Due to their clear benefits in maintaining sinus rhythm and in the conversion of acute pharmacological atrial fibrillation, Class IC antiarrhythmic drugs (primarily flecainide and propafenone) are increasingly widely used for rhythm control in patients with atrial fibrillation. These drugs are a first-line treatment option for atrial fibrillation patients with normal cardiac structure. However, they pose greater risks for patients with ventricular hypertrophy, heart failure, and coronary artery disease. To ensure safe medication use, a key clinical measure is to frequently perform 12-lead ECGs, both at rest and during exercise, to monitor and diagnose adverse reactions such as drug-induced malignant arrhythmias.

The mechanism of action of Class IC drugs is to inhibit the conduction of cardiac electrical signals by blocking sodium ion channels in cardiomyocytes, thereby calming the disordered atrial fibrillation. However, this may come at the cost of slowing normal cardiac conduction. The *American Journal of Cardiology* reported that after patients took the medication, 12-lead electrocardiograms showed a 17% to 29% prolongation of the PR interval and an 11% to 27% prolongation of the QRS complex. As early as the 1980s, the medical community discovered that type IC antiarrhythmic drugs carried the risk of inducing ventricular proarrhythmias. Initially, these drugs were used to suppress premature ventricular contractions and were thought to potentially help patients with frequent ventricular ectopic rhythms after myocardial infarction, thereby preventing sudden cardiac death. The Cardiac Arrhythmia Suppression Trial (CAST study), published in the *New England Journal of Medicine* in 1991, showed that type IC drugs not only lacked protective effects but also increased the risk of sudden cardiac death in patients after myocardial infarction. Following this study, the clinical use of type IC drugs declined sharply. However, in the past 30 years, with the accumulation of clinical experience, their safety and practicality in atrial fibrillation patients with completely normal cardiac structure have been re-evaluated.

To achieve precise medication monitoring, clinical practice must rely on meticulous monitoring of electrocardiogram waveform intervals. Electrocardiogram (ECG) monitoring involves detecting the start, end, and peak amplitude of various waveforms (such as the P wave, QRS complex, and T wave) on an electrocardiogram (ECG), and calculating key parameters such as the PR interval and QRS duration. Statistical analysis of these characteristic segments based on these parameters allows for the diagnosis of the subject's cardiac activity, which is crucial for the treatment of heart diseases. Human cells generate electrical signals (biocellular electricity) during their life activities. ECG signals extracted from the body surface are weak signals against a noisy background and are unstable. Clinically, to obtain high-quality, highly stable ECG waveforms, a 12-lead ECG system is routinely used to comprehensively capture and amplify changes in myocardial bioelectricity. Through continuous statistical analysis of these characteristic segments, doctors can make accurate diagnoses of the subject's cardiac electrical activity, thereby quantifying drug efficacy and mitigating risks.

Regarding the use of IC50 drugs, the *World Journal of Cardiology* and related clinical guidelines provide clear standards for resting and exercise ECG monitoring. When starting medication for the first time or increasing the dosage, a baseline 12-lead electrocardiogram (ECG) should be performed after the blood drug concentration reaches steady state. If the resting ECG shows a QRS duration prolonged by more than 25% compared to the baseline before medication, the drug dosage must be halved; if the QRS duration remains prolonged by more than 25% after dosage reduction, discontinuation of medication is recommended.

In summary, Class IC antiarrhythmic drugs have unique clinical advantages in rhythm control for patients with atrial fibrillation. However, in clinical application, strict patient selection and precise monitoring of ECG signals are essential. Through close coordination of 12-lead resting ECG and exercise stress ECG, clinicians can convert weak bioelectrical signals into quantifiable conduction interval parameters, thereby minimizing the risk of sudden cardiac death while using Class IC drugs and ensuring the safety and efficacy of atrial fibrillation drug therapy.