Is Baroreflex Function an Adequate Predictor of Orthostatic Tolerance?

Abstract

In this study we evaluated the magnitude and frequency of arterial baroreflex engagement during orthostatic stress before and after octreotide acetate, a somatostatin analog that improves orthostatic tolerance. Method Baroreflex sensitivity (BRS), the slope of the R-R interval and systolic blood pressure relationship, and baroreflex effectiveness index (BEI), the percentage of spontaneous SBP ramps that elicit a compensatory baroreflex-mediated change in RRI, were determined during orthostatic stress. Healthy, young subjects (n = 52; ages 18–37) were tilted upright either at 70° or at intervals of 15°, 30°, and 50°. Tilt testing was conducted before and after intravenous administration of 1.7 μg/kg of octreotide. Results There was a significant decrease in RRI, SBP, and BRS with increasing orthostatic stress (p<0.0001). At the same time, there was a significant increase in BEI (p=0.0018). After octreotide, orthostatic tolerance (measured as median tilt time to presyncope) increased (p=0.001), but BRS and BEI were unaffected. Conclusion Passive tilt reduces arterial baroreflex sensitivity and increases the frequency of baroreflex engagement. However, orthostatic tolerance can be manipulated independently of baroreflex function as measured by BRS or BEI. These findings suggest that arterial baroreflex function may not be a suitable predictor of orthostatic tolerance in all circumstances. (Supported by NASA NNJ04HF45G).