Bradycardia

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Introduction

Bradycardias are defined as symptomatic heart rhythm disorders resulting in a inappropriately low heart rhythm during during physiologic conditions. Generally this results in a cut-off value of <60 beats per minute, however variation of normal heart rate may exist. For instance, during sleep and in athletes the heart rate can be as low as 40 beats per minute.

Bradycardia can be caused by a variety of intrinsic and extrinsic causes.

Fortunately the human heart has a couple of backup mechanisms that can help the heart in case of severe bradycardia. These escape mechanisms can occur in every part of the heart (i.e. atrium, AV, node, ventricle). In general, the rate of the escape mechanism is faster when the escape rhythm is located higher in the conduction system.

To understand the pathophysiologic basis of most conduction disorders or disorders of impulse formation it is important to understand the physiology of cardiac conduction and mechanisms of arrhythmia as detailed in the general cardiac arrhythmia section [Link].

[BOX] Abbreviations AV: atrioventricular bpm: beats per minute [BOX]

Disorders of Conduction and Impulse Formation

Sinus Node Dysfunction

Sinus Bradycardia

Sinus bradycardia is a slow sinus rhythm of <60 beats per minute.

Sinus Node Exit Block

In the case of sinus node exit block, an impulse generated from the sinus node is blocked at one of the exit sides of the sinus node. However impulse formation is not affected, therefore the interval between subsequent beats should be similar to n=x times the P-P interval.

Sinus Arrest

If the sinus node has a problem with impulse formation it is defined as a sinus arrest. There can be the appearance of a irregular rhythm, however sinus P-waves are clearly present. In comparison with the sinus node exit block, there is no relation with a previous P-P interval.

Sick Sinus Syndrome

Sick sinus syndrome is an denoter of diseases of inappropriate sinus node responses. These encompass for instance an inappropriate response after tachycardia due to overdrive suppression (which can result in long pauses) or an inadequate response to exercise. A common clinical manifestation of disease of the sinus node is the bradycardia-tachycardia syndrome; where alternating bradycardia and tachycardia arise.

AV-Block

First Degree AV Block

Technically a AV-delay and not a AV block, 1st degree AV block is defined as a prolonged interval between atrial and ventricular activation (>200ms). This delay results from disease in the AV-node or His-Purkinje system. An AV block is not the cause of bradycardia, because every atrial impulse results in conduction to the ventricles.

Second Degree AV Block

Mobitz I (Wenkebach)

The Mobitz type I block is characterized by a progressively increased P-Q interval until atrial activation is blocked in the AV-node. Thereafter conduction is restored and this cycle repeats itself. Usually Mobitz type II block is located at a the atrioventricular node and rarely deteriorates to a more severe conduction block.

Mobitz II

When atrial activation is blocked, without progressivly increasing P-Q interval a Mobitz Type II AV block is present. This sudden failure of AV conduction is an omen of severe conduction disease in usually infra-Hision part of the atrioventricular conduction system.

Third Degree AV Block

Third degree AV block is complete block of conduction between atria en ventricle. Atrial and ventricular rhythm are complete dissociated.