It is an abnormal heart rythm in which ventricles of the heart depolarized too early.
During foetal development few special strands of myocardial fibres make connections between the atria and ventricular musculature outside the normal conducting system. After birth, these strands either disappear or become functionless:
In Pre excitation syndrome these strands persist. Because these fibres partly or completely bypass the normal conducting system they are called Accessory pathways. These pathways may connect right atrium with the ventricles, left atrium with the ventricle or connect the atrial or ventricular septae on right or left side.
There are three main forms of Pre excitation syndrome.
ECG shows normal P wave, a short P-R interval < .12 sec, QRS duration is wide > .12 sec, there is initial slurring of QRS complex called Delta Waves. WPW syndrome is of two types :
(I) Type A where excitation travels along left accessory pathway giving rise to right ventricular enlargement or RBBB pattern in V1 and;
(II) Type B where excitation travels along right lateral accessory pathway producing a pattern of LBBB. This is, however, a crude method of classification.
ECG shows short P-R but normal QRS duration. This also causes Tachyarrhythmia.
About 50% of the patients will have no symptom. Remaining 50% may present with supraventricular tachyarrhythmia such as atrioventricular re-entrant tachycardia, atrial flutter and atrial fibrillation from time to time. Asymptomatic cases are diagnosed during routine ECG check up.
ECG shows (a) short P-R interval, (b) wide QRS duration,
(c) Delta wave. In Lown-Ganong-Levine syndrome (b) and
(c) are absent.
Some cases may develop PAT and one third will develop atrical fibrillation.
In symptomatic cases prophylactic antiarrhythmic therapy may be given which slows the conduction rate and prolong the refractory period of bypass tract. Flecainide, disopyramide or amiodarone may be given. Digoxin and Verapamil should be avoided as they increase the conduction in bypass tract. Radiofrequency transvenous catheter ablation of the bypass tract is the treatment of choice. Atrial fibrillation and flutter should be managed as given before avoiding digoxin, calcium channel blocker and B-blocker.
PRE EXCITATION SYNDROME
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