Sporadic transient atrioventricular block and slow heart rate in nervous pointer dogs

Two behaviorally distinct strains of pointer dogs, one bred for nervousness (pronounced freezing behavior, withdrawal behavior), the other bred for normal behavior, exhibit prominent differences in their heart rates and incidence of sporadic occurrences of atrioventricular heart block. Of 67 nervous-line dogs, 70 per cent showed second degree AV-block on at least one occasion, and their heart rates averaged 65 beats/min. Only 15 per cent of 52 normal-line dogs displayed AV-block (generally fewer instances per dog) and their heart rates averaged 110 beats/min. Fifth generation nervous-line puppies, two to six weeks old, exhibited occasional AV-block even at heart rates averaging 183 beats/min. The incidence and “severity” increased as the puppies aged. Dogs crossbred from second and third generation parents of the two main lines show intermediate incidences of AV-block and intermediate heart rates. The incidence of AV-block is predominant in females. This is interpreted as a sex-influenced, agedependent, polygenic mode of inheritance. Humoral factors probably account for the sex-influence, and psychological stress probably also plays a role. No sex difference is noted in heart rates.