Physiological and clinical comparison of active stand and head-up tilt tests in Postural Orthostatic Tachycardia Syndrome (POTS)

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