There can also be delays in transmission of the electrical impulse anywhere in the system, including the SA node, the atria, the AV node, or in the ventricles. Some aberrant impulses cause normal variants of the heart rhythm and others can be potentially life threatening. Some examples include:
First-degree atrioventricular (AV) block, or first-degree heart block, is defined as prolongation of the PR interval on an electrocardiogram (ECG) to more than 200 msec.
Type 1 Second-degree AV block, also known as Mobitz I or Wenckebach periodicity, is almost always a disease of the AV node. Wenckebach published a paper in 1906 on progressively lengthening PR intervals that was later classified as Type I in Mobitz’s 1924 paper. Thus, both “Mobitz type I” and “Wenckebach block” refer to the same pattern and pathophysiology.
Type 2 Second-degree AV block, also known as “Mobitz II,” is almost always a disease of the distal conduction system (His-Purkinje System). Mobitz II heart block is characterized on a surface ECG by intermittently nonconducted P waves not preceded by PR prolongation and not followed by PR shortening. There is usually a fixed number of non-conducted P waves for every successfully conducted QRS complex, and this ratio is often specified in describing Mobitz II blocks. For example, Mobitz II block in which there are two P waves for every one QRS complex may be referred to as “2:1 Mobitz II block”.
In the case of 2:1 block (2 P waves for every QRS complex) it is impossible to differentiate type I from type II Mobitz block based solely on the P:QRS ratio or on a pattern of lengthening PR intervals. In this case, a lengthened PR interval with a normal QRS width is most likely indicative of a type I-like pathology, and a normal PR interval with a widened QRS is most likely indicative of a type II-like pathology.
Third-degree atrioventricular block (AV block), also known as complete heart block, is a medical condition in which the nerve impulse generated in the sinoatrial node (SA node) in the atrium of the heart does not propagate to the ventricles.
XVII. JUNCTIONAL RHYTHMS
PJCs are not often seen in individuals with a healthy heart, but it can occur as result of emotional stress, consumption of caffeine, nicotine, and alcohol, or for no apparent cause. PJCs may occur when AV tissue is irritated or as a result of the following conditions: hypoxia.
A junctional rhythm occurs when the electrical activation of the heart originates near or within the atrioventricular node, rather than from the sinoatrial node. Because the normal ventricular conduction system (His-Purkinje) is used, the QRS complex is frequently narrow.
An accelerated junctional rhythm (rate >60) is a narrow complex rhythm that often supersedes a clinically bradycardic sinus node rate (see images below). The QRS complexes are uniform in shape, and evidence of retrograde P wave activation may or may not be present.
Junctional tachycardia is a form of supraventricular tachycardia characterized by involvement of the AV node. It can be contrasted to atrial tachycardia.
It is a tachycardia associated with the generation of impulses in a focus in the region of the atrioventricular node due to an A-V disassociation.
Wandering atrial pacemaker (WAP) is an atrial arrhythmia that occurs when the natural cardiac pacemaker site shifts between the sinoatrial node (SA node), the atria, and/or the atrioventricular node (AV node).