The anatomy that causes the problem
Brachycephalic dogs, pugs, French bulldogs, English bulldogs, Shih Tzus, Pekingese, were selectively bred for a skull shape that compresses the face inward. The bones shortened. The soft tissue did not. What remains is a normal volume of tissue crammed into a fraction of the space: narrowed nostrils (stenotic nares), an elongated soft palate that droops into the throat, and a trachea that in many individuals is measurably smaller in diameter than in dogs of equivalent body weight. A 2018 study published in PLOS ONE, which examined 700 brachycephalic dogs across the UK, found that 58 percent had at least two of these anatomical abnormalities simultaneously. In pugs specifically, the nares are so frequently narrowed that stenosis is considered a breed-standard feature rather than an anomaly, meaning the dogs most people regard as healthy-looking are often the most compromised.
