EQUINE ECHOCARDIOGRAPHY

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EQUINE ECHOCARDIOGRAPHY 
 
Colin C. Schwarzwald, Dr. med. vet. 
The Ohio State University 
2004 
 
 
Introduction 
 
Over the last 25 years, echocardiography has emerged to become a standard diagnostic 
procedure in equine cardiology. Development of M-mode echocardiography in the mid-1970s 
and introduction of 2D real-time echocardiography in the 1980s allowed evaluation of internal 
cardiac structure, size and function in horses. Development of Doppler echocardiography lead to 
the ability to assess blood flow characteristics in the equine heart. Echocardiography is essential 
for diagnosing cardiac diseases in horses as well as in other species. 
The aim of this presentation is to give an overview on the general principles of equine 
echocardiography. Indications for an echocardiographic examination, its clinical relevance, and 
current limitations of equine echocardiography will be discussed and illustrated by case 
examples. Possible future developments will be outlined. 
 
 
Technical requirements 
 
The ultrasonographic equipment should include a low-frequency phased-array or 
annular-array sector transducer (2 - 3.5 MHz) with a maximal penetration of at least 26 to 30 
cm, in order to image the entire heart of an adult horse from one cardiac window. A high frame 
rate (at least 15 images/second = 15 Hz) is required for real time imaging of the moving 
structures of the heart. Simultaneous recording of an ECG allows exact timing of the images. 
 
 
Principles of equine echocardiography 
 
If possible, the horse should not be sedated prior to the echocardiographic examination. 
While structural information is still valid in a sedated horse, some cardiac dimensions (e.g. end-
systolic left ventricular diameter, interventricular septal thickness, and free wall thickness) and 
indices of cardiac function (e.g. fractional shortening and fractional area change) will be altered. 
The following imaging modes are used in routine echocardiography: Real time two-
dimensional (2D) or B-mode (“brightness” mode), M-mode (motion mode), color flow Doppler, 
pulsed-wave (PW) Doppler, and continuous-wave (CW) Doppler. A systematic approach using 
standardized image planes (identified by means of specific intracardiac landmarks) is important 
for comparison of studies over time and for comparison among different examiners. 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, respectively, are 
displayed to the right side of the screen. Standardized views obtained from a normal horse are 
shown in figures (a) to (i). 
Two-dimensional imaging (2D/B) 
 
1. Right parasternal long-axis views: 
The echocardiographic examination usually 
starts with the right parasternal long-axis views. The 
transducer is positioned in the right fourth intercostal 
space at a level slightly above the olecranon. The 
transducer is slightly rotated clockwise (1 o’clock). 
The three standard long-axis views are obtained by 
angling the transducer caudally (a), straight (b), and 
cranially (c), respectively. Slight changes in 
transducer placement, rotation, and angulation may 
be necessary to obtain standard views. 
 
Fig. a: The four-chamber view shows the 
right atrium (RA), tricuspid valve (TV), right 
ventricle (RV), interventricular septum (IVS), left 
atrium (LA), mitral valve (MV), and left ventricle 
(LV). 
Fig. b: The left ventricular outflow tract 
(LVOT) view shows the right atrium (RA), 
tricuspid valve (TV), right ventricle (RV), 
interventricular septum (IVS), left ventricle (LV), 
left atrium (LA), aortic valve (AV), aortic root (AR), 
and pulmonary artery (PA). 
Fig. c: The right ventricular inflow and 
outflow tract view shows the right atrium (RA), 
tricuspid valve (TV), right ventricle (RV), pulmonic 
valve (PV), pulmonary artery (PA), and the aortic 
root (AR). 
 
 
The right parasternal long-axis views allow: 
• Subjective evaluation of the size of the heart and the cardiac chambers, the structure and 
thickness of the chamber walls, the structure and function of the valves, the myocardial 
function, and the relationship between the cardiac chambers and the large vessels. 
• Measurement of the diameter of the aorta and pulmonary artery, diameter of the left atrium, 
area of the left ventricle in systole and diastole, and calculation of the fractional area 
shortening and ejection fraction. 
• Detection of blood flow disturbances using color flow Doppler echocardiography (see 
below). 
LV 
RA 
RV 
LA 
AR 
PA 
AV 
TV 
IVS 
b
LV
LA 
RV 
RA IVS 
TV 
MV a
c
RV RA 
PA 
TV 
PV 
AR 
2. Right parasternal short-axis views: 
The right parasternal short-axis views are 
obtained by rotating the transducer 90 degrees 
clockwise (4 c’clock). The three standard short-axis 
views are obtained by angling the transducer 
ventrally (d), straight (e), and dorsally (f), 
respectively. Slight changes in transducer placement, 
rotation, and angulation may be necessary to obtain 
standard views. 
 
