Veterinary cardiac auscultation made easy

Cardiac auscultation is an important part of the physical examination. And an accurate physical examination is the principal source of information about a patient's condition. Combined with a thorough history, this information is used to guide further diagnostics.

Since you are usually the first member of the clinical team to examine the patient, it's very important for you to understand the cardiac sounds when you perform the initial evaluation. You should have no problem hearing most cardiac murmurs and relaying your findings to the doctor in an organized, concise fashion. A standard nomenclature has been developed to simplify the process, which includes a grading scale for rating murmur loudness, descriptors of murmur quality and location, and names for transient cardiac sounds. These terms are often misused, so it's important to clarify their proper usage. While we will focus on murmur and heart sounds, keep in mind that you will also need to listen for arrhythmias, some of which might be transient.

What you need

First, let's discuss the right equipment. It's worthwhile to invest in a top-quality stethoscope for good cardiac auscultation. The adage, "You get what you pay for," is true when purchasing a stethoscope. Select a reputable manufacturer and a high-quality model.

Features of a good stethoscope include short (25 inches or less) double-lumen tubing, a brass or high-quality steel head with both large and small diaphragms, and a bell. The diaphragm allows for easy auscultation of high-frequency sounds, and the bell is designed for soft, low-frequency sounds.

A separate bell side is more useful in veterinary medicine than a combined diaphragm and bell side because of the patient's hair. With the combination stethoscope head, extra pressure must be applied to "get through" the hair coat. The extra pressure allows the hair to rub on the diaphragm, making it difficult to hear the low-frequency sounds made audible by the bell.

Using the bell side requires a very light touch, as too much pressure will tense the skin, causing it to act like a diaphragm. A neonatal stethoscope is helpful with puppies and kittens. Your stethoscope should also have comfortable earpieces. Electronic stethoscopes are available, but their high cost is prohibitive for most veterinary technicians.

Where to listen

During the auscultation, you should be completely focused on the heart sounds. The most important part of the stethoscope is the bit between the earpieces. You should take your time, since rushing will virtually guarantee missing subtle sounds. Your auscultation should be done in a relatively quiet place to ensure background noises do not interfere.

To begin the auscultation, start by palpating the precordium, or the outside of the thorax over the area of the heart. Locate the heartbeat against the chest wall. Known as the apical impulse, it is normally felt on the left hemithorax at about the fifth to sixth intercostal space. Some breed variation in the location of the apical impulse is normal. A stronger apex beat on the right hemithorax is abnormal. If the apex beat is faint or absent, it may be due to obesity or pericardial effusion.

During palpation of the precordium, you should be sensitive to palpable thrills. A thrill is felt when a murmur is so strong that the turbulence associated with the murmur can be felt through the chest wall. The sensation is quite characteristic and feels like a buzzing on your fingers. Take care to distinguish a strong apical impulse from a true thrill.

Now it's time to listen. Cardiac sounds can be divided into two groups: transient sounds and murmurs.

Transient heart sounds

Transient sounds are short in duration. This category includes the normal heart sounds as well as abnormal sounds. Murmurs are indicative of turbulent blood flow in the heart and are typically longer in duration.

The first and second heart sounds (S1, S2) are considered normal heart sounds in all animals and are commonly referred to as the lub and dub. In equine and bovine patients, additional normal sounds, identified as S3 and S4, can normally be heard and are associated with early ventricular filling and atrial contraction, respectively. In small animals, however, S3 and S4 are always considered abnormal and are commonly referred to as gallops. Gallops can be difficult to hear at higher heart rates and are most commonly recognized in cats with hypertrophic cardiomyopathy. Any time you hear three heart sounds in small animals, it is abnormal.

Other transient sounds that can be heard include split sounds and systolic clicks. Split sounds are heard when two valves, either the atrioventricular (AV) valves or the semilunar valves, close at slightly different times. In veterinary medicine, it is usually S2 that splits in the presence of bundle branch blocks. Systolic clicks are the sound of AV valve prolapse. During systole, as the ventricular pressure rises, a weakened mitral or tricuspid valve can suddenly buckle into the atrium, causing a high-pitched snap sound. This snapping is heard between the S1 and S2 sounds and is often confused with a gallop because "three heart sounds" are heard.

To differentiate gallops from systolic clicks, remember that clicks tend to be louder. Carefully consider timing to distinguish the gallop from the click. Systolic clicks will be heard most commonly in small-breed dogs predisposed to valvular endocardiosis, such as Cavalier King Charles spaniels. Eventually, the prolapsing valve will leak, and a murmur will develop, masking the sound of the click. Consequently, systolic clicks are not commonly identified.

