In 2003, ACC/AHA/ESC published guidelines for the treatment of supraventricular tachyarrhythmias, and in January 2005, the Chinese Cardiovascular Journal published guidelines for the treatment of supraventricular tachyarrhythmias in China. In this article, we describe the modern diagnosis and treatment strategies of bypass-related tachycardia in the context of the guidelines. 1.Diagnosis of bypass-related tachycardia Pre-excitationsyndrome or WPW (Wolf-Parkinson-White) syndrome is an abnormal atrioventricular conduction phenomenon due to incomplete closure of the fibrous annulus of the mitral and tricuspid valve annulus during embryonic development and the appearance of a myocardial connection between the atrial and ventricular muscles across the atrioventricular sulcus and In addition to the atrioventricular node, an additional conduction pathway, or atrioventricular bypass, is formed. The supraventricular impulse can partially or fully excite the ventricles in advance via the AV bypass. The atrioventricular bypass may form a foldback loop with the normal atrioventricular pathway, or the bypass may act as a bystander to allow rapid conduction of supraventricular arrhythmic excitation from the bypass to the ventricles, resulting in bypass-related tachycardia. The typical atrioventricular bypass is manifested as a preexcitation wave on the body ECG, and its ECG detection rate in the general population is 0.15 to 0.25%. The incidence of bypass in first-degree relatives of patients with atrioventricular bypass is approximately 0.55%. Bypasses usually resemble normal myocardial fibers with fast, nondecreasing conduction, sometimes with intermittent conduction. Bypasses that can be transmitted anteriorly are called “dominant preexcitation” and have preexcitation waves on the ECG, the degree of which depends on the speed of transmission of the bypass to the ventricles relative to the AV node. Bypasses that are only retrograde are called “occult preexcitation” and their presence cannot be confirmed by conventional electrocardiography. Dominant bypasses are usually both antegrade and retrograde. Bypasses with only anterograde function are rare, and approximately 8% of bypasses have only anterograde or retrograde decremental conduction. Bypasses are classified according to their location in the mitral and tricuspid annulus, their conduction characteristics (decremental or not), and whether they have an anterior or retrograde conduction function. The diagnosis of preexcitation syndrome should have both preexcitation ECG manifestations and/or tachyarrhythmias. (1) Paroxysmal atrioventricular foldback tachycardia: It accounts for about 95% of all bypass-related tachycardias. AVRT is divided into cis-transmissive and retrotransmissive types. cis-transmissive foldback impulses are transmitted from the atrium to the ventricle via the AV node and back from the ventricle to the atrium via the bypass, and the ECG shows narrow QRS wave group tachycardia, which is easily confused with other types of supraventricular tachycardia. The retrograde type accounts for 5-10% of the preexcitation syndrome, in which the retrograde impulse is transmitted from the atrium to the ventricle along the bypass, and then retrograde through the atrioventricular node or another bypass, and the ECG shows wide QRS wave group tachycardia, which is easily confused with ventricular tachycardia and supraventricular tachycardia with bundle branch block. (2) Persistent junctional retrograde tachycardia: This is a rare type of insidious slow-conducting bypass. When the impulse travels very slowly backwards through the bypass to the atrium and reaches the AV node again, the AV node has already left the inactive phase, and the impulse can travel down through the AV node again and form a restless tachycardia. According to electrophysiological examination and ablation treatment, most of the bypasses causing PJRT are located near the coronary sinus orifice, which is the same location as the slow pathway of the AV node double pathway and should be distinguished from the fast-slow AV node tachycardia. A small number of bypasses are located in the anterior septum or free wall. (3) Patients with preexcitation syndrome may also have a combination of arrhythmias such as atrial fibrillation, atrial flutter, AVNRT, or atrial tachycardia. Once these supraventricular tachyarrhythmias are transmitted anteriorly via the bypass, although the bypass is not a critical part of the tachycardia folding loop as a bystander, the specific conductivity of the bypass leads to a rapid ventricular response that is the root cause of later cardiac insufficiency or even sudden death in patients. The preexcitation syndrome combined with atrial fibrillation has an incidence of 11% to 39%, which is significantly higher than the general population and is a potentially fatal arrhythmia. In the case of atrial fibrillation, if the bypass antegrade induction period is short, the rapid ventricular response due to excitation conduction to the ventricles may metastasize into ventricular fibrillation. Possible mechanisms for the development of AVRT include increased atrial pressure due to rapid or sustained AVRT, myocardial ischemia, abnormal sequence of atrial excitation, and altered neurohumoral factors, which may cause AVRT to metastasize into atrial fibrillation; premature ventricular excitation via atrioventricular bypass, which may trigger atrial fibrillation by falling into the vulnerable period of the atria; multiple bypasses are more likely to trigger AVRT, which increases the likelihood of AVRT metastasizing into atrial fibrillation. Multifactorial analysis showed that age, gender and history of syncope were independent risk factors for preexcited syndrome with AF. Preexcitation syndrome combined with atrial flutter is rare, accounting for only 4% of cases. Whether atrial excitation in patients with preexcitation syndrome occurs via atrial bypass or normal atrioventricular pathways depends on whether these conduction pathways are in a nonstimulated phase at the time of arrival of atrial excitation. Atrial flutter or atrial fibrillation can occur when ventricular excitation travels backwards through the bypass to the atria, coinciding with a period of post-excitation vulnerability of the atrial muscle. The abnormal bypass is usually more receptive to rapid and continuous excitation than the normal atrioventricular pathway, so the preexcitation combined with atrial flutter is mostly caused by the bypass excitation of the ventricles resulting in wide QRS wave group tachycardia, and the fast ventricular rate can cause severe hemodynamic disturbances, which is potentially dangerous. These ECG changes can be easily confused with other arrhythmias such as ventricular tachycardia and supraventricular tachycardia combined with bundle branch block. Preexcitation syndrome can lead to ventricular fibrillation. Although rare clinically, it can be the first manifestation in patients with asymptomatic preexcitation syndrome, the incidence of which is not known. Epidemiological surveys in children show an annual incidence of 0.0635%. The mechanism of ventricular fibrillation caused by preexcitation syndrome: persistent AVRT triggers atrial fibrillation; short effective atrial induction period (<250 ms) and rapid ventricular rate due to fast conduction velocity; humoral factors such as increased secretion of catecholamines; inappropriate application of antiarrhythmic drugs (e.g., drugs to shorten the bypass induction period), etc. 2. Risk stratification of sudden death in patients with preexcitation syndrome The incidence of sudden death in patients with preexcitation syndrome ranges from 0.15 to 0.39%. The risk stratification of patients with preexcitation syndrome can be initially performed by non-invasive methods such as body surface ECG, ambulatory ECG, exercise ECG, drug test and combined with medical history. The potential value in identifying high-risk patients must be considered in balance with the risk of complications associated with catheter ablation procedures (approximately 2%) due to the low positive predictive value of invasive electrophysiological examinations that are not suitable for routine screening of asymptomatic patients. With improved catheterization techniques, the low complication rate of electrophysiological testing outweighs the benefits compared to the potential dangers of pre-excitation.