Response to Two Standardized Exercise Tests in Dogs with Different Cephalic Biotypes

Dogs are classified according to their total cephalic index into three biotypes: dolichocephalic, mesocephalic, and brachycephalic. The latter has emerged due to the deliberate selection of extreme phenotypic traits during breeding, which has intensified the expression of associated conformational defects and led to several medical disorders. The Brachycephalic Obstructive Airway Syndrome (BOAS) is a respiratory condition directly linked to these conformational traits. Dogs affected by BOAS present a wide range of clinical signs, including respiratory noise, exercise intolerance, syncope episodes, or even sudden death. This study aimed to evaluate craniofacial anatomical differences and similarities among dogs of different cephalic biotypes (dolichocephalic, mesocephalic, and brachycephalic) and to determine how two exercise tolerance tests-a 6 min walk and a 1000 m walk-influence physiological parameters. Eighty dogs from different breeds were included and classified according to their biotype. Morphometric data from the head, body, and limbs were obtained. Additionally, physiological parameters, including heart rate, respiratory rate, blood pressure, oxygen saturation, and rectal temperature, were evaluated before and after the tolerance tests. The results indicated that dogs tolerated both exercise tests. Dolichocephalic and mesocephalic dogs showed a greater tolerance to or greater respiratory adaptation during walking. Despite the brachycephalic biotype, a wide dispersion at a distance of 1000 m, indicating that those with a higher BOAS grade did not require emergency medical assistance during the tests. However, evidence of rostral shortening (<38 mm), together with facial foreshortening and measurements ≥ 20 mm for necks, chest circumference, and nasal fold, suggested a higher risk of airway obstruction in brachycephalic dogs diagnosed with BOAS grades 2 and 3 compared to dolichocephalic and mesocephalic dogs. This anatomical conformation was associated with significant alterations in physiological parameters including heart rate, respiratory rate, oxygen saturation below 90%, and temperature, which did not return to baseline values 10 min post-exercise. This showed significant differences between the biotypes in the distance in the 1000 m test (H = 11.74; p = 0.0028) and between the subdivisions (p = 0.0389), where G3 covered less distance than G2 (699.1 m vs. 932.77 m. These findings suggest that extreme brachycephalic conformation impairs the respiratory function and leads to thermoregulatory inefficiency, potentially compromising the animals' survival under physical stress. Moreover, the application of safe walking tests and non-invasive morphometric measurements is suggested to facilitate prompt diagnosis of BOAS.