Murmurs

Murmurs are the sound of turbulent blood flow in the hemocardiac structures and are the most common abnormal cardiac sounds you will hear. In a normal heart, blood flow moves in a laminar fashion. That is, all the blood moves in a smooth unidirectional motion. Turbulence is created when the blood moves in multiple directions at varying velocities all at once. The turbulence creates vibrations, which you can hear with a stethoscope or, in extreme cases, you can feel with your fingers as a thrill. The grade of the murmur is not always correlated with the severity of disease. Radiographic and echocardiographic examinations are necessary for a complete diagnosis and prognosis. However, accurate isolation and identification of the murmur can limit the list of rule-outs.

Knowing how to describe what you hear is the first step. Murmurs can be described by their location, timing, and loudness. As you will see, by using this scheme, you might tell your veterinarian that you hear a grade 3 systolic murmur that is heard best over the mitral valve, or you might describe the same murmur as a left apical grade 3 systolic murmur; both are correct since they describe where you heard the murmur (left side over the mitral valve or left apex), the loudness (grade 3), and the timing (systolic).

Location

Murmur location is described as either right or left hemithorax and by valve area of the point of maximal intensity (PMI). The PMI is the location where a murmur is heard the loudest. This term is often mistakenly used to mean the apex beat.

The mitral valve is best heard at the location of the left apex beat (Figure 1). From there, move the stethoscope cranial and dorsal roughly one intercostal space in each direction. You will then be over the aortic valve. Next, move approximately one half intercostal space cranial and ventral to locate the pulmonic valve. Before proceeding on to the right hemithorax, you should also listen deep in the axillary region, where the great vessels cross. This area is the best site for hearing the continuous murmur of patent ductus arteriosus (PDA). Now move to the right hemithorax. The tricuspid valve is located on the right side, directly across from the mitral valve. Last, move the stethoscope to the cranial dorsal region of the right hemithorax for murmurs of ventricular septal defects.

Figure 1: A view of the left side of a dog showing the approximate valve location for auscultation. Auscultation should begin at the mitral valve (M) where the PMI (point of maximal intensity) is normally located, and then proceed to the aortic (A) and pulmonic valves (P), respectively. (GV = the great vessel area for hearing patent ductus arteriosus, or PDA).

A simplified method of describing PMI location is to identify a murmur ventral to the costochondral junction (the mitral valve area) as apical and one that is dorsal to the costochondral junction (the aortic and pulmonic valve regions) as basilar and the side of the chest (right vs. left).

Timing

The timing tells much about which condition might be causing an abnormal sound. Timing is the period of the cardiac cycle in which the sound occurs. Sounds can be systolic, diastolic, or continuous. The periods of the cardiac cycle are identified by the normal heart sounds S1 and S2. The S1 sound indicates the closing of the AV valves and signals the beginning of systole. The S2 sound is associated with the closure of the semilunar valves (aortic and pulmonic) and indicates acoustically the end of systole or the beginning of diastole. The period between S2 and the next S1 is diastole.

Murmurs that last the entire duration of systole are holosystolic and may mask S1 and S2. A murmur that occurs in systole, breaks in sound, and returns again in diastole is called a systolic/diastolic, or to and fro, murmur. Continuous murmurs are present throughout both periods of the cardiac cycle but never break in sound. The to and fro murmur has a sound reminiscent of sawing wood, where the continuous murmur is more like a washing machine.

Loudness

Table 1: Scale for loudness of heart murmurs

The established scale for cardiac murmur rates the loudness of a murmur from 1 to 6, with 6 being the loudest (Table 1). Some subjectivity is allowed in the system based on the listener's skill and hearing capabilities. The general idea is still transmitted, understanding that a grade 5 murmur is very different than a grade 2.

Pitch and quality

When discussing murmurs, you can characterize the pitch and quality of the sounds to more completely describe murmurs. Pitches are usually described as high or low, while the quality of the murmur refers more to the changes in tone. The quality of murmurs can be displayed with a phonocardiogram that can show the shape of the tone. These shapes are the plateau, crescendo, decrescendo, and crescendo-decrescendo, or diamond-shaped (Table 2 on page 18). Some murmurs have characteristic sounds, such as subaortic stenosis, which commonly has a crescendo or crescendo-decrescendo quality at the left heart base. These murmurs are sometimes referred to as ejection murmurs and tend to start slightly after the onset of systole or stop just before the end. In opposition, the murmur of mitral regurgitation is typically a plateau shape with a low, harsh pitch.

Table 2: Pitches and quality of heart murmurs

Organic vs. physiologic

Murmurs can be classified in two groups: organic and physiologic. Organic murmurs are present due to a change or abnormality in the architecture of the cardiac structures. Some murmurs exist in the presence of a structurally normal heart and are known as physiologic, functional, or innocent. These murmurs may not be related to heart disease at all. An innocent murmur is a term used for soft murmurs heard in puppies or kittens less than 16 weeks of age that the animals normally outgrow. Functional or physiologic (these terms are interchangeable) murmurs can be heard in patients with fever, anemia, or metabolic high output conditions (e.g., hyperthyroidism) or in patients of advanced athletic training. You will hear physiologic or innocent murmurs best on the left hemithorax over the aortic valve, as they are usually created by a small amount of turbulence at the aortic valve. These murmurs are of medium to low intensity, typically a grade 3 or less. As the underlying condition that creates the murmur is treated, the murmur should disappear completely.

