Introduction to equine cardiology (Proceedings)

Primary cardiovascular disease is relatively uncommon in horses.  This may be real, or it may be an indication that cardiac disease is not recognized readily in many horses. Subtle, early signs of cardiovascular disease are often overlooked and identification of mild problems may be difficult.  On the other hand, identification of more severe problems may be fairly obvious, but treatment may be impractical or impossible.

At NCSU, the most common reason for referral for a cardiac workup is detection of a heart murmur or arrhythmia. Often the murmur or arrhythmia is detected on prepurchase or insurance exam, and the veterinarian is being asked to predict future performance ability or safety. 

However, other potential signs of cardiac disease that may warrant referral include decreased performance or exercise intolerance, inappropriately high heart rate during exercise, increased time to cool out after exercise or for heart rate to return to its resting level, increased resting heart rate, an irregular heart rate, poor pulse quality, increased capillary refill time, a prominent jugular vein or jugular pulse, ventral or limb edema, and/or cough.

Thorough examination of the cardiovascular system includes careful physical examination, echocardiogram, and electrocardiogram if an arrhythmia is present. Additional tests such as blood pressure measurement, thoracic radiographs, etc, may be warranted in certain cases, but often are unnecessary.  Laboratory tests that may be useful, depending on the problem, include CBC, serum chemistry panel, measurement of cardiac troponin, and blood cultures. Therapeutic monitoring is available for some drugs, including digoxin and quinidine.

Physical examination:

When cardiac disease is suspected, it is tempting to focus on auscultation of the heart. However, it is also important to assess mucous membranes for color and capillary refill time, palpate pulse quality, feel the extremities for warmth, and notice whether there is generalized venous filling, edema, or a jugular pulse. Cardiac auscultation should then be performed carefully in a quiet environment. The heart should be ausculted over all 4 major valve areas.

It is possible to hear 2-4 of the 4 heart sounds normally, and it may vary which heart sounds can be heard at different spots on the chest wall. The 4 normal heart sounds that can be heard in the horse are S1 (atrioventricular valve closure), S2 (semilunar valve closure), S3 (rapid ventricular filling), and S4 (atrial contraction). S1 and S2 are the “lup” and the “dup”.  When all four heart sounds are heard, the sequence is S4, S1, S2, S3, pause for diastole, S4, S1, S2, S3, pause for diastole, etc.  However, you may also hear S4, S1, S2, pause.... or S1, S2, S3, pause....These may sound like “gallup” rhythms, but are not considered to be abnormal in the horse.

Murmurs

Cardiac murmurs are caused by turbulent blood flow.  If you listen carefully, you may find low intensity murmurs to be fairly common in horses, especially if you listen right after exercise or when the horse is excited. The key is to determine whether the murmur is an innocent "flow murmur" or if it represents actual pathology. And if there is pathology, it is important to determine whether or not the heart is compensating adequately for the problem.

This is where cardiac ultrasound, or echocardiography, has greatly improved our ability to diagnose problems and predict future performance or quality of life. However, even without echocardiography, certain conclusions can be made by carefully listening to the murmur and accurately describing it. In general, murmurs are more likely to adversely affect performance if they are loud and if they radiate widely (eg can be heard over several valve areas or on both sides of the chest, etc.)

Murmurs are generally characterized by location in the cardiac cycle (systolic vs diastolic), timing (early, mid, late or pan), shape or frequency (band, crescendo, decrescendo), intensity, and point of maximal intensity. There are two grading systems for describing intensity of murmurs. One uses 5 grades, the other uses 6. They are pretty similar except the 5-grade system combines two of the middle grades of the 6-grade system into one. Most people are familiar with the 6-grade system.

In this system, a grade 1 murmur is very localized, barely audible, and heard only after careful listening in a quiet environment. A grade 2 murmur is also low intensity, but is fairly easily heard with careful listening. A grade 3 murmur has moderate intensity and is easily heard. A grade 4 murmur is loud, but you cannot feel a thrill (ie you cannot palpate a vibration on the chest wall when you place your hand over the heart). A grade 5 murmur is a loud murmur with a thrill, and a grade 6 murmur can be heard even when you hold the stethoscope a centimeter off the chest wall.

