Compensation to exsanguination hypotension in healthy conscious dogs

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Abstract
In some earlier studies on exsanguination hypotension in conscious dogs, reduction in systemic arterial pressure to shock levels was accompanied by a transient tachycardia during the removal of blood, but the heart rate returned to level, at or near control values during extended periods with the mean arterial pressure between 40 and 60 mm Hg. This observation stimulated a series of experiments on five healthy conscious dogs in which transient hypotension was induced by withdrawing blood from the region of the right atrium to determine which mechanisms were dominant in the compensatory reaction. A surprising degree of variability in response was encountered, such that tachycardia was the main response on some occasions, increased peripheral resistance on others, and in still others, several mechanisms appeared to play a role. Similar variability in the response to exsanguination have been reported in human subjects. These observations suggest that the baroceptor reflexes are not simple servo controls and their role in everyday cardiovascular responses should be re-examined.

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