Fig. d: Short-axis view of the left ventricle 
(LV). Papillary muscle (PM), interventricular septum 
(IVS), left ventricular free wall (LVW), right 
ventricle (RV). 
Fig. e: Short-axis view of the mitral valve 
(MV). Right ventricle (RV), interventricular septum 
(IVS). 
Fig. f: Short-axis view of the aortic valve 
(AV). Left atrium (LA), right ventricle (RV), 
pulmonary artery (PA). 
 
 
The right parasternal short-axis views allow: 
• Subjective evaluation of the size of the heart and 
the cardiac chambers, the structure and thickness 
of the chamber walls, the structure and function 
of the valves, and the myocardial function. 
• Measurement of the diameter of the left ventricle 
in systole and diastole and evaluation of left 
ventricular systolic function using M-mode 
echocardiography (see below). 
• Detection of blood flow disturbances using color 
flow Doppler echocardiography (see below). 
 
 
3. Left parasternal long-axis view: 
Images from the left cardiac window should be obtained when left-sided murmurs were 
detected, the entire heart could not be imaged from the right window, additional views of certain 
structures are required, or alignment with blood flow needs to be optimized for Doppler 
examinations. 
The transducer is positioned at a level slightly above the olecranon. The three standard 
long-axis views are obtained from the fifth (g), fourth (h), and third (i) intercostal space, with 
the transducer oriented perpendicular to the chest wall. Slight changes in transducer placement, 
rotation, and angulation may be necessary to obtain standard views. 
 
 
LV 
IVS 
RV 
LVW 
PM 
d 
IVS 
RV 
MV 
e 
LA
RV AV
PA 
f 
 
LV
RV
IVS 
LVW 
k 
 
 
 
 
 
 
 
 
Fig. g: Left atrium (LA), mitral valve 
(MV), left ventricle (LV), and right ventricle (RV) 
imaged from the left cardiac window. 
Fig. h: Left ventricular outflow tract 
imaged from the left cardiac window. Left ventricle 
(LV), aortic valve (AV), aortic root (AR), right 
ventricle (RV). 
Fig. i: Right ventricular outflow tract 
imaged from the left cardiac window. Pulmonary 
artery (PA), pulmonic valve (PV), right ventricle 
(RV), tricuspid valve (TV), right atrium (RA), aortic 
root (AR). 
 
 
M-mode imaging 
 
M-mode echocardiography shows the cardiac structures on the cursor line displayed over 
time. Motion of the heart and the valves can be recorded with a very high resolution and is not 
limited by frame rate. Recordings are usually obtained from the right parasternal short-axis 
views. Left parasternal short-axis views can be used if the maximal depth displayed by 
ultrasoundmachine does not allow imaging of the whole heart from the right side. 
 
 
 
Fig. k: M-mode 
image of the left ventricle 
(LV). Interventricular septum 
(IVS), left ventricular free 
wall (LVW), and right 
ventricle (RV). 
 
M-mode is routinely used for measurement of the left ventricular diameter, left 
ventricular free wall, and septal thickness at the chordal level in systole and diastole. 
Measurement of the right ventricular diameter is considered inaccurate and highly variable in 
this view, and is therefore not recommended. The fractional shortening, an indicator of left 
ventricular systolic function (contractility), can be calculated using the following formula: 
 
FS% = (LVIDd - LVIDs) / LVIDd x 100. 
LV
LA
MV 
RV 
g h 
AR 
LV 
RV
AV 
i 
RV 
PA 
RA 
AR 
PV 
TV 
Doppler echocardiography 
 
Doppler echocardiography is used to estimate blood flow velocities using the shift in 
ultrasound frequency which occurs after the ultrasound waves have been reflected by moving 
red blood cells (Doppler principle). 
Color flow Doppler is routinely used to screen for disturbances of normal blood flow in 
a specified area of interest (e.g. around a valve or in areas with congenital defects). Direction of 
blood flow, its velocity, and its characteristics (e.g. turbulent flow vs. laminar flow) can be 
assessed using a color-coded display superimposed to a 2D (for location) or M-mode (for 
timing) image. Traditionally, flow directed toward the transducer is coded red, flow away from 
the transducer is coded blue, and turbulent flow is colored green. 
In pulsed-wave (PW) Doppler and continuous-wave (CW) Doppler modes, blood 
flow velocity is displayed versus time. One important limitation is that Doppler studies must be 
performed with the ultrasound beam parallel to the direction of blood flow (max. 20° angle), in 
order to avoid underestimation of flow velocities. Very high flow velocities can be measured 
accurately in CW Doppler mode only. 
Pressure gradients between two chambers (e.g. between the left and right ventricle in the 
presence of a VSD, or between the right atrium and the right ventricle in the presence of 
tricuspid regurgitation) can be estimated using the modified Bernoulli equation: 
 
dp = 4 v2 
(dp = pressure difference; v = peak flow velocity) 
 
