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A wheeze is a continuous, coarse, whistling sound produced in the respiratory airways during breathing. For wheezes to occur, some part of the respiratory tree must be narrowed or obstructed, or airflow velocity within the respiratory tree must be heightened. Wheezing is commonly experienced by persons with a lung disease; the most common cause of recurrent wheezing is asthma, a form of reactive airway disease.

The differential diagnosis of wheezing is wide, and the cause of wheezing in a given patient is determined by considering the characteristics of the wheezes and the historical and clinical findings made by the examining physician.

Inflamed airways and bronchoconstriction in asthma. Airways narrowed as a result of the inflammatory response cause wheezing.

Common causes of wheezing are:

• Asthma
• Chronic obstructive pulmonary disease
• Medication-induced bronchoconstriction
• Pulmonary edema
• Tracheobronchitis, and
• Vocal cord dysfunction
• anaphylaxis

Less common causes of wheezing include:

• Amyloid deposition
• Carcinoid syndrome
• Chondromalacia
• Descending aortic aneurysms
• Postlobectomy bronchial torsion
• Postradiation stenosis
• Tracheo-broncheal tumors
• Tracheal stenosis
• Tracheomalacia
• Aspiration (foreign particles or foods)

Wheezes occupy different portions of the respiratory cycle depending on the site of airway obstruction and its nature. The proportion of the respiratory cycle occupied by the wheeze roughly corresponds to the degree of airway obstruction.^1,2 Bronchiolar disease usually causes wheezing that occurs in the expiratory phase of respiration. The presence of expiratory phase wheezing signifies that the patient's peak expiratory flow rate is less than 50% of normal.³ Wheezing heard in the inspiratory phase on the other hand is often a sign of a stiff stenosis, usually caused by tumors, foreign bodies or scarring. This is especially true if the wheeze is monotonal, occurs throughout the inspiratory phase (ie. is "holoinspiratory"), and is heard more distally, in the trachea. Inspiratory wheezing also occurs in hypersensitivity pneumonitis.⁴ Wheezes heard at the end of both expiratory and inspiratory phases usually signify the periodic opening of deflated alveoli, as occurs in some diseases that lead to collapse of parts of the lungs.

The location of the wheeze can also be an important clue to the diagnosis. Diffuse processes that affect most parts of the lungs are more likely to produce wheezing that may be heard throughout the chest via a stethoscope. Localized processes, such as the occlusion of a portion of the respiratory tree, are more likely to produce wheezing at that location, whence the sound will be loudest and radiate outwardly. The pitch of a wheeze does not reliably predict the degree of narrowing in the affected airway.⁵

Wheezing can also occur in people who are not in shape. Wheezing is not a severe problem. The more often you run, the easier breathing gets.^{[citation needed]}

A special type of wheeze is stridor. Stridor — the word is from the Latin, strīdor⁶ — is a harsh, high-pitched, vibrating sound that is heard in respiratory tract obstruction. Stridor heard solely in the expiratory phase of respiration usually indicates a lower respiratory tract obstruction, "as with aspiration of a foreign body (such as the fabled pediatric peanut)."⁷ Stridor in the inspiratory phase is usually heard with obstruction in the upper airways, such as the trachea, epiglottis, or larynx; because a block here means that no air may reach either lung, this condition is a medical emergency.

• Rales
• Rhonchi