Brachycephalic refers to animals with a shortened, flattened skull that compresses the airway, nasal passages, and facial structures. The term comes from Greek: brachys (short) + kephalē (head). Breeds like Pugs, French Bulldogs, and Persian cats are brachycephalic. The resulting breathing difficulties are collectively called brachycephalic obstructive airway syndrome (BOAS).
The physical changes that create the characteristic flat face also compress the airway in multiple places simultaneously: the nares (nostrils) are often narrowed and can barely flare during breathing; the soft palate is frequently elongated and falls into the airway during inhalation; the trachea may be narrower than normal; and the throat tissue is more redundant. Any one of these would cause some breathing difficulty; most brachycephalic animals have several problems occurring at once.
The result is that brachycephalic animals must work significantly harder to breathe even at rest. Signs include: constant loud breathing and snoring, exercise intolerance, overheating rapidly, blue-tinged gums (cyanosis) during excitement or exertion, sleep apnea, and in severe cases, collapse. Heat is particularly dangerous — these breeds can develop fatal heatstroke from temperature and humidity combinations that would be safe for other dogs.
Surgical correction is available and can significantly improve quality of life: widening the nares, shortening the soft palate, and removing excess throat tissue. The procedure is most beneficial when performed in young adult animals before secondary changes from years of straining occur. Breeding selection toward more moderate facial structure is the only long-term solution to the health problems in these breeds.