Overview.
Variable preexcitation syndrome is also known as Mahaim-type preexcitation syndrome. It is a group of syndromes in which sinus node excitation is transmitted down Mahaim’s fiber, resulting in the presence of delta waves on the electrocardiogram and a widening of the QRS wave trains with normal P-R intervals, with or without supraventricular tachycardia.
Etiology
It is mainly due to the presence of an additional conduction pathway between the atria and the ventricles, which has a much faster conduction velocity than normal atrioventricular conduction. Before atrial excitation through normal atrioventricular conduction reaches the ventricles, it rapidly reaches the ventricles through the additional atrioventricular conduction pathway, thus prematurely agitating part of the ventricular myocardium. The additional conduction pathways that have been demonstrated histologically include Kent’s bundle, James’ bundle, and Mahaim’s fiber. It has been suggested that Mahaim fibers are extremely thin fibrous tissues that are shorter than Kent bundles and are more common in children, decreasing with age. It is rare in adults and can be found in the normal heart.
Symptoms
Similar to typical preexcitation syndrome. When not combined with tachycardia, there may not be any clinical symptoms; if combined with tachycardia, symptoms such as palpitations, dizziness and chest tightness may occur. However, since all patients with this disease have reverse atrioventricular tachycardia, which is a wide QRS wave cluster tachycardia with a wide left bundle branch block pattern, this may cause some difficulties in differential diagnosis, and it may have a greater hemodynamic impact than cis-type atrioventricular tachycardia, and the corresponding symptoms may be more obvious. The symptoms may be more pronounced.
Examination
1. Electrophysiologic characteristics of atrial bundle bypass (Mahaim fiber)
(1) Slow conduction velocity This is the most prominent electrophysiologic feature of the atrial bundle bypass. The conduction time of the atrioventricular bundle bypass is mostly >150 ms, whereas the conduction time via the paraventricular bundle (Kent bundle) is 30-40 ms, and the conduction time via the AV node (A-H phase) is <150 ms. This slow conduction makes the electrocardiogram show: (1) normal or prolonged P-R interval; (2) left bundle branch block is often accompanied by one degree of atrioventricular block; (3) when supraventricular tachycardia occurs, the A-V interval is longer.
(2) Only anterograde conduction None of the atrioventricular bundle bypass circuits discovered so far have a reverse conduction function, only an interatrial anterograde conduction. This feature makes the patients with atrial bundle bypass with supraventricular tachycardia have atrioventricular node retrograde conduction, i.e., the QRS wave is wide and aberrant with a left bundle branch block pattern.
(3) Relatively short duration Compared with the atrioventricular node duration, the atrioventricular bundle bypass has a relatively short duration. When early supraventricular excitation is transmitted down the atrioventricular node, the excitation is blocked during the atrioventricular node duration, and then the excitation is transmitted along the atrioventricular bundle bypass, with a shorter duration, and then retrograde transmission through the atrioventricular node, resulting in reverse AV refractory tachycardia.
(4) Decremental conduction The atrioventricular bundle bypass is similar to the AV node in that it also has decremental conduction. When a faster frequency supraventricular atrial stimulus is applied, the original 1:1 downward conduction of the atrioventricular bundle bypass can be changed to a ventricular-type downward conduction. Decremental conduction occurs.
(5) ATP can block its conduction Injection of adenosine triphosphate (ATP) can block conduction in the AV node, but has no effect on conduction in the bypass, as a result of excitation of the vagus nerve. The conduction of the atrioventricular bundle bypass is affected by ATP, as evidenced by the temporary loss of its only anterograde function after ATP injection.
(6) There may be no or few δ waves. Because the end of the atrial bundle bypass is directly fused to the terminal end of the right bundle branch, the surface ECG may be free of δ waves, and fewer δ waves may be seen in the bypass downstream from other Mahaim fibers.
2. Characteristics of esophageal atrial pacing in the atrial bundle bypass (Mahaim fibers)
(1) QRS wave With the advancement of atrial preperiodic stimulation, the atrium may enter the period of nonresponsiveness, and supraventricular excitation is transmitted down the atrial bundle bypass, and the QRS wave appears to resemble the pattern of left bundle-branch block, and the V1 lead is still in the rS type.
