Cardiac Arrhythmias: Difference between revisions

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''Sébastien Krul, MD''
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= General Introduction =
= General Introduction =
To facilitate diagnoses and treatment of cardiac arrhythmias a basic understanding of the physiological cardiac action potential and cardiac conduction system is required.  
A basic understanding of the physiological cardiac action potential and cardiac conduction system facilitates diagnosis and treatment of cardiac arrhythmias. The effects and side-effects of anti-arrhythmic drugs are depended on the influence on ion channels involved in the generation and/or perpetuation of the cardiac action potential.


==Cardiac Action Potential==
==Cardiac Action Potential==
The cardiac action potential is a result of ions flowing through different ion channels. Ion channels are passages for ions (mainly Na<sup>+</sup>, K<sup>+</sup>, Ca<sup>2+</sup> and Cl<sup>-</sup>) that facilitate movement through the cell membrane. Changes in structure of these channels can open, inactivate or close and control current inside and outside the myocytes. Due to different activity and expression of ion channels, the various parts of the cardiac conduction channel have slightly different action potential characteristics. Ion channels are mostly a passive passage, where movement of ions are caused by the electrochemical activity gradient. In addition to these passive ion channels a few ATP-dependent channels exist to fine-tune the action potential. These changes of membrane potential produce and action potential lasting a few hundreds of milliseconds. Disorders is single channels can lead to arrhythmias, as seen in the section [[Primary_Arrhythmias]]. The action potential is conducted throughout the heart by the depolarization of the immediate environment of the cells and through intracellular coupling with gap-junctions. The communication pores are located in cell to cell adhesion structures, the intercalated disks.
The cardiac action potential is a result of ions flowing through different ion channels. Ion channels are passages for ions (mainly Na<sup>+</sup>, K<sup>+</sup>, Ca<sup>2+</sup> and Cl<sup>-</sup>) that facilitate movement through the cell membrane. Changes in the structure of these channels can open, inactivate or close these channels and thereby control the flow of ions into and out of the myocytes. Due to differences in expression of the type and structure of ion channels, the various parts of the cardiac conduction system have slightly different action potential characteristics. Ion channels are mostly a passive passageway, where movement of ions is caused by the electrochemical gradient. In addition to these passive ion channels a few active trigger-dependent channels exist that open or close in response to certain stimuli (for instance acetylcholine or ATP). These changes in the membrane potential produce and action potential lasting a few hundreds of milliseconds. Disorders in single channels can lead to arrhythmias, as seen in the section [[Primary_Arrhythmias]]. The action potential is propagated throughout the myocardium by the depolarization of the immediate environment of the cells and through intracellular coupling with gap-junctions.  
 
In summary during the depolarization, sodium ions (Na<sup>+</sup>) stream into the cell followed by a influx of calcium (Ca<sup>2+</sup>) ions (both from the inside (sarcoplasmatic reticulum) and outside of the cell). These Ca<sup>2+</sup> ions cause the actual muscular contraction. Shortly thereafter potassium (K<sup>+</sup>) ions flow out of the cell, causing repolarization. During repolarization the ion concentration returns to its precontraction state, due to the passive efflux of K<sup>+</sup> and active exchange of Na<sup>+</sup> with Ca<sup>2+</sup>. In detail the action potential can be divided in five phases:


In summary during the depolarization, sodium ions stream into the cell followed by a influx of Ca<sup>2+</sup> ions (both from the inside (sarcoplasmatic reticulum) and outside of the cell). These Ca<sup>2+</sup> ions cause the actual muscular contraction. Shortly thereafter K+ ions stream out of the cell. During repolarization the ion concentration returns to its precontraction state. The action potential can be divided in five phases:
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[[File:AP.png|thumb|500px|The cardiac action potential and relevant ion channels.]]
[[File:AP.png|thumb|500px|The cardiac action potential and relevant ion channels.]]
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