Introduction: Cardiovascular autonomic neuropathy (CAN) is a clinically significant yet frequently underdiagnosed complication of Type 2 diabetes mellitus (T2DM), associated with substantially elevated risks of cardiac arrhythmia, silent myocardial ischaemia, and all-cause mortality. Heart rate variability (HRV) has been validated internationally as a sensitive, non-invasive, and reproducible tool for detecting early autonomic dysfunction. Despite the escalating diabetic burden in Bihar, India, locally relevant evidence on HRV-based autonomic profiling remains critically sparse, underscoring the need for regionally contextualized research. Methods: A hospital-based cross-sectional observational study was conducted at the Department of Medicine, Shyamlal Chandrashekhar Medical College and S.P.N.M. Hospital, Khagaria, Bihar, over six months from July to December 2025. A total of 168 confirmed T2DM patients aged 30–70 years were recruited through consecutive sampling. Short-term five-minute resting electrocardiographic HRV recording was performed in the supine position, with extraction of time-domain parameters including SDNN, RMSSD, and pNN50, and frequency-domain parameters including LF power, HF power, and LF/HF ratio, following Task Force (1996) standardized guidelines. Biochemical parameters including HbA1c, fasting plasma glucose, and lipid profile were retrieved from certified hospital laboratory records. Statistical analysis was performed using IBM SPSS Version 25.0, with Spearman's correlation and independent samples t-test applied as appropriate. Results: CAN was identified in 76 participants (45.2%). All time-domain and frequency-domain HRV parameters were significantly attenuated in the CAN-positive group compared to the non-CAN group (p < 0.001 for all indices). Disease duration demonstrated the strongest inverse correlation with SDNN (r = −0.512; p < 0.001), followed by HbA1c (r = −0.468; p < 0.001). The LF/HF ratio was markedly elevated in the CAN group (3.46 ± 0.94 vs. 2.18 ± 0.68; p < 0.001), indicating pronounced sympathovagal imbalance. Conclusion: HRV analysis effectively identified subclinical autonomic dysfunction in 45.2% of T2DM patients, with disease duration and glycemic control emerging as the strongest modifiable determinants. Integration of standardized HRV assessment into routine diabetic follow-up protocols is strongly warranted at tertiary care hospitals in resource-limited settings across India.

Heart Rate Variability, Type 2 Diabetes Mellitus, Cardiovascular Autonomic Neuropathy, Autonomic Dysfunction, Sympathovagal Balance.