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Time-honored indices of classical, closed loop arterial blood pressure (BPa) control have recently been supplemented by analyses thought to be sensitive to both closed and open loop mechanisms. These newer tools include, among others, power spectral analyses of heart rate (HR) and BPa, and the cross-correlation between changes in BPa and changes in HR or R-R interval. The „baroreceptor effectiveness index‟ (BEI) is significantly larger in supine, healthy individuals as compared to diabetic patients with peripheral neuropathy. Such findings give promise that BEI may provide a means to assess the development of autonomic neuropathy in diabetic or in other at-risk patients. The cross-correlation between BPa and heart rate in man includes a peak where the two are positively correlated with HR changes leading pressure changes, suggestive of open loop control, and a sharp nadir where the two are negatively correlated and BPa changes precede HR changes, suggestive of closed loop control. In rat the positive cross-correlation depends upon intact spinal sympathetic pathways. The magnitudes of both the positive and negative cross correlations are weaker in tetraplegic patients as compared to neutrally intact subjects. Both peaks remain relatively strong in healthy subjects during the challenge of head up tilt, but decline precipitously in tetraplegic patients. The recognition that both closed and open loop mechanisms are operative and functionally important suggests multiple additional possible etiologies for cardiovascular dysfunctions, including hypertension. The advent of non-invasive means to quantify autonomic function promises new insights into control of the cardiovascular system in health and disease. Closed and Open Loop Mechanisms Controlling Arterial Blood Pressure in Waking State in Human and Experimental Animal David C. Randall 1 and David R. Brown 1 1 Department of Physiology, College of Medicine, University of Kentucky, Lexington, KY

Closed and open loop mechanisms controlling arterial blood pressure in waking state in human and experimental animal