., Moise, N. S., and Moses, B. L. Recommendations for Standards in Transthoracic Two-Dimensional Echocardiography in the Dog and Cat. Echocardiography Committee of the Specialty of Cardiology, American College of Veterinary Internal Medicine. Journal of Veterinary Internal Medicine 1993;7:247-52.

Cornell, C. C., Kittleson, M. D., Della, Torre P., Haggstrom, J., Lombard, C. W., Pedersen, H. D., Vollmar, A., and Wey, A. Allometric Scaling of M-Mode Cardiac Measurements in Normal Adult Dogs. J Vet Intern Med. 2004;18:311-21.