Our respiratory system enables us to take up oxygen from the air and to get rid of carbon dioxide that our body produces during various metabolic processes. The respiratory system consists of a series of air passages that connect the nasal cavities to the lungs, which are the sites where the gas exchanges with the blood occur. Normal functioning of the respiratory system depends on the proper functioning of the airways and the lungs.
The air passages allow the movement of air into and out of the lungs. During the inhalation phase of the breathing process, air that enters the nasal cavities passes through a series of air passages, including the pharynx, larynx (also called the voice box), trachea (also known as the windpipe), bronchi, and bronchioles to finally reach the alveoli in the lungs. The alveoli (also known as the air sacs) are the place where the gas exchange with the blood occurs in the lungs.
The oxygen from the inhaled air is taken up by hemoglobin in the red blood cells, and the carbon dioxide from the blood is released back into the alveoli. During the exhalation phase of the breathing process, the carbon dioxide released into the alveoli flows back through the series of air passages and exits through the nasal passages. The series of air passages that allow the flow of air into and out of the lungs are arranged like the branches of a tree.
Hence, these air passages are also sometimes referred to as the tracheobronchial tree. The air passages in the tracheobronchial tree are thin-walled tubes that are lined with mucus membranes. The lumen of the air passages is controlled by the contraction of the smooth muscles that lie within the walls of these tubes. A narrowing of the lumen of the air passages (caused by smooth muscle contraction or inflammation and swelling of the airway passages) can adversely affect the flow of air into and out of the lungs.
Obstruction of the airway passages can also occur due to the presence of some foreign body, tumor, or abscess within these tubes. Disruption of airflow caused by narrowing or obstruction of the air passages results in the production of abnormal breathing sounds. Stridor and wheezing are two such types of abnormal breathing sounds.
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What is stridor?
A stridor is a rough and high-pitched sound that is sometimes described as a crowing sound. This sound is produced during both inspiration (or inhalation) and expiration (or exhalation) phases of the breathing cycle. However, stridor is more prominent during the inspiratory phase. The high-pitched sound of stridor is loud enough to be heard without the aid of a stethoscope. A stridor can be caused by both physical and functional abnormalities of the air passages. This type of abnormal breathing sound is most commonly heard in people who have infections of the respiratory tract, allergies, tumors, and disorders of the vocal cords.
What is wheezing?
A wheezing sound is also a high-pitched sound that is often described as a whistling sound. Unlike stridor, wheezing is most prominent during the expiration or exhalation phase of the breathing cycle. Although the sound may be loud enough to be heard by the unaided ear, a stethoscope is usually required to clearly hear the wheezing sound. A wheezing sound is typically caused by a narrowing of the air passages (especially the lower air passages). This type of abnormal breathing sound is commonly heard in people who suffer from asthma.
Stridor and wheezing are two different types of abnormal breathing sounds. However, they are often confused with one another. Even the causes of these two conditions may overlap in some cases. Therefore, other accompanying signs and symptoms (along with clinical test results) must be taken into account before reaching a final diagnosis.
Causes of Wheezing and Stridor
Read more on whistling breathing sounds.
Asthma is a very common respiratory allergy that is characterized by wheezing sounds during breathing. Asthma attacks can be triggered by exposure to a variety of factors, including pollen, dust, nuts, dairy, humidity, cold, and intense exercise.
Acute inflammation of the respiratory passages due to infections can cause stridor. Acute bronchitis is also characterized by fever and productive cough.
Severe allergic reactions (anaphylaxis) to various allergens (such as insect bites, shellfish, certain drugs) can cause constriction of the airways and abnormal breathing sounds. An anaphylactic reaction requires immediate medical intervention to restore normal breathing. Delay in seeking emergency treatment could be fatal.
Wheezing and stridor could occur in bronchiectasis due to excessive accumulation of mucus in the airways.
Childhood respiratory infections
Bronchiolitis and croup are common childhood respiratory infections that can cause stridor. Bronchiolitis occurs due to an infection with the respiratory syncytial virus (abbreviated as RSV). The symptoms of bronchiolitis, such as shallow and rapid breathing, are typical of a “chest infection”. Croup is characterized by stridor and a cough that sounds like barking. This condition is caused by a swelling of the larynx and trachea.
Chronic obstructive pulmonary disease
Chronic bronchitis and emphysema are two examples of chronic obstructive pulmonary disease (commonly abbreviated as COPD). Long term exposure to irritants, such as cigarette smoke, air pollutants, and dust, is the main cause of chronic obstructive pulmonary disease. This condition is characterized by wheezing, stridor, excessive mucus secretion in the airways, and breathing problems.
Wheezing could also occur due to side effects or allergic reactions to certain drugs, such as aspirin and blood pressure medications.
Stridor can also occur due to obstruction of the airways caused by the presence of laryngeal or endobronchial tumors. In some cases, lung cancer can also result in abnormal breathing sounds.
Obstruction to the passage of air due to the presence of some foreign object in the airways may result in abnormal breathing sounds, such as wheezing. If the foreign object is large, a person may go into respiratory distress and require immediate oxygen administration.
Gastrointestinal reflux disease
Reflux of gastric acid in gastrointestinal reflux disease can irritate the airways and cause abnormal breathing sounds, in addition to other symptoms.
Inflammation of the epiglottis, which is a structure that prevents food and drink from entering the respiratory passages, can occur due to bacterial infections (for example, strep throat). Swelling caused by inflammation of the epiglottis can disrupt the normal passage of air in the respiratory tract.
Constriction of the air passages frequently occurs upon exposure to airborne irritants (such as fumes, smoke, chemicals, and dust). A constriction of air passages is usually accompanied by stridor and wheezing.
Paralysis of vocal cords
Paralysis of the vocal cords can result in stridor. Vocal cord paralysis could occur due to neck injuries, tumors, viral infections, and neurological disorders.
Congenital defects in the structure of the tracheobronchial tree can be the cause of stridor in some cases. A common example of such a congenital defect is laryngomalacia.
Congestive heart failure
Congestive heart failure is characterized by accumulation of fluid in the lungs and other tissues in the body. Breathing becomes difficult and abnormal breathing sounds (such as stridor and wheezing) may be present chronically.
Swelling of the larynx
An audible stridor may occur after intubation of the larynx, which may cause swelling and spasm of the larynx.
Pulmonary embolism causes blockage of blood vessels within the lungs. The disruption of blood flow in the lungs results in severe pain in the chest, coughing, and breathing difficulties. Stridor or wheezing may also accompany pulmonary embolism.