Breathing is not a completely silent process. There are certain characteristic sounds associated with normal breathing. These breathing sounds are produced by the passage of air through the various respiratory passages, such as nasal cavity, pharynx, larynx, trachea, and bronchi. As these sounds travel the overlying tissues, like the chest wall, its characteristics change.
Bronchial Breathing Sounds
The movement of air through the large air passages produces a rustling or a blowing type of sound. This is because the air flow is more turbulent in large chambers. These rustling and blowing sounds are more predominant when the air passes through parts of the upper respiratory tract, which include the nasal cavity, pharynx (commonly known as the throat), larynx (also known as the voice box), and the trachea.
Breathing consists of two phases: inspiration and expiration. During inspiration, air from the outside atmosphere is taken into the lungs. During expiration, air contained in the lungs is expelled out into the atmosphere. Both inspiration and expiration stages of breathing produce sounds. Under normal conditions, the sounds produced by inspiration are louder than the sounds produced by expiration. These normal breathing sounds are also referred to as bronchial breathing sounds.
However, in some pathological conditions, the sounds produced by expiration are louder than the sounds generated during inspiration.
Vesicular Breathing Sounds
Breathing sounds soften once the air enters the bronchial system in the lungs. The flow of air through the upper respiratory tract resonates in the lung tissue, and produces a soft, wispy sound. This sound is more pronounced during the inspiration phase. The expiration phase is almost silent. These sounds, produced by the movement of air in the lung passages, are known as vesicular breathing sounds. Vesicular breathing sounds produced in the lungs are of lower amplitude and quality than bronchial breathing sounds produced in the upper airways.
Read more on abnormal breathing sounds.
Diminished Breathing Sounds
Under certain conditions, the normal vesicular breathing sounds may get diminished. This is usually a localized effect. The location where the sound is muffled becomes evident when compared to the sounds in the rest of the lung. Comparison of sounds in both lungs can also help in identifying the region where the sound is getting diminished, provided only one lung is affected by the underlying pathology.
The causes for diminished vesicular breathing sounds can either be impaired conduction of sound through the overlying tissue or an impairment in the flow of air through the air passages. Impaired sound conduction through the overlying tissues can occur in the following conditions:
- Increased thickness of the chest wall: The chest wall may be excessively thick in certain individuals, such as obese or muscular individuals. This reduces the sound conduction through the chest wall.
- Pleural effusion: Pleural effusion refers to the accumulation of fluid in the space around the lungs.
- Pleural fibrosis: Pleural fibrosis refers to a thickening of the tissue surrounding the lungs.
- Pneumothorax: Pneumothorax refers to the accumulation of air around the lungs.
Impaired air flow through the respiratory passages (also known as reduced ventilation) could occur in the following conditions:
- Airway obstruction: Obstruction of the airways could be due to lodging of some foreign body within the air passages, especially in the bronchus.
- Reduction of lung elasticity: The capacity of the lungs to expand and contract is compromised in conditions such as chronic obstructive pulmonary disease (commonly abbreviated as COPD).
- Collapsed lung: Lung collapse may occur due to airway obstruction (also known as obstructive atelectasis). Obstructive atelectasis may occur in lung cancer.
Read more on heavy chest.
Loud Breathing Sounds
Normal breathing sounds are not usually loud. However, abnormally loud breathing sounds can sometimes be heard over the lungs. These sounds are clearly audible, and are similar to bronchial breathing sounds with respect to their nature and volume. This similarity suggests that the vocal resonance is pronounced in such cases. Voice transmission tests are generally required in order to distinguish these loud breathing sounds from the normal bronchial breathing sounds (as determined by auscultation or bronchophony).
Causes of Loud Breathing Sounds
The following are some of the conditions that can cause loud breathing sounds:
- Consolidation of lung tissue: This is commonly seen in pneumonia.
- Lung fibrosis: Fibrosis may affect only a part of the lung tissue (technically known as localized pulmonary fibrosis).
- Collapsed lungs: Lung collapse can occur in conditions such as tension pnemothorax and non-obstructive atelectasis.
The vibrations created when a sound is produced due to air flow (phonation process) are referred to as vocal resonance. This vocal resonance can be heard throughout the respiratory tract and in the adjacent tissue structures. Vocal resonance is an important factor in diagnosing abnormal breathing sounds. Hearing the vocal resonance over the lung through auscultation (bronchophony) is helpful in establishing the presence of any pathological loud breathing sounds. This can also be confirmed through tactile fremitus or touch.
Voice Transmission Tests
Voice transmission tests are used to check for any defects in the conduction of breathing sounds through the lung tissue and the fluid that surrounds the lungs. Changes in the conduction of sound could occur due to accumulation of fluid around the lungs, changes in the structure of the lung and the surrounding tissues, and any obstructions within the airways.
The procedure for voice transmission test consists of requesting the patients to say a phrase like “ninety nine” while listening to the sounds in the chest with the help of a stethoscope (a procedure known as auscultation). Under normal conditions, the sound produced by saying the words “ninety nine” is indistinct. The sound also reverberates. This is because the normal lung tissue attenuates the high-pitched sounds while transmitting the low-pitched sounds.
Therefore, only some sounds are heard. In case of consolidated lung tissue, higher frequency sounds are no longer attenuated. Therefore, the sounds of the words “ninety nine” will be heard clearly. In these cases of consolidated lung tissue, even whispering the words would produce audible sounds over the lung tissue. This increase in the loudness of whispered sounds heard through auscultation is technically referred to as “whispering pectoriloquy”.
Alternatively, vocal resonance is significantly diminished (or even absent) in cases where the space surrounding the lung tissue is affected (such as in pleural effusion, pleural thickening and lung collapse).