Estimation of pressure gradients is a very useful tool for evaluation of hemodynamic 
consequences of cardiac disease. For example, Doppler echocardiography can be used for 
assessment of hemodynamic significance of ventricular septal defects (pressure difference 
between right and left ventricle), or for detection of pulmonary hypertension (if tricuspid 
regurgitation or pulmonic insufficiency are present). 
 
 
Use of echocardiography in equine cardiology 
 
Indications and clinical use of echocardiography in horses 
 
Horses can suffer from congenital cardiac malformations, valvular disease, myocardial 
disease, other aquired cardiac defects, and cardiac arrhythmias. Heart murmurs are frequently 
detected on cardiac auscultation in horses. Although heart murmurs may indicated valvular 
disease or congenital malformations, they are also found in a large number of clinically normal 
horses and foals. Differentiation of physiologic (functional) from pathologic heart murmurs can 
be difficult or impossible based on physical examination and auscultation alone. Cardiac 
arrhythmias occur frequently in horses, either as primary disorders of impulse generation and 
conduction, or secondary to underlying structural cardiac disease. Detection of any underlying 
cardiac disease may have important implications on prognosis and treatment of arrhythmias. 
Occasionally, cardiac disease can lead to very unspecific clinical signs such as poor 
performance or fever. 
Echocardiography can be used to identify a cardiac disease, make the correct anatomical 
diagnosis, assess hemodynamic consequences, provide important prognostic data, monitor 
progression of the disease, and identify complications of a known diagnosis. 
 
The following table lists the most important indications for echocardiography in horses: 
 
Indication Clinical use and significance of echocardiography 
Evaluation of heart 
murmurs 
Differentiation between physiologic flow murmurs and 
pathologic murmurs (e.g. valvular regurgitation), assessment of 
clinical significance of pathologic murmurs 
Dysrhythmias Detection of underlying cardiac disease (e.g. mitral 
regurgitation and left atrial dilatation with atrial fibrillation) 
Suspected congenital 
defects 
Evaluation of heart murmurs, unexplained cyanosis, 
dysrhythmias, or signs of heart failure in neonates 
Exercise intolerance / 
poor performance 
Detection of cardiac disease 
Muffled heart sounds Detection of pericardial effusion 
Fever of unknown origin Detection of endocarditis 
Unexplained collapse / 
episodic weakness 
Detection of cardiac disease 
Clinical signs of congestive 
heart failure 
Cause of heart failure, assessment of severity, monitoring of 
progression and response to treatment 
Severe respiratory disease Detection of pulmonary hypertension, detection of patent 
foramen ovale in foals with respiratory disease 
 
 
Possibilities, limitations, and a glimpse to the future 
 
Echocardiography is a well established diagnostic method in equine medicine, and 
allows for diagnosis of heart disease, assessment of cardiac function, and better understanding 
of the normal physiology of the heart. Animal size, anatomical characteristics of the equine 
thorax, and technical features of (human) echocardiography equipment sometimes limit the 
ability to image the equine heart. While this is usually less problematic for standard 2D imaging, 
determination of blood flow velocities using Doppler imaging techniques is often difficult, due 
to the inability to achieve adequate alignment with blood flow. 
Alignment for Doppler echocardiographic studies of left ventricular inflow and outflow 
can be improved by using transesophageal echocardiography. Although this technique has 
been described in horses, it is not routinely used in a clinical setting. As echocardiographic 
technology advances, it may be possible that this technique will be applied for the evaluation of 
cardiac function in resting and exercising horses. 
Post-exercise (stress) echocardiography is occasionally used for evaluation of cardiac 
function in athletic horses. It is performed during the first 2 to 3 minutes following high-
intensity exercise, preferably on a treadmill. Detection of myocardial dysfunction and wall 
motion abnormalities after exercise has been associated with poor performance and exercise 
intolerance in athletic horses. However, there are currently no standard guidelines on how to 
assess post-exercise studies in horses, and evaluation remains quite subjective. 
Newer technologies such as Tissue Doppler Imaging may find some applications in 
equine echocardiography and may allow for better assessment of diastolic heart function, 
detection of myocardial dysfunction, and wall motion analysis at rest and after exercise. Clinical 
applications of this technique are still being developed in humans, and tissue Doppler imaging 
has not been applied yet to horses. 
Further work needs to be done to establish these newer techniques for use in clinical 
routine echocardiography in horses. 
 
