Autoregulation and regional blood flow of the dog during hemorrhagic shock

In 42 anesthetized dogs, we studied autoregulation and regional blood flow in the adrenal gland during hemorrhagic hypotension at 70, 50, and 30 mmHg of mean arterial pressure and in trimethaphan camsylate (TMP)-induced hypotension at 50 mmHg. Blood flow was measured by nonradioactive microspheres. Renal blood flow was significantly reduced at 50 mmHg, but total adrenal blood flow did not significantly decrease until the blood pressure fell below 30 mmHg. The autoregulatory response was clearly different between the kidney and adrenal gland. Cortical blood flow decreased significantly at 50 mmHg, but medullary blood flow increased in contrast to the reduction of blood pressure below 70 mmHg and returned at 30 mmHg to the flow rate seen in normotensive control. TMP induced a similar response in the medulla to that at 50 mmHg of hemorrhagic hypotension, and atropine reversed this TMP-induced change. Opposite microvascular responses were observed during hypotension: medullary blood flow increased and cortical flow decreased. This medullary vascular response is considered to contribute to the maintenance of total adrenal blood flow during hemorrhagic hypotension.