Mitral and tricuspid regurgitation

The most common murmur you will hear is mitral regurgitation. These murmurs are extremely common in cats and small-breed dogs. You will hear this murmur as systolic and over the left apex of the heart. The loudness is variable, ranging grade 2 to 6. Echocardiographic and thoracic radiographic examinations are required to fully assess these dogs. Many small-breed dogs have grade 3 to 4 murmurs with no evidence of congestive heart failure. Mitral regurgitation can be caused by endocardiosis, endocarditis, volume overload, or dysplasia.

Tricuspid regurgitation is almost identical to mitral regurgitation in quality, pitch, and timing. Tricuspid murmurs are loudest in the right hemithorax. Unfortunately, it is often difficult to tell if the murmur on the right is mitral regurgitation radiating or if it is true tricuspid regurgitation. In some cases, a subtle difference in pitch or quality can be a clue to true tricuspid regurgitation, but an echocardiographic examination is the best diagnostic test for conformation.

Subaortic and pulmonic stenosis

Murmurs of subaortic and pulmonic stenosis are also important murmurs since they are caused by a congenital heart defect. You may find it difficult to separate the two by auscultation alone. The valves are close to one another and sound the same. You will hear subaortic stenosis murmurs in golden retrievers, Newfoundlands, boxers, and other large-breed dogs. You will hear pulmonic stenosis in bulldogs, small-breed dogs, terriers, Labrador retrievers, and boxers. Subaortic and pulmonic stenosis murmurs are heard at the left heart base. In this case, the grade of the murmur may correlate better with the severity of disease, but not always. Doppler echocardiography is required to determine the exact valve in question and the severity of the obstruction.

Patent ductus arteriosus

As previously stated, in the extreme left axillary region, you will hear the continuous murmur of the patent ductus arteriosus (PDA). Patients with PDA routinely have a moderate to severe amount of mitral regurgitation. Combined with the systolic portion of the PDA, it is easy to miss the diastolic portion of the murmur. When you hear a continuous murmur in the left axillary region, a PDA is virtually assured. Again, echocardiography is used to confirm the diagnosis and to potentially guide treatment options. This is the one congenital heart defect that you can almost always diagnose by auscultation.

Ventricular septal defects

In patients with ventricular septal defects, you will hear a loud (grade 4 to 6) right basilar systolic murmur. The ventricular septal defect presents an instance in which the grade of the murmur is inversely related to the effect on the heart. The loud murmur implies normal ventricular chamber pressures and functions and has a better prognosis. As the heart starts to fail, the murmur actually grows softer. The ventricular septal defect and tricuspid regurgitation are the common right-sided murmurs you will hear (Table 3).

Table 3: Most common heart murmurs based on type and location

Relative pulmonic stenosis

You may hear a fairly loud (grade 3 to 4) left basilar systolic murmur in the presence of a ventricular septal defect. This murmur, known as relative pulmonic stenosis, sounds much like pulmonic stenosis. This is the sound of an increased volume of blood passing through a normal-sized orifice. The blood shunting through the defect to the right ventricle is added to the normal volume of blood returning from the right atrium. Together these represent an increased volume trying to pass through the pulmonic valve. Since the amount of time the heart has to move this extra volume remains unchanged, it must increase the blood velocity thereby creating turbulence and a murmur. In the case of an atrial septal defect, the shunt itself does not make a murmur, and if the volume of the shunt is not sufficient to create relative pulmonic stenosis, you will hear no murmur.

Cardiac rhythm

During your auscultation, be sure to note the cardiac rhythm. Pauses, skips, bursts, rapid changes in rate, or asynchrony are indicative of arrhythmias. Rhythms, such as atrial fibrillation, are so completely asynchronous they are distinctive, and you can practically diagnose them by auscultation alone. This arrhythmia is often described as having the sound of tennis shoes in the dryer because of the chaotic rhythm. Premature complexes sound like skips in the rhythm and may be ventricular or atrial in origin. During this phase of the examination, you should palpate an arterial pulse in the patient to detect any pulse deficits created by any arrhythmia. A pulse deficit is detected when a heart sound is heard but no corresponding pulse is felt. The femoral pulse is most easily accessible in dogs and cats. A facial artery can be used in horses or cattle.

Conclusion

With some practice, you can become proficient at cardiac auscultation. Just remember, only by actually listening carefully to every patient, especially normal ones, can you improve your skills.

H. Edward Durham Jr., CVT, LATG, VTS (Cardiology), is a technician at the College of Veterinary Medicine Veterinary Medical Teaching Hospital at the University of Missouri in Columbia, Mo. He is also a charter member of the Academy of Internal Medicine for Veterinary Technicians and serves on its executive board as the Director at Large-Cardiology.