Head up tilt screening in healthy non‐fainters: relationships with other measures of autonomic function?

Head up tilt (HUT) is a meaningful diagnostic tool for baroreflex dysfunction or autonomic failure. We sought to determine if baroreflex sensitivity (BRS), the rise in plasma catecholamines, or 24‐hr ambulatory HR variability (HRV) would correlate with HR and BP during HUT. We performed 60° HUT for 5 min followed by 2 modified Oxford trials in 46 young healthy males and females. Blood pressure (brachial artery) and HR (ECG) were recorded, while arterial epinephrine (Epi) and norepinephrine (NE) were assayed during the final 30 s of HUT. HUT increased HR (mean ± SD: 63 ± 9 to 79 ± 12), MAP (89 ± 7 to 93 ± 8), Epi (28 ± 13 to 61 ± 45), and NE (111 ± 57 to 242 ± 85) (p< 0.05 for all). Individuals with a high 24‐hr HRV had a greater increase in HR during HUT (r= 0.41, P<0.05). Interestingly, a negative correlation was observed between BRS and the NE response to HUT (r= −0.36, p<0.05), such that individuals with low NE response to tilt also had a high BRS. The Epi response was not correlated with any of the measures. These findings suggest that subjects with high vagal activity (high BRS) require a lower sympathetic response (NE) to maintain BP during orthostatic stress. Supported by: K23 RR17520, HL 46493, K12 RR023263 ‐03