Body mass index and orthostatic haemodynamic responses during head-up tilt testing

Purpose To examine the association between body mass index (BMI) and the severity of orthostatic hypotension (OH) during head-up tilt (HUT) testing using continuous phasic haemodynamic measurements in a large cohort of patients with unexplained falls, blackouts, and (pre)syncope. Methods We conducted a retrospective observational cohort study of consecutive HUT tests performed between 1998 and 2024 at a single tertiary centre. Adults referred for HUT testing were included, and analyses were restricted to patients with a measurable systolic blood pressure change during tilt. Participants were categorised by World Health Organization BMI groups. Outcomes included change in systolic blood pressure (ΔSBP), diagnosis of OH (≥ 20 mmHg systolic fall), and heart-rate response during tilt. Results Among 6,610 participants (mean BMI 25.8 ± 4.7 kg/m²), 3% were underweight, 46% normal weight, 34% overweight, and 17% obese. Median ΔSBP was − 18 mmHg (IQR − 30 to − 9) and differed significantly across BMI categories (p = 0.008). Underweight participants had the greatest median systolic BP fall (–23.5 mmHg) compared with overweight participants (–17 mmHg, p = 0.01). OH was most frequent in the underweight group (58%) and similar across other BMI categories (46–50%, p = 0.004). Heart-rate responses did not differ meaningfully between BMI groups. Conclusion Low BMI was associated with greater orthostatic systolic blood pressure drops, while higher BMI did not confer measurable protection. These findings suggest that the obesity paradox does not extend to autonomic haemodynamic responses and that low BMI may serve as a marker of vulnerability to orthostatic hypotension.