Thoracic auscultation and percussion are two of the most useful and economical procedures of a physical examination. Many clues as to both respiratory and cardiac function and disease can be obtained through proper auscultation. Differences in the intensity and character of sounds can be useful in distinguishing underlying thoracic pathology. Auscultable sounds originate as mechanical vibrations within compressible media that are then transmitted through the tissues as sound waves. Differences in the acoustical density of tissues result in attenuation, reflection, and refraction of these sound waves. Breath sounds originate in the large airways due to turbulence in airflow. The character of the sound that is heard at the surface is determined by the factors influencing the production of the sound, and acoustical characteristics of the intervening tissues. Normal breath sounds are classified as bronchial, bronchovesicular, and vesicular. Abnormal, or adventitious, sounds are classified as crackles, wheezes, stertor, and stridor. The intensity and character of breath sounds help determine the location and pathology of thoracic disease.

History of Thoracic Auscultation

Auscultation, or listening to the sounds within the body, is a fundamental examination procedure in clinical medicine. There are several historical references to auscultation in medical literature.

You shall know by this that the chest contains water and not pus, if in applying the ear during a certain time on the side, you perceive a noise like that of boiling vinegar. Hippocrates (c. 460 to c. 370 B.C.)

With each movement of the heart, when there is the delivery of a quantity of blood from the veins to the arteries, a pulse takes place and can be heard within the chest. William Harvey (1578 to 1657)

I have been able to hear very plainly the beating of a man’s heart… who knows, I say, but that it may be possible to discover the motions of the internal parts of bodies… by the sound they make. Robert Hooke (1635 to 1703)

Early auscultation was performed by placing one’s ear directly on the patient’s body. It was not until the early 1800’s that the stethoscope was “discovered” by Rene Laennec (1781 to 1826) who used a rolled cylinder of paper to listen to a patient’s chest. Since then, the stethoscope has evolved into its current form and has become one of the most cost effective diagnostic instruments available to clinicians.

Principles of Sound Production and Transmission

The goal of clinical auscultation is to relate the sounds heard to the underlying pathology of the tissues examined. In order to accomplish this goal, it is necessary to understand the basic principles of sound production and transmission. Sound is produced by vibrations in air or other compressible media that are transmitted as waves and provide the stimulus for the subjective sensation of hearing. The sound pitch is determined by the frequency (cycles per second or Hz) and the intensity or loudness is determined by the amplitude of the pressure waves. The sound quality is determined by the complexity of the pressure wave.

Different compressible media have different acoustical properties that affect the transmission of sound. Dampening of the sound wave results in decreased amplitude and thus decreased sound intensity. Different media will dampen vibrations at different rates and over different frequencies. Reflection occurs at the interface of two media with different acoustical properties resulting in decreased intensity of the transmitted sound wave. At interfaces with closely matched acoustical properties, there will be little attenuation of the sound wave. As an example, sounds produced by vibrations in the heart muscle and valves are readily transferred across the similar tissue of the chest wall. However, when the acoustical properties of the tissues differ, such as air filled lung and muscular thoracic wall, much of the sound wave is reflected and sound intensity decreases. By interpreting the intensity, pitch and quality of the sound heard upon auscultation, one can make inferences about the health and pathology of the underlying organs and tissues.

Stethoscopes

The stethoscope is a relatively simple device consisting of a chest piece to collect the sound waves, tubing to transmit the sound, and earpieces to direct the waves into the examiner’s ears. While there are many varieties of stethoscopes on the market, the differences in performance are often subtle and will vary with the individual using the stethoscope. While there are some acoustical benefits to the more expensive stethoscopes, there are many acceptable inexpensive stethoscopes. The chestpiece should have both a diaphragm and a bell for accentuating high and low pitched sounds respectively. The length of the tubing is often a consideration as shorter tubing has less attenuation of sound intensity, however, longer tubing may provide better access in large animals. In addition, a chestpiece with a thin profile is more easily placed in the axilla of large animals for auscultation of the heart. It is this clinician’s opinion that the most important aspect of a stethoscope is comfortable, well fitting earpieces. This will vary among users and can often make the biggest difference in overall sound transmission and quality than any other factor.

Cardiac Auscultation

Complete cardiac auscultation involves the assessment of cardiac rate, rhythm, and sound character. Normal cardiac rates for several species are listed in table 1. Normal sinus rhythm is the most common cardiac rhythm in all of the routine domestic species except dogs where normal sinus arrhythmia is often observed. Normal sinus arrhythmia is sometimes observed in normal horses, llamas, sheep, goats, and in fasting cattle.