 
Case examples 
 
The principles of echocardiography in horses, normal and abnormal findings in a variety 
of cardiac diseases, the clinical relevance of echocardiographic findings, and current limitations 
of equine echocardiography will be discussed and illustrated using case examples. 
 
 
Selected References 
 
• Boon JA: Manual of Veterinary Echocardiography. Williams&Wilkins, Baltimore, 1998.• Reef VB: Equine Diagnostic Ultrasound. W.B. Saunders, Philadelphia, 1998. 
• Marr C: Cardiology of the Horse. W.B. Saunders Company, London, 1999. 
• Patteson MW: Equine Cardiology. Blackwell Science, Oxford, 1996. 
• Blissitt KJ, Marr CM, Rossdale PD, and Green RE: Equine Cardiovascular Medicine. 
Equine Veterinary Journal, Suppl. 19, September 1995. 
• Bonagura JD, Herring DS, Welker F: Echocardiography. Veterinary Clinics of North 
America Equine Practice, August 1985, 311-333. 
• Otto CM: Textbook of Clinical Echocardiography. W.B. Saunders, Philadelphia, 2000. 
• Oh JK, Seward JB, Tajik AJ: The Echo Manual. 2nd Ed. Lippincott Williams&Wilkins, 
Philadelphia, 1999. 
 
Addendum 
 
Normal ranges of selected cardiac measurements in adult horses. 
 
Measurement Mode and 
Imaging plane 
Normal Range 
mean (min-max) 
Population References
LVIDs (cm) M RSXC 7.5 (5.8-8.8) THB and THB cross 1, 3 
LVIDd (cm) M RSXC 11.9 (9.7-13.4) THB and THB cross 1, 3 
IVSs (cm) M RSXC 4.4 (3.2-5.6) THB and THB cross 1, 3 
IVSd (cm) M RSXC 3.0 (2.3-3.7) THB and THB cross 1, 3 
LVWs (cm) M RSXC 4.0 (3.0-5.4) THB and THB cross 1, 3 
LVWd (cm) M RSXC 2.3 (1.7-3.4) THB 3 
FS (%) M RSXC 38.1 (29.0-47.0) THB and THB cross 1, 3 
LAD (cm) 2D LLX 
End-diastole 
Mid-atrium 
11.9 (9.4-14.5) 
max. 13.5 
max. 14.0 
THB and THB cross 
THB and STB 
Larger horses 
2, 3 
4 
4 
AoD (cm) 
(Sinus of 
Valsalva) 
2D RLX 
End-diastole 
8.3 (7.3-9.9) 
 
THB and THB cross 2, 3 
PAD (cm) 2D RLX 6.1 (5.2-8.0) THB 3 
LA:Ao 2D 1.4 (1.2-1.7) THB and THB cross 2 
PA:Ao 2D 0.7 [PA < Ao] THB and THB cross 2, 3 
 
The normal ranges were summarized based on the listed references. Please see references for 
detailed description of the imaging planes and anatomical landmarks used for standardized 
measurements. 
 
LVID - left ventricular internal diameter; IVS - interventricular septum; LVW - left ventricular 
free wall; FS - fractional shortening of the left ventricle; LAD - left atrial diameter; AoD - 
diameter of the aorta; PAD - diameter of the pulmonary artery; s - systole; d - diastole; 
M - M-mode ; 2D - two-dimensional echocardiography ; RLX - right long axis; RSXC - right 
short axis at chordal level; LLX - left long axis. 
 
References: 
1 Long KJ et al. Standardized imaging technique for guided M-mode and Doppler 
echocardiography in the horse. Equine Vet J 1992; 24: 226-235 
2 Voros K et al. Measurement of cardiac dimensions with two dimensional echocardiography in 
the living horse. Equine Vet J 1991; 23: 461 
3 Patteson MW et al. Echocardiographic measurements of cardiac dimensions and indices of 
cardiac function in normal adult Thoroughbred horses. Equine Vet J 1995; Suppl 19: 18-27 
4 Virginia B. Reef: Equine Diagnostic Ultrasound. W.B. Saunders Company, Philadelphia, 
1998. 
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