The bypass refers to the conduction pathway between the atria and ventricles other than the AV node, and is generally considered to be a remnant of the myocardial tissue that remains during the separation of the atria and ventricles during embryonic development. The bypass often crosses the mitral or tricuspid annulus and has a conduction function that can form a foldback loop with the normal atrioventricular conduction system – the AV node, bundle branches, etc. – resulting in tachycardia. Depending on the location of the bypass, the bypass can include mainly left-sided bypass and right-sided bypass, and can be classified as fast bypass or slow bypass depending on whether the conduction has a decreasing character. Bypasses can have antegrade conduction, retrograde conduction, or both directions. Bypasses with only reverse conduction are called “occult bypasses” because they do not appear on the surface ECG. Because bypass conduction is usually faster than AV node conduction, in the presence of a bypass with antegrade conduction, the electrical impulse from the sinoatrial node is transmitted to the atrial level and travels not only down the AV node to the ventricles, but also down the bypass to the ventricles, the latter being faster than the former and preexciting some of the myocardium. The latter is faster than the former and preexcites part of the myocardium. This is a characteristic feature on the body electrocardiogram, so the bypass with antegrade conduction is also called “dominant bypass”. Both dominant and recessive bypasses provide the basis for the formation of a fold loop with the normal AV conduction pathway, meaning that patients with dominant or recessive bypasses do not necessarily have a history of paroxysmal supraventricular tachycardia. The presence of a dominant bypass with a history of episodes of supraventricular tachycardia is called “pre-excitation syndrome” or “WPW syndrome”. It should be noted that many physicians, including some electrophysiologists, are accustomed to refer to an ECG showing a dominant bypass as a “pre-excitation syndrome,” which patients and families need to understand; the definition of “pre-excitation syndrome” in this section follows a strict written definition. In addition, some physicians sometimes refer to supraventricular tachycardia with an overt bypass as “overt preexcitation” and to supraventricular tachycardia with an overt bypass as “recessive preexcitation”; in fact, the term “recessive preexcitation” is strictly speaking. The term “recessive pre-excitation” is not strictly scientific, but it is generally understood. The majority of bypass-mediated paroxysmal supraventricular tachycardias are located between the atria and the ventricles (supraventricular tachycardias with atrioventricular nodal regression can be considered to be located inside the AV node), and this part of the tachycardia is therefore also called “atrioventricular regression tachycardia. Atrioventricular tachycardia can be divided into two types: antegrade and retrograde. During an episode of forward atrial tachycardia, electrical impulses are transmitted from the normal atrioventricular conduction system (atrioventricular node, Hirschsprung’s bundle, bundle branches, etc.) down to the ventricles, and then backwards along the bypass to the atria, where they are transmitted in a continuous loop in the folding loop. In reverse atrioventricular tachycardia, the electrical impulses travel in the opposite direction from the forward direction, down the bypass to excite the ventricles, and then back up the normal atrioventricular conduction system to excite the atria. The QRS wave group representing ventricular excitation on the ECG is generally narrow during an episode of this type of tachycardia, whereas the QRS wave group on the ECG must be wide during an episode of reverse atrioventricular tachycardia. As mentioned earlier, the AV node has a natural protection against downward transmission of electrical impulses of excessive frequency to the ventricles, whereas the bypass is not a normal conduction bundle and does not have this protective function. Also as mentioned earlier, the frequency of electrical impulses at the atrial level in atrial fibrillation is very fast, and if a dominant bypass is combined, a large proportion of these electrical impulses will be transmitted from the bypass down to the ventricles and the heart will lose the filtering protection of the AV node; as a final result, the number of ventricular beats in atrial fibrillation combined with a dominant bypass may be very fast, and in particularly severe cases, the interval between beats may be very short, which is a dangerous situation This is a dangerous situation, and in a small number of patients it may turn into fatal ventricular fibrillation. The annual incidence of sudden cardiac death in patients with preexcitation syndrome is estimated to be around 0.15%. In fact, patients with preexcitation syndrome alone, even if it is not sufficient to diagnose preexcitation syndrome (preexcitation syndrome in the strict sense of the term is defined by the presence of a clinical history of supraventricular tachycardia), are at the same risk as patients with preexcitation syndrome when combined with atrial fibrillation. Given the possible risks in patients with preexcited syndrome combined with atrial fibrillation, ablation should be considered in all patients with preexcited syndrome. There is no unanimous opinion on whether