title: Cardiac autonomic function and vascular profile in subclinical hypothyroidism: Increased beat-to-beat QT variability.
abstract: BACKGROUND
  Patients with subclinical hypothyroidism (SH) may have higher incidence of coronary heart disease and autonomic dysfunction.
  
  
  METHODS
  Prospective case control study.
  
  
  OBJECTIVE
  To evaluate beat-to-beat QT variability and vascular stiffness in patients with SH compared to normal controls.
  
  
  METHODS
  We compared linear and nonlinear measures of cardiac repolarization liability using beat-to-beat QT intervals derived from the surface electrocardiogram during supine posture and vascular indices including pulse wave velocity and ankle-brachial index (ABI) during supine posture between female patients with SH and age- and sex-matched normal controls. Spectral analysis was done at very low frequency (LF) (0.003-0.04 Hz), Low frequency (LF) (0.04-0.15 Hz), and high frequency (HF) (0.15-0.4 Hz). The HF represents vagal regulation (parasympathetic) and LF represents both parasympathetic and sympathetic regulation.
  
  
  RESULTS
  We recruited 58 women with a mean age of 31.83 ± 8.9 years and 49 controls with mean age of 32.4 ± 9.9 years (P= NS). QT variability index (QTvi) was higher in cases compared to controls (P= 0.01). The ratio of LF/HF of R-R interval which is an index of sympathovagal tone was significantly more in cases compared to controls (P= 0.02). The difference in the left minus the right ABI was significant between cases and controls (P= 0.03).
  
  
  CONCLUSIONS
  The cases had lower parasympathetic activity as compared to controls, and there was a predominance of sympathetic activity in cases. QTvi may be an important noninvasive tool in this group of patients to study the risk of cardiovascular mortality.
authors.affiliation: Department of Endocrinology, M.S. Ramaiah Medical College, Bengaluru, Karnataka, India.
authors.forename: Pramila
authors.initials: P
authors.lastname: Kalra
authors.affiliation: Department of Psychiatry, University of Alberta, Edmonton, Alberta, Canada.
authors.forename: Vikram K
authors.initials: VK
authors.lastname: Yeragani
authors.affiliation: Centre for Diabetes and Endocrine Care, Bangalore Diabetes Hospital, Bengaluru, Karnataka, India.
authors.forename: K M
authors.initials: KM
authors.lastname: Prasanna Kumar
keywords: Ankle-brachial index; autonomic dysfunction; subclinical hypothyroidism