Bradycardia: Difference between revisions

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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.  
Bradycardias are symptomatic heart rhythm disorders resulting from an inappropriately low heart rhythm due to inappropriate slow impulse formation or conduction delay of the cardiac impulse in the myocardium or conduction system during physiologic conditions. These two problems can lead to a slow heart rate, a bradycardia. Generally the definition of bradycardia is a heart rate of <60 beats per minute. However, a normal variation of heart rate exists<cite>Spodick</cite>. For instance, during sleep and in athletes the heart rate can be as low as 40 beats per minute.<cite>Talan</cite>


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].
Bradycardia can be caused by a variety of intrinsic and extrinsic causes. The most common intrinsic cause is ageing, but ischemic heart disease, infiltrative diseases or surgery can also result in conduction disorders.<cite>Ferrer, Mangrum, ESC</cite> 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.
 
Complaints from bradycardia result from an insufficient capacity of the heart to supply the body with blood. Complaints of palpitations, syncope or heart failure may result from bradyarrhythmias, but often vague symptoms like dizziness, exercise intolerance or fatigue may be more prominent<cite>Mova</cite>. A causal relation between complaints and the bradycardia should be established and reversible causes should be identified (for instance use of certain drugs).  


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).


=Disorders of Conduction and Impulse Formation=
=Disorders of Conduction and Impulse Formation=
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===Sinus Bradycardia===
===Sinus Bradycardia===
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.
Sinus bradycardia is a slow sinus rhythm of <60 beats per minute<cite>Spodick</cite>. Sinus bradycardia can be physiological, as in athletes or during sleep<cite>Ector2</cite>. Commonly sinus bradycardia is caused by medication, ischemia or neuro-mediated bradycardia, such as in a vasovagal reaction<cite>ESC</cite>. Furthermore metabolic diseases can cause bradycardia, e.g. hypothermia or hypothyroidism.
 
[[File:Sinusnode.svg|thumb|500px|Sinus node dysfunction.]]
[[File:Sinusnode.svg|thumb|500px|Sinus node dysfunction.]]


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Like AV-nodal conduction disorders (see below) multiple subtypes can be distinguished:
Like AV-nodal conduction disorders (see below) multiple subtypes can be distinguished:
* <b>Second degree Type I:</b> (Wenkebach) SA exit block: the P-P interval progressively shortens prior to the pause
* Second degree Type I (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
* Second degree Type II SA exit block: 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)  
* Third degree SA exit block: absence of P waves, but still impulse formation at the level of the sinus node (can only be diagnosed with an sinus node electrode, during electrophysiological evaluation)  


===Sinus Arrest===
===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. Often an ectopic pacemaker takes over lower in the conduction system, but the new rate varies slightly from the old one.
If the sinus node has a problem with impulse formation it is defined as a sinus arrest. There can be the appearance of an irregular rhythm, however sinus P-waves are clearly present, between intervals of no rhythm or an escape rhythm. 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===
===Asystole===
Asystole is the lack of cardiac activity eventually leading to immediate death.
Asystole is the lack of cardiac activity eventually leading to immediate death.  


===Sick Sinus Syndrome===
===Sick Sinus Syndrome===
Sick sinus syndrome is a denoter of diseases of inappropriate sinus node responses. These encompass for instance:
Sick sinus syndrome is a denoter of diseases of inappropriate sinus node responses <cite>Ferrer</cite>. These encompass for instance:
* An inappropriate response after tachycardia due to overdrive suppression (which can result in long pauses)
* An inappropriate response after tachycardia due to overdrive suppression (which can result in long pauses)
* An inadequate response to exercise.  
* An inadequate response to exercise.  
* Bradycardia-tachycardia syndrome; where alternating bradycardia and tachycardia arise.
* Bradycardia-tachycardia syndrome; where alternating bradycardia and tachycardia arise.


==AV-Block==
==AV-Block==
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===First Degree AV Block===
===First Degree AV Block===
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.
Technically an AV-delay and not an 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===
===Second Degree AV Block===
The second degree AV block can be divided in two separate entities depending on the clinical characteristics of the conduction disorder. If conduction to the ventricle is conducted in a 2:1 fashion; that is if after every second P-wave there is no conduction to the ventricle, it is not possible to distinguish between the two types and a severe kind of conduction block should be assumed. If two sequential P-wave are not followed by a QRS-complex the term malignant block is used, as this could lead to or be an indication of a total block.
The second degree AV block can be divided in two separate entities depending on the clinical characteristics of the conduction disorder. If conduction to the ventricle is conducted in a 2:1 fashion; that is if after every second P-wave there is no conduction to the ventricle, it is not possible to distinguish between the two types and a severe kind of conduction block should be assumed. If two sequential P-wave are not followed by a QRS-complex the term malignant block is used, as this could lead to or be an indication of a total block.
* <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 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.<cite>Simons, Strasberg</cite>
* <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.  
* <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.<cite>Simons, Zipes, Donoso</cite>


===Third Degree AV Block===
===Third Degree AV Block===
Third degree AV block is complete block of conduction between atria en ventricle. Atrial and ventricular rhythms are complete dissociated.
Third degree AV block is complete block of conduction between atria en ventricle. Atrial and ventricular rhythms are complete dissociated.<cite>Levine</cite>
 
==Paroxysmal AV block==
Paroxysmal atrioventricular block (PAVB), is characterized by a sudden and unexpected block of the atrial impulse. Due to the delayed emerge of an escape rhythm, these patients often present with syncope. However, if a escape rhythm is established patients may present themselves without symptoms. Two different variations of the PAVB are commonly distinguished;
 
 


==Ventricular Conduction Block==
==Ventricular Conduction Block==
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=References=
=References=
<biblio>
<biblio>
# Epstein2012 pmid=23255456
# Epstein pmid=23255456
# Epstein2008 pmid=18483207
# Ferrer pmid=5695590
# Wilkoff pmid=18551743
# Talan pmid=7083929
# Vardas pmid=17726042
# Mangrum pmid=10706901
# Strasberg pmid 7471363
# Donoso pmid 14118480
# Zipes pmid 378457
# Levine pmid=13356435
# Kay pmid=6461235
# Kastor pmid=1089890
# Dreifus pmid=6826942
# Ector pmid=6191291
# Langberg pmid 2598419
# Kim pmid=11230857
# Glikson pmid=9388104
# Shaw pmid=4005079
# Choksk pmid=2360528
# Mymin pmid=3762641
# Stevenson pmid=2299071
# James pmid=14451031
# Friedberg pmid=14206803
# Dhingra pmid=4817704
# McAnulty1 pmid=7088050
# McAnulty2 pmid=619828
# Spodick pmid=1529897
# Ector2 pmid=6147639
# Mova pmid=19713422
# Simons pmid=9894656
# Wiens pmid=6741841
# Lichstein pmid=6176956
# Kinley pmid=7503472
# Krahn pmid=15125724
# Hoffman pmid=3536438
# ESC isbn=9780199566990
# ESC isbn=9780199566990
# Braunwald isbn=1437703984
# ECGpedia http://en.ecgpedia.org
# ECGpedia [http://en.ecgpedia.org ECGpedia]
# Robles isbn=9789031313983
# Spodick pmid=16957456
# Wellens isbn=9781416002598
# Elsherrif PMCID: PMC2877697

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