Impact of Smoking on Heart Rate Variability: A Cross-sectional Study of Healthy Individuals in a Tertiary Care Center

including coronary artery disease, diabetes, and psychiatric disorders.9,10 Heart rate variability is typically analyzed using time-domain and frequency-domain indices: time-domain indices such as SDNN (standard deviation of NN intervals), RMSSD (root mean square of successive RR interval differences), and pNN50 (% of NN intervals differing by more than 50 ms) predominantly reflect parasympathetic (vagal) activity.On the other hand, frequencydomain indices such as LF (low frequency), HF (high frequency), and LF/HF ratio provide a detailed understanding of sympatheticparasympathetic balance.A higher LF/HF ratio is indicative of sympathetic dominance or vagal withdrawal.[10][11][12] IntroductIonCigarette consumption is the leading cause of preventable deaths worldwide and is associated with numerous adverse health outcomes.1 Cigarette smoke contains over 7,000 toxic chemicals, including nicotine, carbon monoxide, formaldehyde, hydrogen cyanide, and various free radicals.Among these, nicotine primarily modulates the autonomic nervous system (ANS) by stimulating nicotinic acetylcholine receptors in both the central and peripheral nervous systems.2,3 This stimulation results in increased release of catecholamines (epinephrine and norepinephrine) from both the adrenal medulla and sympathetic nerve terminals, leading to elevated sympathetic tone, tachycardia, vasoconstriction, and elevated blood pressure.4,5 The autonomic imbalance caused by chronic smoking is a critical pathophysiological mechanism behind its adverse cardiovascular effects.An increase in sympathetic drive and concurrent suppression of parasympathetic (vagal) activity may be a hallmark of chronic nicotine exposure, promoting arrhythmogenesis, endothelial dysfunction, increased myocardial oxygen demand, and progression of atherosclerosis.[6][7][8] Heart rate variability (HRV) has emerged as a reliable, noninvasive method to evaluate cardiac autonomic regulation.HRV reflects beat-to-beat variations in heart rate and is modulated by the balance between sympathetic and parasympathetic inputs to the sinoatrial node.A higher HRV indicates a healthy, responsive autonomic system, while a lower HRV is considered a predictor of increased morbidity and mortality in various clinical settings, 1-