In the horse, a low intensity (grade 1-2) systolic murmur heard very localized over the aortic valve area may be a “flow” murmur. This means the sound is produced by a large cardiac output going through a normal aortic valve. It is more evident when cardiac output is increased, such as when the horse has just been exercising or is excited. It can also occur if the viscosity of the blood is low, such as with anemia or hypoproteinemia, or if cardiac output is increased with fever. The most common causes of pathologic systolic murmurs in horses include mitral or tricuspid valve regurgitation.

Although a systolic murmur could also theoretically be caused by a stenotic semilunar valve, acquired valve problems in horses are almost always regurgitant, not stenotic.  Ventricular septal defect and some other congenital problems can also cause a systolic murmur. A widely radiating systolic band shaped murmur, heard best on the left side where the girth goes is most likely caused by mitral regurgitation.  Systolic murmurs heard best on the right side of the chest are most likely tricuspid regurgitation or a VSD. The presence of a jugular pulse would make tricuspid regurgitation more likely.

Diastolic murmurs can be caused by semilunar valve regurgitation or atrioventricular valve stenosis. However, to reiterate, stenotic lesions are almost non-existant in horses.  By far the most common diastolic murmur in horses is a decrescendo murmur heard best on the left side of the chest and is caused by aortic valve regurgitation. 

The severity of regurgitation is better predicted by the strength of the pulse than by the intensity of the murmur. With aortic valve regurgitation, arterial pressure falls off more rapidly and diastolic pressure is lower, making the difference between systolic and diastolic blood pressures greater. This makes the pulse pressure that you feel seem stronger, even though mean arterial pressure is unchanged. Some refer to this as a BB or water hammer pulse.

Arrhythmias

Arrhythmias are very common in horses and not always pathologic. In fact, the most commonly encountered arrhythmia, 2nd degree atrioventricular block, is most often caused by high resting vagal tone in relaxed, fit horses. The heart beats 3-4 times, pauses, then beats 3-4 times again. If you listen carefully, you can predict when the next “dropped beat” will occur, and you will be able to hear S4 without S1, S2, or S3 following it. 

It is also common to hear arrhythmias right after exercise, as the heart rate slows back down to its normal resting rate. This is thought to be caused by imbalances in sympathetic and parasympathetic tone, and the arrhythmia goes away once resting heart rate has returned.  Other arrhythmias encountered in horses include atrial fibrillation (which results in an irregularly irregular heart beat) and atrial or ventricular premature contractions. Electrocardiography is necessary to properly identify arrhythmias.

Electrocardiography

Electrocardiography records the electrical events of the cardiac cycle.  In equine medicine, it is primarily important in the diagnosis of arrhythmias.  In fact, it is the only way to definitively identify an arrhythmia. However, most experts will agree that ECGs are not useful for measurement of heart size or Mean Electrical Axis in equine medicine, as they are in small animal medicine. This is because much of the equine heart is electrically silent when measured on the body surface. Vector analysis is not much use in the horse for determining right vs left hypertrophy, because of the following properties of equine hearts:

            1.         Explosive depolarization

a.        Due to the Purkinje network anatomy

            2.         Ventricular septum

b.        The bulk of the QRS complex recorded on the surface of the body was due to depolarization of the septum only.

            3.         Bulk of myocardium silent to ECG

c.        Due to multiple cancellation of signals

Therefore, although it is possible to record the usual 6 leads in horses, most clinicians usually only record what is called the "base-apex" lead because they are only interested in determining the rhythm or characterizing an arrhythmia.  The base-apex lead passes current from the right jugular groove or withers area to the left cardiac area, maximizing the orientation of the ECG current to the overall flow of current through the heart.

Components of electrical conduction (same as in small animal):

·         P wave-atrial depolarization, the S4 is synchronous with atrial contraction just after onset of the P wave.

·         P-Q or P-R interval-the electrically silent period representing conduction through the A-V node, bundle of HIS, bundle branches and purkinje fibers.

·         QRS waves-ventricular depolarization, S1 begins around the middle of QRS and initiates ventricular systole.

·         S-T interval-time from completion of ventricular depolarization, to termination of repolarization.  

·         T wave-ventricular repolarization, occurs just prior to S2 and is at the conclusion of ventricular systole.

·         T-P interval-the sinus node depolarization is an electrically silent event preceding the P wave.

