If conduction in the dysfunctional fascicle also fails completely, complete heart block ensues.Myotonic dystrophy (DM) is a group of inherited neuromuscular disorders with an autosomal dominant pattern of distribution. Trifascicular block is present when bifascicular block is associated with first degree heart block. The left posterior fascicle is fairly stout and more resistant to damage, so right bundle branch block with left posterior hemiblock is rarely seen. Right bundle branch block with left anterior hemiblock is the commonest type of bifascicular block. The electrocardiogram shows right bundle branch block with left or right axis deviation. Left posterior hemiblock is characterised by a mean frontal plane axis of >90° in the absence of other causes of right axis deviation.īifascicular block is the combination of right bundle branch block and left anterior or posterior hemiblock. Left anterior hemiblock is characterised by a mean frontal plane axis more leftward than −30° (abnormal left axis deviation) in the absence of an inferior myocardial infarction or other cause of left axis deviation. Abnormal ventricular depolarisation is associated with secondary repolarisation changes, giving rise to changes in the ST-T waves in the right chest leads.īlock of the left anterior and posterior hemifascicles gives rise to the hemiblocks. These terminal deflections are wide and slurred. Thus the later part of the QRS complex is abnormal the right precordial leads have a prominent and late R wave, and the left precordial and limb leads have a terminal S wave. As left ventricular depolarisation is complete, the forces of right ventricular depolarisation are unopposed. The wave of depolarisation then spreads to the right ventricle through non-specialised conducting tissue, with slow depolarisation of the right ventricle in a left to right direction. The left ventricle depolarises in the normal way and thus the early part of the QRS complex appears normal. When conduction in the right bundle branch is blocked, depolarisation of the right ventricle is delayed. In most cases right bundle branch block has a pathological cause though it is also seen in healthy individuals. High degree atrioventricular block, which occurs when a QRS complex is seen only after every three, four, or more P waves, may progress to complete third degree atrioventricular block. 2:1 atrioventricular block is difficult to classify, but it is usually a Wenckebach variant. The block is often at the level of the bundle branches and is therefore associated with wide QRS complexes. The PR interval is constant, though it may be normal or prolonged. There is intermittent failure of conduction of P waves. Mobitz type II block is less common but is more likely to produce symptoms. The PR interval then returns to normal, and the cycle repeats. The initial PR interval is normal but progressively lengthens with each successive beat until eventually atrioventricular transmission is blocked completely and the P wave is not followed by a QRS complex. Mobitz type I block (Wenckebach phenomenon) is usually at the level of the atrioventricular node, producing intermittent failure of transmission of the atrial impulse to the ventricles. There are three types of second degree block. Some P waves are not followed by a QRS complex. In second degree block there is intermittent failure of conduction between the atria and ventricles. They take over when normal impulse formation or conduction fails and may be associated with any profound bradycardia. The pause is unrelated to the length of the P-P cycle.Įscape rhythms are the result of spontaneous activity from a subsidiary pacemaker, located in the atria, atrioventricular junction, or ventricles. Sinus arrest occurs when there is transient cessation of impulse formation at the sinoatrial node it manifests as a prolonged pause without P wave activity. The pauses are the length of two or more P-P intervals. Sinoatrial block is characterised by a transient failure of impulse conduction to the atrial myocardium, resulting in intermittent pauses between P waves. The commonest electrocardiographic feature is an inappropriate, persistent, and often severe sinus bradycardia. The possible electrocardiographic features include persistent sinus bradycardia, periods of sinoatrial block, sinus arrest, junctional or ventricular escape rhythms, tachycardia-bradycardia syndrome, paroxysmal atrial flutter, and atrial fibrillation.
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