Table 1. Normal heart rates of several species

Auscultation of the heart sounds is an important aspect of cardiac examination. There are four normal heart sounds that may be heard (Figure 1). The first heart sound (S₁) coincides with atrioventricular valve closure at the beginning of systole and is loudest over the left apex of the heart. The sound is louder, longer, and lower-pitched than the second heart sound. The second heart sound (S₂) is associated with aortic and pulmonic valve closure at the beginning of diastole. It is heard best over the heart base on the left and right sides. The third heart sound (S₃) occurs during early diastole and is due to vibrations in the ventricle that occur during passive filling of the ventricles. It is a low frequency; low-amplitude sound that is heard best at the apex of the heart. The fourth heart sound (S₄) occurs in late diastole at the time of peak atrial contraction and is associated with rapid filling of the ventricle following atrial contraction. S₁ and S₂ are normally heard with each cardiac cycle while auscultation of S₃ and S₄ are variable for both species and individuals.

Figure 1. Association of heart sounds with ECG

While evaluating the heart sounds, it is important to listen for murmurs and assess their location, timing, sound quality, and intensity. Location is assessed relative to the heart valves. Timing is made in reference to systole or diastole. The quality of the murmur (Table 2) may be regurgitant (plateau, holosystolic), ejection (crescendo-decrescendo,), blowing (decrescendo, usually diastolic), or machinery (continuous). The murmur intensity is often evaluated on a 5 or 6 point scale (Table 3). It is important to note that the intensity of the murmur does not always correlate with the severity of the cardiac defect.

Table 2. Classification of the sound quality of cardiac murmurs

Murmur Type	Murmur Quality			Common Causes
	S4     S1		S2   S3
Regurgitant				Ventricular-septal defect AV valve insufficiency
Ejection				Atrial septal defect Aortic or pulmonic stenosis
Blowing				Aortic insufficiency Pulmonic insufficiency
Machinery				Patent ductus arteriosis

Table 3. Classification of the intensity grade of a cardiac murmur

Pulmonary Auscultation

Pulmonary auscultation involves the assessment of respiratory rate and breath sound character. Breath sounds originate as air moving within the airway causes vibrations in the airway walls. These vibrations are then transmitted as sound waves through the thoracic tissues. The velocity of airflow, turbulence, and acoustical properties of the surrounding tissues influence the character and intensity of the sound. Breath sounds originate only in the trachea and bronchi (airways >2mm diameter) and not in the terminal bronchioles where air velocity is too slow to produce audible sound waves. The origination of the sound is influenced by air velocity that can be affected by respiratory rate, depth, and airway diameter. Higher velocity airflow creates more turbulence and thus increases sound intensity. Sound is transmitted through the pulmonary field via the lung tissue and not through the small airways. Thus, it is the acoustical property of the lung parenchyma and body wall that determines the transmission and final character of the breath sound heard upon auscultation of the thorax.

Table 4. Normal respiratory rates of several species

Classification of Breath Sounds

The character of normal breath sounds is influenced by many factors including species, ventilatory pattern, body condition, and underlying pulmonary pathology. It is important to auscult the trachea along with the thorax. Breath sounds heard over the thorax are normally of lower intensity than tracheal sounds (except in some sheep, goats, and camelids). Normal breath sounds should be described with additional modifiers indicating loudness/intensity, pitch/frequency and anatomical location. Increased or decreased intensity of normal breath sounds can occur with several types of pulmonary dysfunction or pathology (Table 5). The term “harsh” breath sounds is an often, and poorly, used descriptor for lung sounds that most people use to refer to normal breath sounds of increased intensity.

1.  Normal Breath Sounds are divided into three categories based on their sound character and anatomic location. The terminology for these sounds can be misleading as it implies an anatomical source of the sounds. Rather, the different sounds heard at the different anatomical locations are due to differential attenuation of the sounds that are produced in the larger airways.

a.  Bronchial Breath Sounds are heard over the trachea and hilar region of the lung field. They have a prominent inspiratory and expiratory component.

b.  Bronchovesicular Breath Sounds are intermediate between bronchial and vesicular breath sounds. They consist of a soft inspiratory sound with a short expiratory sound that is more prominent than vesicular sounds.

c.  Vesicular Breath Sounds are heard in the periphery of the lung field. They consist of a soft inspiratory sound and a short soft expiratory sound.

