Echocardiography

There have been few studies of the normal echocardiogram of neonatal horses [1,2]. For more practical details go to Ultrasonography of the Fetus and the Neonate.

I. Potential Information

Potential Information to be gained from the M-mode, two-dimensional (2D) or Doppler echocardiogram includes:

1. Cardiac anatomy, including atrial and ventricular septa and walls, valves, and great vessels. Combined with IV injection of saline containing microbubbles, right-to-left intracardiac shunting disorders can be demonstrated -2D is superior to M-mode for this purpose.
2. Cardiac chamber size and wall thicknesses - M-mode is easier to measure and the technique is considered to be the gold standard of echocardiographic measurements.
3. Cardiac function - Combining measurements of wall thickness, systolic and diastolic chamber dimensions, and LV shortening fraction [SF% = (LVED-LVFES)/LVED X 100] allows evaluation of myocardial contractility and recognition of volume overload or pressure overload states.
4. Evaluation of blood flow - Doppler echocardiography is the most accurate non-invasive means of velocity measurements.

II. Echocardiogram of the Normal Neonatal Horse

1. Normal M-mode echocardiographic measurements reported in Thoroughbred and pony foals are shown in Tables 1-5 [1,2]
2. Normal 2D echocardiography is now routinely performed in every neonatal ICU. Reproducible images can be obtained from both right and left intercostal transducer positions. Right intercostal long-axis views are useful for evaluating the right and left atria and ventricles, mitral, tricuspid and aortic valves, atrial and ventricular septa. Right intercostal short-axis views are useful for evaluating cardiac contractility in M-mode, evaluation of the aortic and pulmonary arteries and semilunar valves. Left caudal intercostal long-axis and 4 chamber views are useful for evaluating both atria and ventricles, mitral and tricuspid valves, and atrial and ventricular septa. Left cranial intercostal short-axis views are useful for evaluating the aortic and pulmonic valves, pulmonary artery, ascending aorta and, in some cases, the ductus arteriosus.
3. Doppler Echocardiography [3,4]
   Doppler echocardiography is used for non-invasive blood flow evaluation of the heart and great vessels. It has several types: Continuous-wave (CW), Pulsed-wave (PW), and Color-flow (CF). CW Doppler is used to accurately determine peak velocities of blood flow. CF Doppler is a special form of pulsed wave Doppler, in which Doppler signals appear as color-coded pixels superimposing the 2D or the M-mode image within the Doppler window. The frequencies are color coded, and flow to the transducer appears as red, flow away from the transducer appears as blue. The lighter colors refer to higher velocities.
4. Contrast Echocardiography [3,4]
   Contrast echocardiography involves the creation of microbubbles in the circulation using injectable agents (carbon dioxide, 0.9% NaCl, 5% Dextrose, indocyane dye). These agents are usually mixed with the blood of the patient and injected back to a vein. The technique allows imaging the acoustic difference between the microbubbles and the patient’s blood, and used to visualize, right to left shunts.
5. Transesophageal Echocardiography[3,4]
   Transesophageal Echocardiography (TEE) can be performed in foals with standard TEE transducers. The technique provides a superior visualization of the great vessels, and can be carried out when transthoracic approach failed to image a particular structure.

III. Echocardiographic Examination

Echocardiographic examination can be useful in evaluating the following types of neonatal disorders [5]:

1. Congenital cardiac anomalies, including simple defects such as aortic or pulmonic stenosis, atrial or ventricular septal defects, mitral and tricuspid dysplasia, and complex or multiple defects. The severity of a pressure or volume overload state may be estimated by the degree of ventricular hypertrophy or dilation present.
2. Myocardial depression (decreased contractility), indicated by normal to increased LV end-diastolic and increased end-systolic dimension or volume, and resulting decreased fractional shortening (FS).
3. Hypovolemia, indicated by decreased atrial and ventricular diastolic dimension or volume.
4. Primary pulmonary/pleural disorders, in which serious cardiovascular disease can usually be confidently excluded as the cause of clinical signs.
5. Inflammatory processes within the heart or adjacent organs (endocarditis, pericarditis) can be evaluated.
6. Traumatic injuries (heart or great vessel lacerations, cardiac tamponade) can be imaged usually just immediately before death.