Wheezes—which are associated with such conditions as bronchospasm, airway thickening caused by mucosal swelling or muscle hypertrophy, inhalation of a foreign body, tumor, secretions, and dynamic airway compression—are musical sounds generated by air passing through a bronchus so narrowed as to be almost closed. The bronchus walls oscillate between closed and barely open positions; these oscillations generate audible sounds.

Wheezes are musical, relatively high-pitched, continuous sounds with frequencies of ≥400 Hz and a duration of ≥250 milliseconds. Their duration is long enough to carry an audible pitch similar to a musical tone that may often sound like hissing. Wheezes are described by their timing within the respiratory cycle; they may be heard during inspiration or expiration or continuously throughout the respiratory cycle. Wheezes can be further described as localized or diffuse, episodic or chronic.

Polyphonic wheezes are thought to be caused by the dynamic compression of large airways during expiration. In healthy individuals, polyphonic wheezes can sometimes be auscultated during a maximal forced expiration when the dynamic compression occurs simultaneously in all airways. No regional variations exist in airway resistance or in lung compliance; consequently, polyphonic wheezes are heard throughout the lungs during expiration.

Polyphonic wheezes are loud, musical, and widely transmitted; when heard during regular breathing, they indicate widespread airflow obstruction.

In patients with widespread airflow obstruction (as in asthma, chronic obstructive pulmonary disease, chronic bronchitis), elastic recoil properties, peripheral airway resistance, and airway mechanics are altered throughout both lungs. Together, these abnormal changes affect the timing of dynamic airway compression within the bronchi, which alters normal sound production. For example, during sequential forceful expirations, the central bronchi are compressed first when elastic recoil of the airways is low or when peripheral airway resistance is high. This compression produces a series of sounds, beginning with a monophonic wheeze and quickly followed by bitonal sounds. Soon, the full complement of sounds that make up polyphonic wheezes are audible through the stethoscope. These multiple musical tones begin simultaneously and maintain a constant pitch that rises sharply at the end of expiration. (♦Sound 92)