Figure 1.

Definition of the degree of nostril stenosis in brachycephalic dogs. Representative nostrils of French bulldogs with different degrees of stenosis. (A) Open nostrils: nostrils are wide open; (B) Mild stenotic nostrils: slightly narrowed nostrils but the lateral nostril wall does not touch the medial nostril wall. Immediately after the exercise tolerance test (ETT), the nostril wings should move dorsolaterally to open on inspiration. (C) Moderately stenotic nostrils: the lateral nostril wall touches the medial nostril wall at the dorsal part of the nostrils and the nostrils are only open at the bottom. Immediately after the ETT, the nostril wings are not able to move dorsolaterally and there may be nasal flaring (ie, muscle constraction around the nose trying to enlarge the nostrils); (D) Severely stenotic nostrils: nostrils are almost closed. The dog may switch to oral breathing from nasal breathing with stress or very gentle exercise such as playing.