Low body mass index is associated with a positive response during a head-up tilt test

To describe the association between body mass index (BMI) and a positive response during a head-up tilt test (HUT) in patients referred for an investigation of syncope.

Observational study of patients referred for the diagnostic evaluation of syncope. Patients were divided into four groups according to their BMI: <18.5 kg/m(2), 18.5-24.9 kg/m(2), 25-29.9 kg/m(2), and > 30 kg/m(2).

A total of 419 patients were evaluated. The mean age was 43 ± 22 years, and 62% were female. The prevalence of a positive tilt test was different between groups when stratified by BMI (P = 0.01), with a higher proportion of patients with positive tests among those with BMI <18.5 kg/m(2) compared with other groups (P = 0.05). Multivariate analysis also showed that underweight patients had a 3.9 times higher risk for a positive HUT response (P = 0.01); additionally, the use of contraceptive drugs was associated with a protective effect during HUT (odds ratio: 0.35, confidence interval: 0.19-0.45, P = 0.001).

In our sample, changes in BMI are associated with a positive response for HUT, and oral contraceptives seemed to protect against this response. Further studies are needed with larger numbers of patients to corroborate this finding.