Lung sounds are mainly classified into breath sounds and adventitious sounds.
Breath sounds are normal noises that can be heard on the chest wall with breathing. Adventitious lung sounds are abnormal sounds superimposed on the breath sounds and usually indicate some type of respiratory disorder.
Normal breath sounds are of four types
Directly over the trachea, the breath sound is particularly loud and high-pitched.
It is described an as tracheal sound.
The tracheal sound is typical as there is a pause between the inspiratory and expiratory components. The expiratory component is slightly longer.
Bronchial sound is a harsh, high pitched sound with approximately equal inspiratory and expiratory components. This may be heard directly over a major bronchus during normal breathing.
Bronchovesicular sounds are a slight variation to the tracheobronchial sound and are heard just distal to the central airways. They are less intense and lower pitched than bronchial sounds but maintain an equal inspiratory and expiratory components.
The vesicular breath sound is significantly softer in intensity and primarily is an inspiratory sound. The expiratory components of the vesicular breath sounds are normally minimal, only occurring during the initial one third of the expiratory phase.
When vesicular breath sounds are found to be less intense than expected, they are described as diminished or even absent in extreme cases. If the peripheral breath sound increases intensity, it is described as harsh. If the vesicular breath sound takes on more prominent expiratory components, it is described as tracheobronchial.
Mechanisms of Breath Sounds
The exact mechanisms responsible for the production of normal breath sounds are not known. Normal breath sounds are produced regionally within each lung and probably within each lobe. This means that the breath sounds heard over a specific lobe probably are a result of air entry into that underlying lobe.
Changes in lung pathology will alter the sound transmission characteristics of the lung parenchyma. Diseases that increase lung tissue density usually will increase the sound transmission qualities and result in a significant loss in the filtering effect. As a result, transbronchial breath sounds may be heard over areas of consolidation, e.g. lobar pneumonia or atelectasis. An obstructed bronchus will block transmission of the bronchial sounds are lead to absent or markedly diminished breath sounds.
Diminished breath sounds may result from decreases in the sound generation with shallow breathing patterns. This may occur with neuromuscular diseases and other restrictive lung defects.
Diminished breath sounds will also be identified when the sound transmission ability of the lung or chest wall is reduced and results in more filtering of the turbulent flow sounds. This can occur when the lung becomes hyperinflated with pleural disease, or with muscular and obese chest walls.
Adventitious Lung Sounds
These are abnormal lung sounds of different causes. Adventitious sounds can be divided into two categories: continuous and discontinuous. Continuous lung sounds are musical sounds with a constant pitch lasting from very short duration to several seconds. They are more often heard during exhalation and are associated with obstruction of airways.
Discontinuous adventitious lung sounds are intermittent, crackling or bubbling sounds of short duration. These brief bursts of sounds are heard most commonly during inspiration and may be present with both restrictive and obstructive defects.
High-pitched, continuous sounds are described as ‘wheezes” and low-pitched continuous sounds as rhonchi. Crackles are small clicking, bubbling, or rattling sounds in the lungs. They are heard when a person inspires.
The high-pitched, continuous sound heard over the upper airways of a patient with upper airway obstruction is referred to as stridor.
These sounds can be produced by the bubbling of air through airway secretions or by the sudden opening of small airways. Latter sounds are usually coarser.
Sounds due to sudden opening of the airways are inspiratory. Late inspiratory crackles are very suggestive of a restrictive lung defect and indicate a loss in lung volume.
More proximal airways may collapse during exhalation when bronchial wall compliance increases. During the subsequent inspiration, the airways will pop open intermittently and produce different inspiratory sounds.
Continuous adventitious lung sounds are believed to result from airway narrowing, which initially causes rapid airflow past the site of obstruction. The more rapid air flow decreases lateral airway wall pressures, and results in the opposite walls pulling closer together and briefly touching. As a result, flow is briefly interrupted and airway pressure increases. The airway now returns to a more open position permitting airflow to return. The cycle repeats itself rapidly causing vibration of airway walls. This process will continue until insufficient flow occurs as when the patient tires or until the airway obstruction is relieved.
The pitch of the continuous adventitious sounds is determined by the relationship between flow rate and degree of obstruction. Lower flow rates or less obstruction will result in lower pitched sounds.
During expiration, bronchi become progressively smaller and continuous adventitious lung sounds is likely to occur more frequently and for longer periods during expiration.
Small airways have significantly lower flow rates than the larger airways and are much less likely to be a source of expiratory wheezes.
Pleural Friction Rub
Normally, the smooth, moist layers of the pleura slide silently on one another during breathing. Alterations in the pleura from inflammation or fibrin deposits can result in added friction between the pleural layers. The sound produced is usually non-musical and has been compared to the creaking sound of old leather.
Alteration in lung pathology will change the transmission of voice sounds, resulting in either increased or decreased transmission of vocal resonance.
An increase in vocal resonance, known as bronchophony, results in louder and clearer voice sounds over the affected area. This occurs with increases in lung tissue density, as in lung consolidation from pneumonia or atelectasis.
A reduction in vocal resonance occurs when lung tissue density decreases, resulting in more filtering of sound as in emphysema and acute asthma.
Decreased vocal resonance also is noted over areas of the lung separated from the chest wall by pneumothorax or pleural effusion.
Whispering creates high-frequency vibration that are filtered out selectively by normal lung tissue and normally heard as muffled, low-pitched sounds over the chest wall. When consolidation is present, however, the lung loses its selective transmitter quality, and the whispering is transmitted to the chest wall with more clarity. This sign, known as whispering pectoriloquy and is helpful to identify in patients with small or patchy areas of lung consolidation
In patients with chest hair, a crackling noise may be heard from the chest hair rubbing on the diaphragm of the stethoscope. Firm pressure of the stethoscope on the skin, or wetting the hair, will help to eliminate this extraneous sound.
When air is present in the subcutaneous tissue, a crackling noise can be heard when the stethoscope is pressed down over the affected area.
When air is present in the mediastinum and sometimes with a left pneumothorax, a crunching or crackling sound may be heard with each heartbeat and is referred to as a systolic or xiphisternal crunch.
If the patient has fractured ribs or a fractured sternum, the ends on the bone may rub against one another and cause a clicking sound this is referred to bone crepitus.