Complete heart block This occurs when the atrioventricular node does not conduct signals from the atria to the ventricles. The atria and ventricles are at their own respective regular rates without any correlation. Examining carefully the PR interval will demonstrate that the P waves (blue lines) and QRS complexes (red lines) are independent of each other.

Supraventricular tachycardia Most SVTs are due to a reentrant mechanism with rates typically 220-300 bpm. The reentrant circuit comprises the AV node in the antegrade limb and an accessory connection in the retrograde limb. If the accessory connection conducts antegrade in sinus rhythm, ECG will show a short PR interval and delta wave (Wolff-Parkinson-White syndrome).

Ventricular tachycardia This ECG shows a wide complex tachycardia due to an abnormal fast rhythm from the ventricles. VT occurs at a rate ≥120 bpm or 25% bpm faster than the sinus rate and is a potentially life-threatening rhythm. VT may be due to abnormal automaticity secondary to hypoxia, inflammation, electrolyte abnormalities, medications, or due to ion channelopathies such as congenital long QT syndrome.

Long QT syndrome This is a group of inherited disorder of cardiac repolarization caused by ion channelopathies. ECG shows prolonged QT interval and abnormal T waves. Long QT syndrome can lead to syncope and sudden cardiac death due to torsade de points ventricular arrhythmia. There are mutations in at least 17 genes identified so far.

Brugada syndrome The abnormal ECG is characterized by right bundle branch block with ST elevation in right praecordial leads (V1-V3). It is associated with ventricular tachyarrhythmia and is a cause of sudden death in otherwise healthy subjects at night. ECG pattern is dynamic and intermittent. ECG pattern may be unmasked during febrile illness or by administration of a sodium-channel blocker (e.g. flecainide, procainamide, or ajmaline). It has an autosomal dominant inheritance.

Wolff Parkinson White syndrome The ECG shows short PR interval and delta wave, indicating ventricular pre-excitation in the presence of an accessory pathway. In patients with ventricular pre-excitation, about 20-30% will develop supraventricular tachycardia with peak incidence in infancy and in the second decade of life.