Comparison of active stand and head-up tilt table testing for the diagnosis of orthostatic intolerance syndromes

Background: Orthostatic intolerance (OI) causes signs (e.g., elevated heart rate (HR) or reduced blood pressure (BP)) and symptoms (e.g., light-headedness) when standing, which are relieved by lying down. Testing involves measuring HR and BP during a postural challenge (head-up tilt test (HUTT)/active stand (AS)). Variability of testing outcomes is possible due to the lack of testing guidelines and physiological differences between tests. The aim of this thesis was to determine if there is variation between AS and HUTT in the context of OI testing. 

Methods: This thesis used various methods to compare AS to HUTT, including a systematic literature review. We also conducted a controlled pairwise study, accounting for pre- and during-study confounders, with a healthy population (n = 12; 23 ± 7 years; 75% female). Cardiovascular responses were measured using continuous and intermittent monitoring (Finapress, electrocardiogram (ECG), and brachial cuff) during a 40-minute AS and HUTT. After 40-minutes, bilateral leg cuffs occluded the individuals’ lower limbs for 3 minutes, and responses were recorded after release. Additionally, a 5-minute slow HUTT (slower transition from supine to upright) was conducted. 

Results: The 10 papers included in our literature review, revealed high variability between populations, protocols, and results measured. Our controlled experiment revealed inter-test differences in maximum HR (p < 0.05) and minimum BP in AS compared to baseline (p < 0.001). Leg cuff occlusion release showed systolic blood pressure (SBP) and diastolic blood pressure (DBP) drops (−8±6mmHg; −6±4mmHG (p < 0.0014)) and elevated HR (14±5bpm (p < 0.05)). No significant difference in the effect of HUTT transition speeds on HR or BP was observed. 

Conclusions: The literature was heterogenous leading to difficulty in comparing HUTT and AS tests. Our controlled experiment demonstrated inter-test variability in maximum HR, while the leg cuff produced HR and BP changes comparable to exogenous protocols, indicating a physiologically meaningful response. Additionally, HUTT transition speeds is not a confounder. OI testing need clear guidelines based on a rigorous comparison of techniques, which should involve both diagnosed and healthy populations. This clarity would enable quicker and more accurate diagnosis of OI.