2.  Adventitious Breath Sounds are abnormal sounds that are superimposed upon normal breath sounds and indicate underlying pulmonary pathology. They are divided into two broad categories, crackles and wheezes, which are then characterized as to their loudness/intensity, pitch/frequency, anatomical location, and timing within inspiration or expiration.

a.  Crackles (rales) are abnormal discontinuous explosive sounds associated with the sudden opening of airways that are either collapsed due to surrounding inflammation or blocked due to collections of fluid or inflammatory exudate. Crackles heard throughout inspiration are associated with large or widespread airway disease. Crackles heard at the end of inspiration are usually associated with small airway disease since these are the last airways to open.

i.  Low-pitched crackles are associated with airway secretions and are often altered by coughing.

ii.  High-pitched crackles are associated with the opening of collapsed peripheral airways.

iii.  Pleural friction rubs are abnormal, low-pitched, discontinuous sounds associated with roughened pleural surfaces and indicate pleuritis.

b.  Wheezes (rhonchi) are abnormal, continuous sounds with a musical character. They are believed to result from the vibrations of airway walls in close contact. The pitch of the wheeze is determined by the stiffness of the opposing tissues and is not necessarily related to airway diameter. Monophonic wheezes suggest a single site of airway obstruction while polyphonic wheezes are suggestive of more generalized pulmonary pathology. Wheezes are often heard during expiration due to dynamic compression of the airways. Wheezes heard at the end of inspiration suggest the opening of smaller, inflamed airways. Stridor is an abnormal continuous monophonic musical sound generally focused over extrathoracic airways and therefore heard primarily during inspiration. It is often associated with laryngeal or tracheal disease.

c.  Stertor is a poorly defined and inconsistently used term that refers to a sonorous snoring sound without the musical quality of stridor. This can also describe a harsh discontinuous crackling sound in the trachea or larynx suggestive of accumulation of secretions, swollen, or edematous tissue within the upper airways.

Figure 2. Diagrammatic representation of breath sounds

Table 5. Causes of abnormal breath sounds

Thoracic Percussion

Acoustic percussion is a method to evaluate the consistency of tissues by interpretation of the sounds elicited by striking the body surface in the area of interest. Tissue density can be qualitatively assessed based on the pitch, loudness and duration of the induced sound (Table 6)

Table 6. Sound characteristics during acoustic percussion

In small animals, thoracic percussion is usually performed by placing the finger of one hand (pleximeter) flat in the intercostal space over the lung field and briskly tapping it with the middle finger of the opposite hand (plexor In large animal species, the procedure is usually performed by placing a pleximeter (i.e., wood block, metal block, or spoon) in the intercostal space and hitting it with a rubber reflex hammer (plexor). Wood blocks tend to create better resonance and are preferred as pleximeters. This procedure is performed over the entire lung field and differences in sound quality are assessed. Acoustic percussion can assess tissues to a depth of approximately 7 cm and detect lesions as small as 5 cm diameter. Note that the cardiac silhouette lies in the cranial ventral portion of the lung field and manifests as an area of dullness. Enlargement of the cardiac silhouette, or variation in the tympany over the lung field may indicate thoracic disease (Table 7). Thoracic percussion is a relatively simple technique to develop with practice and can provide very useful clinical information.

Table 7. Causes of abnormalities observed with thoracic acoustic percussion

References

1.  Chang, L. 1987. Development and use of the stethoscope in diagnosing cardiac disease. American Journal of Cardiology. 60:1378-1382.

2.  Curtis, R. A., L. Viel, S. M. McGuirk, O. M. Radostits, and F. W. Harris. 1986. Lung sounds in cattle, horses, sheep and goats. Canadian Veterinary Journal. 27:170-172.

3.  Kotlikoff, M. I., and J. R. Gillespie. 1983. Lung sounds in veterinary medicine part I. Terminology and mechanisms of sound production. Compend. Contin. Educ. Pract. Vet. 5:634-639.

4.  Kotlikoff, M. I., and J. R. Gillespie. 1984. Lung sounds in veterinary medicine part II. Deriving clinical information from lung sounds. Compend. Contin. Educ. Pract. Vet. 6:462-467.

5.  Roudebush, P. 1982. Lung sounds. J. Am. Vet. Med. Assoc. 181:122-126.

6.  Roudebush, P. 1988. Lung Sounds: Physiological and pathophysiological basis of interpretation, p. 171-174. Annual Veterinary Medical Forum. Omnipress.

7.  Roudebush, P., and J. Ryan. 1989. Breath sound terminology in the veterinary literature. J. Am. Vet. Med. Assoc. 194:1415-1417.

8.  Roudebush, P., and C. R. Sweeney. 1990. Thoracic percussion. J. Am. Vet. Med. Assoc. 197:714-718.

9.  Tyler, J. W., K. L. Angel, H. D. Moll, and D. F. Wolfe. 1990. Something old, something new: Thoracic acoustic percussion in cattle. J. Am. Vet. Med. Assoc. 197:52-57.