Dogs and the heart.

Beware of the dog. A dog is a man's best friend. Few of us can encounter an unfamiliar dog without some change of emotion, and dogs have been an integral part of human society since our earliest beginnings. We are social creatures, and how we interact with each other and our pets has a major impact on our well-being. The effects of the interactions between pets and people is beginning to be studied in a scientific manner, both in epidemiological and laboratory studies. The overall impression is that owning a pet is associated with better health and a reduced risk of cardiovascular disease. One proposed mechanism for this is that pets reduce anxiety and help to moderate the effects of stress. In this issue Kingwell et al. [1] have examined the effects of an unfamiliar but friendly dog on the cardiovascular 9 responses to a mental stressor. The main finding was a negative one: neither the blood pressure nor the heart rate response differed according to whether or not a dog was present during the course of the study. However, when the subjects were classified as pet owners or non-owners, it was apparent that there were two different response patterns. The investigators used the technique of spectral analysis of heart rate variability, which is thought to provide an index of sympathetic arousal, albeit not a very pure one [2]. The measure they used was the ratio between the low-frequency (peak at 0.1 Hz) and the high-frequency (peak at 0.2 to 0.3 Hz) oscillations of heart period. The data indicated that when the dog was not present, pet owners showed a heart rate profile that suggested a higher level of sympathetic arousal that of non-owners. The presence of the dog had opposite effects in the two groups: in the owners the lowfrequency/high-frequency ratio went down, whereas in the non-owners it went up. The simplest interpretation of this is that the dog owners found the presence of the dog relaxing (whereas the non-owners did not), or may have experienced sympathetic arousal. Hence the overall changes would be null. There are some interesting parallels with cardiovascular reactivity experiments in which the modulating effect of having another person present during the stressful challenge has been examined. In a study [3] of healthy subjects whose heart rate and blood pressure were monitored during a speech task, we found that if the person being spoken to was a friend, the cardiovascular response was smaller than if he or she was a stranger. Furthermore, if the stranger appeared supportive during the speech the response was smaller than if he or she was neutral. The common thread of these studies is the concept that a familiar and comforting presence, or social support, may buffer the cardiovascular responses to stress. We are social animals, and there is a growing body of evidence that people who are socially isolated are at higher risk of cardiovascular disease than those who have close social ties, either with people or pets. The literature on pets as a source of social support is fragmentary but encouraging [4-6]. As with studies of human social support, there have been two general approaches: laboratory studies, in which the effects of the presence of the pet on responses to stressors have been examined, and epidemiological studies, where the clinical outcomes of pet owners have been compared with those of non pet-owners. Both research strategies have their problems. With laboratory studies, the results may fail to generalize across any number of dimensions. Thus, for example, the results may be very different if the pet is someone else's rather than one's own. In addition, there is always the unanswered question as to the generalizability of the acute response to a contrived laboratory stressor to any long-term effects in real life. As with all epidemiological studies, the big problem is the limited interpretability of the data because of the effect of