Mechanisms of cardiac arrhythmias | Fundamentals

The arrhythmias are an abnormality in rate and rhythm of heart.

They may be either bradyaarhythmias or tachyarrhythmias.

There are 3 basic mechanisms by which tachyarrhythmias may occur 
1. Enhanced automaticity 
2. Triggered automaticity 
3. Reentry 

 Enhanced automaticity 

This type of arrhythmias may poccur due to abnormality in the pacemaker cells. Enhanced automaticity may occurif there is increase in slope of the pacemaker potential or the Resting membrane potential becomes less negative. This may happen in case of excessive sympathetic stimulation and mild kyperkalemia

Triggered automaticity 

This occurs in cells which are not having automaticity but automaticy is being triggered by someting. It involves due to generation of after-depolarization
After depolarizations are of two types:

Early after depolarization which occurs during the repolarization phase of cardiac action potential. It occurs whenever action potential gets prolonged. QT interbal prolonation is a sign of prolonationof action potential duration and thus it may lead to early after depolarization and hence predisposes a person to arrhythmias.

Delayed after depolarization occur after the repolarization ends i.e in phase 4 of cardiac action potentia. These after depolarization can lead to generation of impulse before the normal impulse reaches the contractile cells.

Reentry circuits 

Third mechanism for arrhythmias is reentry or movement of an impulse in a circus movement. If an impulse has 2 paths through which it can pass and these conducting paths differe electrically such that one is fast conducting path and th eother is slow conducting path, in that case of renetry of impulse may occur. Reeentry may occur due to an anatomical defect (anatomical reentry) or may be due to functional difference in the electrical propery of the cells (functional reentry).

Wolff-Parkinson syndrome is a type where arrhthmias occur due to anatomical reentry.