1. Cardiovascular responses to graded increments of lower body negative pressure were studied in non-diabetic subjects and in patients with diabetes mellitus.

2. In all subjects, low levels of negative pressure (which did not affect significantly systemic arterial pressure) induced forearm vasoconstriction, suggesting normal function of the ‘low pressure’ cardiopulmonary baroreflex. However, in some diabetic patients the response to higher levels of negative pressure was abnormal, and it seems likely that although afferent mechanisms were intact there was impairment of efferent vasoconstrictor function.

3. Changes in R—R interval were linearly related to changes in systolic blood pressure induced by higher levels of negative pressure. The slope of the relationship was taken as the sensitivity of the ‘high pressure’ arterial baroreflex; diabetic patients showed a reduced sensitivity compared with normal subjects. Furthermore, in diabetic patients, abnormalities of R—R interval control were more common than abnormalities of vasoconstrictor function, suggesting that heart-rate control is impaired earlier than vasomotor function in diabetic autonomic neuropathy.

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