Altered cardiovascular vagal responses in nonelderly female patients with subclinical hyperthyroidism and no apparent cardiovascular disease.

OBJECTIVE

Subclinical hyperthyroidism (SH) has been associated with exercise intolerance, changes in cardiac morphology, atrial arrhythmias and sympathovagal imbalance. The aim of this study was to evaluate the vagal reserve and modulation by a sympathetic stimulus in nonelderly patients with SH without cardiovascular problems.

DESIGN

We carried out a cross-sectional study, comparing data of the heart rate variability (HRV) of SH patients and healthy controls at rest and after vagal and sympathetic stimulation.

PATIENTS

We studied 16 female patients with at least 6 months of SH and 16 healthy female controls with the same median age (40 vs. 34.5 years).

MEASUREMENTS

We used the tilt test, with electrocardiographic record at rest, during the respiratory sinus arrhythmia (RSA) manoeuvre and after tilting, in order to analyse HRV in the frequency domain (%high frequency (HF) and low/high frequency ratio (LF/HF) using Biopotentials Captation System software.

RESULTS

The median TSH level was 0.03 mU/l in patients and 1.37 mUI/l in controls. The median free T4 was 1.37 ng/dl in patients and 1.20 ng/dl in controls. Patients demonstrated a significantly smaller difference between %HF during the RSA and %HF at rest than controls (median -7.5 vs. 36.6, P < 0.001). There was a lower difference between LF/HF ratio after tilting and LF/HF ratio at rest in patients than in controls (1.5 vs. 5.3, P = 0.005).

CONCLUSION

Subclinical hyperthyroidism affects cardiovascular autonomic balance in otherwise apparently healthy nonelderly females by blunting vagal responses.