What are the symptoms and causes of preexcitation syndrome?

The heart is an important organ in the human body, and the frequency of its beating also plays an important role. Preexcitation syndrome is related to the health of the heart. This disease is generally manifested as arrhythmia, which arrives at the atrium during the vulnerable period of the atrium and causes partial excitation of the ventricular muscle. Its diagnostic criteria mainly rely on electrocardiogram. So, what are the symptoms and causes of preexcitation syndrome?

symptom

Simple preexcitation is asymptomatic. Concurrent supraventricular tachycardia is similar to general supraventricular tachycardia. For patients with atrial flutter or atrial fibrillation, the ventricular rate is mostly around 200 beats/min. In addition to discomfort such as palpitations, shock, heart failure and even sudden death may occur. When the ventricular rate is extremely fast, such as 300 beats/min, the heart sounds detected by auscultation may be only half of the ventricular rate on the electrocardiogram, indicating that half of the ventricular excitation cannot produce effective mechanical contraction.

Causes

It is the presence of a congenital atrioventricular additional channel (abbreviated as bypass) outside the normal atrioventricular conduction system. Most patients have no organic heart disease. It is also seen in certain congenital and acquired heart diseases, such as tricuspid valve dysplasia, obstructive cardiomyopathy, etc. Electrophysiological studies have shown that the conduction speed of the bypass is fast, and part of the atrial impulse is quickly transmitted down through the bypass, reaching the ventricular end of the bypass in advance, exciting the adjacent myocardium, thereby causing premature ventricular excitation and changing the normal excitation order of the ventricular muscle. As a result, the QRS complex on the electrocardiogram is deformed, with a pre-excitation wave (δ wave) in the initial part.

The remainder of the atrial impulse can be transmitted along the normal pathway and merge with the ventricular excitation caused by the bypass pathway to form a ventricular fusion wave. The morphology of the ventricular fusion wave is determined by the length of the refractory period of the normal and accessory pathways. If the refractory period of the normal pathway is long, or most of the impulses are conducted along the bypass pathway, the QRS deformity will be obvious; if the refractory period of the bypass pathway is long, the ventricular fusion wave will be close to normal.

There are two conduction pathways between the atrioventricular space in patients with preexcitation syndrome, which makes reentry and reentrant tachycardia prone to occur. When tachycardia occurs, most impulses are transmitted retrogradely through the bypass pathway and then down along the normal channel, so the QRS complex of tachycardia has normal morphology; occasionally, impulses are transmitted retrogradely through the bypass pathway and then down along the normal channel, causing tachycardia, in which the QRS complex appears pre-excited. Patients with preexcitation may also have episodes of atrial fibrillation or atrial flutter, which are mostly caused by retrograde impulses reaching the atria during the atrial vulnerable period.

During atrial flutter and atrial fibrillation, the impulse is concealed in the tissue at the junction, causing most or all of the impulse to be transmitted to the ventricles via the bypass pathway. Atrial flutter or atrial fibrillation with extremely fast ventricular rate and malformed QRS complex may sometimes develop into ventricular fibrillation. Unidirectional blockade of the accessory pathway (mostly downward conduction block) can result in no preexcitation on the electrocardiogram, but repeated episodes of supraventricular tachycardia; electrophysiological studies can confirm that the accessory pathway is involved in the reentry of tachycardia.

Second-degree conduction block of the accessory pathway may result in intermittent preexcitation on the ECG. The following are known bypass pathways, and the same patient may have multiple bypass pathways:

① Atrioventricular bypass (Bundle of Kent). Most of them are located beside the atrioventricular groove or septum on the left and right sides, connecting the atrial muscle and ventricular muscle;

②Atrial node bypass (James pathway). It is the channel between the atrium and the lower part of the atrioventricular node or the atrioventricular bundle, which may be formed by some fibers of the posterior internodal bundle;

③ Knot chamber and bundle chamber connection (Mahaim fiber). It is a pathway connecting the distal end of the atrioventricular node or the proximal end of the atrioventricular bundle or bundle branch with the ventricular septum. Among the three, the atrioventricular bypass is the most common.