Bradycardia: Difference between revisions

Bradycardias are defined as symptomatic heart rhythm disorders resulting in a inappropriately low heart rhythm or loss of conduction during during physiologic conditions. Generally this results in a cut-off value of <60 beats per minute, however variation of heart rate exists. 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. The most common intrinsic cause is ageing, but ischaemic heart disease, infiltrative diseases or surgery can result in conduction disorders. Medication that modifies the excitability of the heart is the most frequent extrinsic cause, however electrolyte and metabolic disorders may influence the heart rate directly or indirect. To understand the pathophysiologic basis of most conduction disorders or disorders of impulse formation it is important to have knowledge about the physiology of cardiac conduction and mechanisms of arrhythmia as detailed in the general cardiac arrhythmia section [Link].

Sinus bradycardia is a slow sinus rhythm of <60 beats per minute. Sinus bradycardia can be physiological, as is athletes or during sleep. Commonly sinus bradycardia is caused by medication, ischemia or neuro-mediated bradycardia, such as in a vasovagal reaction. Furthermore metabolic diseases can cause bradycardia, e.g. hypothermia or hypothyroidism.

 

* <b>Second degree Type I:</b> (Wenkebach) SA exit block: the P-P interval progressively shortens prior to the pause

* <b>Second degree Type II SA exit block:</b> the pause equals approximately 2-4 times the preceding PP interval

* <b>Third degree SA exit block:</b> absence of P waves (can only be diagnosed with an sinus node electrode, during electrophysiological evaluation)  

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. Often an ectopic pacemaker takes over lower in the conduction system, but the new rate varies slightly from the old one.

Asystole is the lack of cardiac activity eventually leading to immediate death.

Sick sinus syndrome is a 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)

* An inadequate response to exercise.  

* Bradycardia-tachycardia syndrome; where alternating bradycardia and tachycardia arise.

Technically an 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.

* <b>Mobitz I (Wenkebach)</b>: 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. A common finding in the Mobitz I block is that the first prolongation of the PR interval is associated with the largest increase in interval. After this first prolongation of the interval, the interval gradually increases. Usually Mobitz type II block is located at the atrioventricular node and rarely deteriorates to a more severe conduction block.

* <b>Mobitz II:</b> When atrial activation is blocked, without progressively 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 is complete block of conduction between atria en ventricle. Atrial and ventricular rhythms are complete dissociated.

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# ECGpedia [http://en.ecgpedia.org ECGpedia]

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