Pre-excitation is an abnormality of atrioventricular conduction in which an impulse travels down an additional channel and excites part or all of the ventricle early, causing premature excitation of part of the ventricular muscle, called “pre-excitation. Pre-excitation is a relatively rare arrhythmia, and the diagnosis mainly depends on the electrocardiogram. The etiology of preexcitation is the presence of a congenital atrioventricular additional channel (referred to as a bypass) outside the normal atrioventricular conduction system. Most patients do not have organic heart disease. It is also seen in certain congenital heart diseases and acquired heart diseases, such as tricuspid valve subluxation and hypertrophic obstructive cardiomyopathy. Clinical manifestations Preexcitation alone is asymptomatic. Complicated supraventricular tachycardia is similar to general supraventricular tachycardia. In case of atrial flutter or atrial fibrillation, the ventricular rate is around 200 beats/min. In addition to palpitations and other discomfort, shock, heart failure or even sudden death may occur. If the ventricular rate is extremely fast, such as 300 beats/min, the auscultatory heart sound may be only half of the ventricular rate on the ECG, suggesting that half of the ventricular excitation cannot produce effective mechanical contraction. The ECG characteristics of each bypass causing preexcitation are as follows: 1. Atrioventricular bypass (1) PR interval (essentially P-δ interval) shortens to less than 0.12 seconds, mostly 0.10 seconds; (2) QRS time limit prolongs to more than 0.11 seconds; (3) QRS wave group is blunt at the beginning and forms a stutter with the rest, which is called preexcitation; (4) secondary ST-T wave changes. The above ECG changes are also classified as type A and B. In type A, both the preexcitation wave and QRS wave group are upward in lead V1, while in type B, both the preexcitation wave and the main wave of QRS wave group are downward in lead V1; the former suggests left ventricular or right ventricular posterior base myocardial preexcitation, while the latter suggests right ventricular anterolateral wall myocardial preexcitation. Although this classification is limited by the variable QRS wave groups due to preexcitation of different parts of the bypass, it helps to distinguish the ventricular end of the bypass in the left or right, anterior or posterior, and is therefore used to this day. 2, Atrioventricular node, Atrioventricular bypass PR interval is less than 0.12 seconds, mostly in 0.10 seconds; QRS wave groups are normal and there are no preexcited waves. This ECG performance is also called short PR, normal QRS syndrome or L, G, L (Lown-Ganong-Levine) syndrome. 3. Nodal ventricle and bundle ventricle connection Normal PR intervals, widened QRS wave groups, and preexcitation waves. In the case of preexcitation syndrome, most of the preexcitation manifestations disappear and the ECG shows supraventricular tachycardia with a normal QRS wave group pattern. In the presence of atrial flutter or atrial fibrillation, it is not uncommon for the QRS to remain preexcited, and the ECG shows atrial flutter or atrial fibrillation with an abnormally wide QRS wave group; the ventricular rate mostly exceeds 200 beats/min and can even reach 300 beats/min. In atrial flutter, 1:1 atrioventricular conduction may be present, and atrial flutter waves may be identified. In atrial fibrillation, the ventricular rhythm is irregular, and after a long interval, individual QRS wave groups may be seen with normal morphology (probably due to prolonged bypass interval and loss of cryptic conduction in the atrioventricular node, where all or most of the impulses are conducted through the atrioventricular node), and atrial fibrillation waves may be identified. Very fast ventricular rates may also be accompanied by frequency-dependent intraventricular conduction changes. Diagnosis 1. Typical preexcitation syndrome (1) P-R interval <0.12 sec, normal P wave; (2) QRS time >0.11 sec; (3) QRS wave group start part becomes blunt, called preexcitation wave or δ wave; (4) secondary ST-TT changes. Clinically, there are two types of preexcitation: type A preexcitation: the preexcitation wave and QRS wave group are upward in lead V1, and their bypasses are located around the left atrioventricular valve ring. type B preexcitation: the preexcitation wave and the main wave of QRS wave group are downward in lead V1 and upward in lead V5 of the left thoracic lead, and their bypasses are located around the right atrioventricular valve ring. 2. Variable preexcitation (1) LGL-type syndrome P-R interval ≤ 0.11 seconds; normal QRS wave group time; no δ wave. (2) Mahaim-type preexcitation P-R interval ≥ 0.12 seconds; QRS integrated wave onset wave with δ wave, but small δ wave; QRS time ≥ 0.12 seconds, but slight widening. In addition to the above ECG features, ECG vectorogram can be used as a diagnostic basis, which is characterized by the beginning part of QRS ring running slowly in a straight line on all surfaces, lasting up to 0.08 sec, and then suddenly turning and continuing to run at normal speed. the QRS ring running time can exceed 0.12 sec. Hirschsprung’s beam electrograms and body surface or endocardial markers are useful in identifying the apex and localizing the bypass, and play an important role in determining whether the bypass is involved in the tachycardia folding loop. V. Treatment No specific treatment is required for preexcitation itself. If supraventricular tachycardia is present, the treatment is the same as that for general supraventricular tachycardia. In case of atrial fibrillation or atrial flutter, if the ventricular rate is fast and the circulation is impaired, synchronous direct current resuscitation should be used as soon as possible. Lidocaine, procainamide, propafenone and amiodarone slow down the conduction of the bypass, which can slow down the ventricular rate or convert atrial fibrillation and atrial flutter to sinus rhythm. Digitalis accelerates bypass conduction, and verapamil and propranolol slow down conduction in the AV node, all of which may significantly increase the ventricular rate and even develop into ventricular fibrillation, and therefore should not be used. In case of frequent episodes of supraventricular tachycardia or atrial fibrillation or atrial flutter, it is advisable to apply the above-mentioned antiarrhythmic drugs for long-term oral prevention of episodes. In recent years, due to the rapid development of percutaneous catheter radiofrequency ablation technology, preexcitation syndrome can be considered firstly by this minimally invasive treatment, and the majority of patients can be cured with good therapeutic results.