Physiological and clinical comparison of active stand and head-up tilt tests in POTS, 2025, Jaiden Uppal et al

Highlights
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Heart rate and Δheart rate were significantly different between the AST and HUT.
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Both the AST and HUT demonstrated a drop in SBP and stroke volume over time.
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Higher proportion of POTS patients met heart rate criteria with HUT than AST.

Abstract
Head-up tilt (HUT) and active stand tests (AST) are used in the diagnosis of Postural Orthostatic Tachycardia Syndrome (POTS), but their relative diagnostic accuracy is unclear. This necessitates a direct comparison under standardized conditions. We aimed to compare the hemodynamic responses and diagnostic accuracy of AST vs. HUT in POTS.

To address this, patients with POTS (n = 60) completed a 10-min AST followed by HUT on the same day. Beat-to-beat hemodynamics were recorded during 10-min supine baselines and each test. Delta values were calculated for each test (upright 1-min averages minus baseline average).

Δ[heart rate] increased significantly over time (1_Min: 28 bpm to 10_Min: 40 bpm; PTime < 0.001), and was greater for HUT (33 bpm vs. 37 bpm; PASTvHUT = 0.01), with significant Time x Condition interaction (38 bpm vs. 42 bpm at10min; PINT < 0.001). Δ[stroke volume] declined over time (1_Min: -18 ml to 10_Min: -32 ml); PTime < 0.001), with no significant test or interaction effects (PASTvHUT = 0.36; PINT = 0.21). Δ[SBP] decreased (1_Min: −0.3 mmHg to 10_Min: −5.7 mmHg); PTime < 0.001) over time, with no test or interaction effects.

Fewer patients met POTS heart rate criteria during the AST (AST: 74 % vs. HUT: 98 %; p < 0.001). Lowering the threshold to 27 bpm for AST narrowed the gap but was still significantly higher for HUT (AST: 83 % vs HUT: 98 %; p = 0.02).

Orthostatic tachycardia differs between AST and HUT in patients with POTS. The proportion of patients with POTS meeting the heart rate diagnostic criteria differs significantly between AST and HUT, a discrepancy that can be mitigated by lowering the heart rate threshold for the AST.
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Could the breathing tests have any effect on the results in the HUT?

They’ve tested the order of the two tests before and didn’t find any differences in a small cohort or 2x16, so the order presumably doesn’t matter.
https://www.s4me.info/threads/compr...n-patients-with-pots-2021-bourne-et-al.22415/