Echocardiography

Echocardiography is by far the most useful tool currently available to equine practitioners to examine equine hearts. An echocardiogram is a recording of high frequency (>20,000 Hz = inaudible) ultrasound waves as they are reflected off the intrathoracic structures (similar to radar or seismic recordings). Every time ultrasound hits tissue of different acoustical impedance (i.e., density) it reflects and refracts. This produces an image of the structures underneath the skin. The greater the difference in acoustic impedance, the greater the reflection.  The amount reflected also depends on the angle of the beam; the more perpendicular the beam, the greater the reflections (echos).

Ultrasound waves are made by passing voltage back and forth across piezoelectric crystals in a transducer. This deforms the crystals, producing oscillations that produce sound waves. The frequency of the oscillations determines the depth of penetration into the tissues, as well as the resolution. In order to get the depth necessary to examine an adult equine heart, the frequency needs to be 2.5 to 3.5 MHz.

Uses of echocardiography

1.        Measurements

a.        Wall thickness

b.        Chamber / vessel dimensions

2.        Myocardial Function

a.        Wall motion

b.        Fractional shortening

3.        Valvular Disorders

a.        Insufficiency / regurgitation

b.        Vegetative or fibrotic lesions

c.        Stenosis (very uncommon in horses)

4.        Anatomical Disorders (Congenital or Acquired)

5.        Contrast Echocardiography

a.        Injection of microbubbles in saline, indocyanine green dye, or blood into the vasculature or heart to observe for shunting

Blood pressure

Blood pressure measurements are probably underutilized in equine medicine. Although direct blood pressure measurements can be difficult to obtain in conscious horses, indirect measurements are relatively easy to obtain. The most common site used for obtaining an indirect blood pressure measurement is the base of the tail. It should be noted that the base of the tail is significantly higher than the base of the heart. Therefore, to be precise, the pressures obtained should be adjusted for the effect of gravity.

Since the specific gravity of blood is about 1.050 to 1.060 and that of mercury about 13.6, a pressure of 1 mmHg should theoretically be added to or subtracted from the weight of the blood for every 13 mm (1.3cm) above or below the heart that the measurement is taken. However, many people use uncorrected tail cuff measurements in the horse. Since the actual pressure measured is not completely accurate by the indirect method, changes in blood pressure within the same individual are often more useful clinically than the actual pressure.

For indirect blood pressure measurement, the cuff size should be selected so that the width is 1/4 the circumference of the tail.  For most adult horses, a 4.5-5.6 cm cuff is usually used for oscillometric devices, while a 10.6 cm cuff is used for ultrasonic flowmeters. Normal indirect blood pressures (unadjusted for height above or below the heart) have been reported as follows:

·         Coccygeal artery pressure:                            86 to 137   /   40 to 72 mmHg.

·         Median artery pressure:                               144 to 194   /   108 to 150 mmHg.

Radiography

Radiographic examination of the heart is limited in adult horses to lateral views. As such, radiographs of the heart are unrewarding. At best, they might give one an idea that the heart looks big. However, thoracic radiographs might be useful in a horse with cardiac disease to look for potential accompanying pulmonary changes, such as distention of pulmonary vasculature, pulmonary congestion or edema.

Both lateral and ventrodorsal thoracic radiographs can be taken in foals. Angiography can also be performed in foals for diagnosis of congenital heart disease.  Angiography allows measurement of oxygen saturation and pressures in various chambers of the heart, pulmonary artery, aorta, etc. Contrast studies can also be performed. However, echocardiography is usually sufficient for diagnosis of congenital problems in foals.

Nuclear angiocardiography

Nuclear scintigraphy overcomes the physical limitations of radiographic contrast cardiography in adult large animals. However, similar to radiographic angiography, nuclear angiography is rarely used since echocardiography can usually adequately reveal a diagnosis and a prognosis.

Uses of nuclear angiocardiography

1.     Identification of

a.           Valvular dysfunction

b.           Intracardiac shunts (ASD, VSD)

c.           Extracardiac shunts (PDA)

d.           Congestive heart failure (prolonged transit)

2.     Measurement of

a.           Cardiac output (indicator dilution principle)

b.           Ejection fraction

c.           Chamber volume

d.           Valvular regurgitation fraction

e.           Pulmonary/ systemic perfusion ratios.