The vasovagal tonus index (VVTI), a time-domain indicator of heart rate variability, was measured in 92 dogs of six breeds (German shepherd dogs, labrador retrievers, cocker spaniels, boxers, bulldogs and cavalier King Charles spaniels). There was a significant difference in VvTi between the six breeds (P=0.003). Brachycephalic dogs had a higher vvTI than other types of dog (P<0.005), and when comparing individual breeds brachycephalic breeds tended to have a higher vvri than non-brachycephalic breeds, although the difference was not always significant. The VVTI was negatively correlated with heart rate (P<0.01) and dogs suffering from congestive heart failure had a lower VVTI than other dogs, whether compared within or between breeds (P<0.001).
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