(2) S2-R2 interval Unlike the frequency-dependent left bundle-branch block that occurs with atrial pacing in the general population, the prolongation of the S2-R2 interval is not obvious with the shortening of the coupled intervals of early stimulation.
3. Electrocardiographic features
(1) Typical electrocardiographic features of traditional variant preexcitation syndrome: (1) P-R interval ≥ 0.12 s. (2) QRS wave widening and distortion. However, it is narrower than in Kent bundle preexcitation syndrome. ③ Pre-excitation wave (δ wave) is present at the beginning of QRS wave, but it is small. Typical ECG patterns are seldom seen in dominant preexcitation syndrome due to Mahaim fibers.
(2) Electrocardiographic features of atrial bundle bypass-type variant of preexcitation syndrome It was previously believed that intracardiac electrophysiologic diagnosis of atrial bundle bypass was difficult, and ECG diagnosis was even more difficult. Guo Ji-hong et al. believe that the body surface electrocardiographic manifestations of atrial bundle bypass have high specificity and can provide more reliable evidence or clues for diagnosis. The electrocardiographic manifestations of atrial bundle bypass preexcitation syndrome are similar to those of the traditional Mahaim fiber preexcitation syndrome, with the following features: ① QRS wave widening and aberration in the form of left bundle branch block pattern. ② δ waves may not be present, and if they are, they are smaller than those in typical WPW syndrome. (iii) The P-R interval is normal. (iv) In the case of concomitant supraventricular tachycardia, a wide QRS tachycardia with left bundle branch block and left deviation of the electrical axis is often present.
To summarize, some scholars believe that if frequency-dependent, intermittent left bundle branch block occurs; or if left bundle branch block is accompanied by fast frequency-dependent conduction delay or Vinzel’s-type conduction with P-R interval, the existence of atrial bundle bypass should be suspected; and if wide QRS tachycardia occurs and left bundle branch block is present with left deviation of the electrical axis, the existence of atrial bundle bypass should be highly suggestive.
(3) Electrocardiographic characteristics of atrial bundle bypass subtype The subtype of atrial bundle bypass refers to the end of atrial bundle bypass directly inserted into the right ventricular free wall near the right bundle branch. Surface electrocardiographic manifestations: ① similar to B-type WPW syndrome; ② QRS wave group widening, there may be δ waves, but smaller than the typical WPW syndrome; ③ P-R interval is normal; ④ due to the atrioventricular node-like conduction characteristics, so it is different from the general WPW syndrome, and may be the Vinzel’s type of conduction. Therefore, when the surface electrocardiogram shows a B-type WPW syndrome pattern with normal P-R interval and fast frequency-dependent delayed conduction or Venn diagram conduction, the possibility of atrial bundle bypass subtype should be suspected; if the tachycardia is accompanied by wide QRS waves and the left bundle branch block pattern is accompanied by a leftward deviation of the electrical axis, the existence of the atrial bundle bypass subtype is highly suggestive.
Diagnosis
Diagnosis can be made on the basis of clinical manifestations and electrocardiographic and electrophysiologic features.
Treatment
1. Treatment of preexcitation syndrome without complications
No treatment is needed, but follow-up observation is required.
2. Treatment of preexcitation syndrome combined with tachyarrhythmia
（Tachyarrhythmias caused by ventricular pre-excitation, especially those with frequent episodes of hemodynamic changes and symptoms, should be treated immediately with medication.
1) Treatment of paracentric (preexcited) atrioventricular tachycardia episodes Same as “Treatment of paroxysmal supraventricular tachycardia.
2) Treatment of pre-excitation syndrome combined with atrial fibrillation (atrial flutter) Some scholars refer to pre-excitation syndrome combined with atrial fibrillation and combined with retrograde atrioventricular tachycardia as pre-excitation syndrome combined with tachyarrhythmia with QRS wave widening. There are a small number of preexcitation syndromes combined with tachyarrhythmias that have abnormally widened QRS waves on the ECG. The urgency of emergency management of an episode depends on the rapidity of the ventricular rate during the tachycardia and the degree of hemodynamic compromise.
