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 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].
The heart being a mechanical pump, complaints from bradycardia result from an insufficient capacity of the pump to supply the rest of the body with blood. Complaints of palpitations, syncope or heart failure may result from conduction disorders. However often vague symptoms for instance like dizziness, exercise intolerance or fatigue may be caused by bradycardia. A causal relation between complaints and the bradycardia should be established and reversible causes should be identified (medication).
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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.
Ventricular Conduction Block
RBBB
The right bundle branch is composed of one fascicle. Right bundle branch block is a unifascicular block of the right bundle and can be found in xx% of normal people. However a new RBBB in a patient with a history of normal ventricular conduction warrens further cardiological investigation.
LAFB
The left bundle branch is composed of two fascicles. One of the fascicle has an anterior location and activates the interventricular septum and the anterior of the ventricle.
LPFB
The second fascicle of the left bundle branch is the posterior fascicle. This fascicle has a posterior location and activates the posterior and lateral part of the ventricle.
LBBB
If the two fascicles of the left bundle branch there is a left bundle branch block, this bifascicular block is uncommon in healthy patients and further cardiologic investigations need to be performed to screen for underlying disease.
Treatment
Fortunately the human heart has a couple of backup mechanisms that can sustain a heart rate 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.
References 1. ECC Textbook of Cardiovascular Medicine (2nd Edition)