Assessment of Cardiac Autonomic Neuropathy in Subclinical Hypothyroidism Using Short-term Heart Rate Variability: A Cross-sectional Study

Abstract

Introduction: The thyroid gland and the hypothalamus are closely linked to the cardiovascular system through the Autonomic Nervous System (ANS). The actions of thyroid hormones target the cardiovascular system, affecting homeostasis, which includes contraction, rhythm, blood flow, and peripheral vascular resistance of the myocardium. To assess the function of the ANS that influences the cardiovascular system, Heart Rate Variability (HRV) is an important and increasingly used tool. Aim: To investigate cardiac autonomic changes as assessed by short-term HRV in Subclinical Hypothyroidism (SCH) and to determine whether there is any association between Thyroid Stimulating Hormone (TSH) levels and the Low Frequency/High Frequency (LF/HF) ratio as well as the Standard Deviation of NN intervals (SDNN). Materials and Methods: A cross-sectional analytical study was conducted at R.G. Kar Medical College and Hospital, Kolkata, West Bengal, India from November 2022 to October 2023. Both males and females aged 18-55 years with newly diagnosed SCH (cases, n=50) and healthy individuals (controls, n=50) were included in the study. HRV was recorded using a Physiograph Polyrite-D instrument equipped with bioamplifiers, four channels, and accessories {Record Mangement System (RMS) latest software-version 1.2.5}. The study variables included anthropometric parameters, biochemical parameters (TSH), and HRV parameters such as SDNN and the LF/HF ratio. Relevant data were collected using standard statistical methods, including Statistical Package for Social Sciences (SPSS) version 20.0, unpaired Student’s t-test, and Pearson’s correlation coefficient. Results: The present study found a significant increase in the LF/HF ratio in SCH patients (1.28±0.75) compared to controls (0.90±0.32). There was a significantly lower SDNN value among cases (40.63±21.89) compared to controls (79.86±15.36). No significant changes were observed in Heart Rate (HR), Systolic Blood Pressure (SBP), or Diastolic Blood Pressure (DBP) between cases and controls. The LF/HF ratio and TSH value showed a positive correlation (r=0.09, p=0.57), while SDNN exhibited a negative correlation with TSH value (r=-0.08, p=0.58) among the cases, but neither correlation was statistically significant. Conclusion: The present study concludes that there is a significant change in the LF/HF ratio in newly diagnosed SCH patients. Therefore, the evaluation of short-term HRV can be utilised as a routine screening test for improved medical care.

Keywords

• autonomic dysfunction
• cardiovascular morbidity
• low frequency/high frequency ratio
• standard deviation of nn interval