3) Poor hemodynamic condition (with severe hypotension, etc.) or still good Electrical cardioversion should be preferred in cases of fast and prolonged arrhythmic tachycardia.
4) If the hemodynamic condition is good and the arrhythmic tachycardia is tolerable, pharmacologic therapy should be tried first. Should be used to extend the atrioventricular bypass should not be prolonged and inhibit its conduction function of the drug. For example, propafenone: often the drug of choice. Procainamide significantly prolongs the anterograde effective response period of the bypass, and can moderately prolong the reverse effective response period and significantly prolong the P-A interval. Amiodarone terminates acute episodes of preexcitation syndrome combined with atrial fibrillation or atrial flutter with very high efficiency. The digitalis preparations trichostatin C (sildenafil) and verapamil (isobarbital) should be contraindicated.
(2) Treatment of interictal period ① For patients with preexcitation syndrome combined with tachycardia who have fewer episodes, shorter duration, less obvious symptoms, and who can recover on their own, treatment is not necessary. However, overwork and other triggering factors should be avoided. If atrial pre-systole, ventricular pre-systole, etc. occurs, you should take propafenone (cardioplegia), mexiletine (slow heart rhythm) and so on to be corrected, which can reduce the number of episodes of tachycardia. ② For intermittent patients with preexcitation syndrome combined with frequent episodes of tachycardia, the maintenance dosage of the effective drugs mentioned above should be taken for a long time to prevent recurrence. Effective preventive drugs can also be screened by cardiac electrophysiologic examination of induced arrhythmias. (iii) In the intermittent period, radical treatment should be used for patients with frequent attacks. Currently, radiofrequency ablation is mostly used, with a very high success rate.
3. Synchronized direct current cardiac electrical resuscitation
Electrical cardioversion (power 100-200J) is effective in terminating atrioventricular tachycardia and preexcitation syndrome combined with atrial fibrillation, and it is especially suitable when the latter is difficult to identify with ventricular tachycardia due to the widening and malformation of the QRS wave in the electrocardiogram by preexcitation and the difficulty in choosing drugs, as well as when there is a significant hemodynamic obstacle due to the tachyarrhythmia. After reentry, drugs are still required for maintenance.
4. Surgical treatment of preexcitation syndrome
Before catheter-based radiofrequency ablation was carried out, surgical treatment of preexcitation syndrome by cutting off or injecting with anhydrous alcohol or local freezing bypass achieved good efficacy and a very high cure rate. However, the surgical method is difficult to be widely used due to high trauma and has been replaced by catheter radiofrequency ablation. Only in some special cases, such as congenital heart disease with preexcitation syndrome or acquired heart disease requiring surgery. Surgery can be considered as a concomitant treatment for pre-excitation syndrome.
5. Catheter-based radiofrequency ablation for pre-excitation syndrome
Transcatheter radiofrequency ablation (RFCA) for the treatment of preexcitation syndrome combined with tachyarrhythmia has been very successful. RFCA is a transcatheter ablation with low-energy radiofrequency current. Generally, it does not lead to myocardial penetration and rarely induces arrhythmia. It can issue radiofrequency current to ablate many times and in many parts of the heart without any feeling or pain.
6. Implantable cardioverter-defibrillator
Implantable cardioverter-defibrillator can be considered when drug treatment is ineffective or catheter radiofrequency ablation fails.
Prevention
Prevention of preexcitation syndrome is mainly to prevent the recurrence of tachycardia, in order to effectively prevent the recurrence of tachycardia, two drugs should be used to simultaneously inhibit the forward and reverse conduction of the refractory pathway, for example, quinidine and propranolol, or procainamide and verapamil can be used together to obtain a better effect.IA, IC drugs amiodarone or sol prolongs the indefiniteness of the atrioventricular (AV) bypass circuit and atrioventricular (AV) node, which can effectively prevent the recurrence of tachycardia. Recurrence. The choice of drugs can be based on clinical experience, or the use of cardiac electrophysiological examination identified as effective drugs. This ensures optimal prevention of recurrence.