Origination of Breath Sounds

Breath sounds can be classified into two categories, either NORMAL or ABNORMAL (adventitious).  Breath sounds originate in the large airways where air velocity and turbulence induce vibrations in the airway walls.  These vibrations are then transmitted through the lung tissue and thoracic wall to the surface where they may be heard readily with the aid of a stethescope.  Normal breath sound production is directly related to air flow velocity and airway lumen architecture.  Air flow velocity is primarily determined by pulmonary ventilation (minute volume     velocity) and TOTAL cross sectional airway area ( area     velocity) at any given level in the lungs.  It is a common misconception that air moving through terminal bronchioles (airways with a diameter <2 mm) and alveoli also contribute to breath sounds.  This is incorrect as the air velocity at this level is too slow (very large total cross sectional area) to produce significant turbulence and sound waves.  However, terminal airway and alveolar disease does modify the breath sounds heard at the surface by either increasing or decreasing the sound transmission through the diseased tissue.  Thus, the sounds that are heard at the periphery of the lung are produced in more central (hilar) regions and are altered in intensity and tonal quality as they pass through pulmonary tissue to the periphery.

Normal Breath Sounds

Bronchial Sounds

Bronchial breath sounds consist of a full inspiratory and expiratory phase with the inspiratory phase usually being louder.  They are normally heard over the trachea and larynx.  Bronchial sounds are not normally heard over the thorax in resting animals.  They may be heard over the hilar region in normal animals that are breathing hard (i.e. after exercise).  Otherwise, bronchial sounds heard over the thorax suggest lung consolidation and pulmonary disease.  Pulmonary consolidation results in improved transmission of breath sounds originating in the trachea and primary bronchi that are then heard at increased intensity over the thorax.

 Click on box to hear a bronchial breath sound

Bronchovesicular Sounds

Bronchovesicular breath sounds consist of a full inspiratory phase with a shortened and softer expiratory phase.  They are normally heard over the hilar region in most resting animals and should be quieter than the tracheal breath sounds.  However, in sheep, goats, llamas, and alpacas, they may be heard throughout the full lung field and are often louder than tracheal breath sounds.  Increased intensity of bronchovesicular sounds is most often  associated with increased ventilation or pulmonary consolidation.

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Vesicular Sounds

Vesicular breath sounds consist of a quiet, wispy inspiratory phase followed by a short, almost silent expiratory phase.  They are heard over the periphery of the lung field.  As stated earlier, these sounds are NOT produced by air moving through the terminal bronchioles and alveoli but rather are the result of attenuation of breath sounds produced in the bronchi at the hilar region of the lungs.  These sounds may be absent or silent in the periphery of normal resting animals.  They are highly variable in intensity depending on the species, ventilation, and body condition.  Increased intensity may be associated with pulmonary consolidation.

   Click on box to hear a vesicular breath sound

 Abnormal Breath Sounds


Crackles are discontinuous, explosive, "popping" sounds that originate within the airways.  They are heard when an obstructed airway suddenly opens and the pressures on either side of the obstruction suddenly equilibrates resulting in transient, distinct vibrations in the airway wall.  The dynamic airway obstruction can be caused by either accumulation of secretions within the airway lumen or by airway collapse caused by pressure from inflammation or edema in surrounding pulmonary tissue.  Crackles can be heard during inspiration when intrathoracic negative pressure results in opening of the airways or on expiration when thoracic positive pressure forces collapsed or blocked airways open.  Crackles are heard more commonly during inspiration than expiration.  They are significant as they imply either accumulation of fluid secretions or exudate within airways or inflammation and edema in the pulmonary tissue.

  Click on box to hear a high pitched crackle


Wheezes are continuous musical tones that are most commonly heard at end inspiration or early expiration.  They result as a collapsed airway lumen gradually opens during inspiration or gradually closes during expiration.  As the airway lumen becomes smaller, the air flow velocity increases resulting in harmonic vibration of the airway wall and thus the musical tonal quality.  Wheezes can be classified as either high pitched or low pitched wheezes.  It is often inferred that high pitch wheezes are associated with disease of the small airways and low pitch wheezes are associated with disease of larger airways.  However, this association has not been confirmed.  Wheezes may be monophonic (a single pitch and tonal quality heard over an isolated area) or polyphonic (multiple pitches and tones heard over a variable area of the lung).  Wheezes are significant as they imply decreased airway lumen diameter either due to thickening of reactive airway walls or collapse of airways due to pressure from surrounding pulmonary disease.

  Click on box to hear a low pitch wheeze


Stridor are intense continuous monophonic wheezes heard loudest over extrathoracic airways.  They tend to be accentuated during inspiration when extrathoracic airways collapse due to lower internal lumen pressure.  They can often be heard without the aid of a stethoscope.  Careful auscultation with a stethoscope can usually identify an area of maximum intensity that is associated with the airway obstruction.  This is typically either at the larynx or at the thoracic inlet.  These extrathoracic sounds are often referred down the airways and can often be heard over the thorax and are often mistaken as pulmonary wheezes.  Stridor is significant and indicates upper airway obstruction.

  Click on box to hear stridor


Stertor is a poorly defined and inconsistently used term to describe harsh discontinuous crackling sounds heard over the larynx or trachea.  It is also described as a sonorous snoring sound heard over extrathoracic airways.  Stertor does not have the musical quality of stridor.  Stertor is significant as it is suggestive of accumulation of secretions within extrathoracic airways.

  Sorry, no sound available for stertor yet