to aggressively perform ablation in patients with a single-pass ECG-confirmed dominant bypass without a history of tachycardia episodes. The argument against ablation is based on the following considerations: although a dominant bypass alone combined with atrial fibrillation has a certain risk, only a minority of patients also have atrial fibrillation; some patients with a dominant bypass may not have tachycardia in their lifetime (supraventricular tachycardia occurs only if a foldback loop is formed between the two paths, sometimes there are 2 paths but no foldback loop is formed), so it can be observed and once tachycardia occurs Tachycardia can be observed and then ablation can be considered once it occurs. The argument in favor of ablation is based on the following considerations: the dominant bypass has a clear risk of sudden cardiac death when combined with atrial fibrillation, most patients with dominant bypass will develop supraventricular tachycardia, and ablation therapy of the bypass is now well established with a low complication rate, so ablation should be actively considered. In practice, ablation is mainly considered based on the following factors: occupation (whether it is an occupation with public safety implications, such as professional driver, pilot, etc.), likelihood of atrial fibrillation (age, other medical history such as hypertension, coronary artery disease, cardiac insufficiency, rheumatic heart disease, etc.), financial ability, and the patient’s own wishes. Ablation may be considered in patients with clinically considered bypass-mediated supraventricular tachycardia (occult bypass), with significant symptoms, where drug therapy is ineffective or where the patient is unwilling to undergo long-term drug therapy; however, as with supraventricular tachycardia with atrioventricular node folding, ablation is currently favored. Ablation of the bypass is more difficult than ablation of the slow pathway because the slow pathway has a relatively fixed anatomic location, but the location of the bypass is variable. The bypass may be located in the left mitral annulus (more common on the left side) or in the right tricuspid annulus, and may be located in the free wall, septum, anterior, or posterior of the annulus; therefore, the most critical aspect of bypass ablation is to determine the location of the bypass by electrophysiologic examination after placement of the appropriate electrode catheter. The location of the bypass can be roughly determined by the characteristic manifestations of the surface ECG, but the location of the occult bypass and the precise determination of the dominant bypass requires manual stimulation of the ventricles with electrical impulses during intracardiac electrophysiological examination. The electrical impulses are conducted retrogradely through the bypass to the atria, where the atrial excitation generates the atrial potential waveform (P wave), the closer to the bypass the earlier the P wave is generated, and the earliest P wave is the location of the bypass. The location of the bypass can also be determined by the earliest preexcitation of the ventricular muscle, but ventricular stimulation is usually required to check the sequence of retrograde P waves. The right bypass requires only puncturing the vein, and the catheter is operated in the right heart system; the left bypass ablation catheter requires access to the left heart system. There are two routes of entry into the left cardiac system: one is by puncturing the atrial septum into the left atrium and then approaching the mitral annulus until the bypass position is reached; the other is by puncturing the femoral artery and the ablation catheter enters the left ventricle retrogradely through the artery to approach the mitral annulus. The choice between the two routes is based on the operator’s custom, the assessment of the risk of septal puncture by the cardiac center where the procedure is performed, and the suitability of the bypass location for septal puncture, with the latter often being the more common choice for domestic physicians. In rare cases, ablation of left-sided bypasses in specific locations is sufficient in the coronary sinus, without the need to puncture the septum and arteries. The ablation catheter arrives at the location of the bypass and begins ablation, usually in 40-60 seconds. The truly accurate and ideal location is usually where the bypass conduction is blocked (“cut off”) within 5-10 seconds of the start of ablation, usually within 2 seconds. The overall success rate of ablation treatment of the bypass is about 95%. The success rate of ablation depends on the site of the bypass, with the highest success rate for left free wall bypass (over 95%) and the lowest for posterior septum and right free wall (90% level). After the first successful ablation of a bypass, the recurrence rate is 7-8%, with recurrence being more common in right free wall and septal bypasses. Bypasses that recover conduction can usually be successfully treated by reablation. Complications of bypass ablation may include valve damage, microembolism, coronary artery injury, and, rarely, cerebrovascular events, in addition to those common to cardiac interventions. Complete AV block occurs in approximately 1% of bypass ablation procedures, mainly in interval bypass ablation (2.5-3%). Overall, the complication rate of bypass ablation ranges from 1 to 4%, and the procedure-related mortality rate is less than 0.2%.