Arrhythmia are abnormal heart rhythms, usually cause by a problem with the way that electrical signals are conducted through the heart.

Your heart beats as a result of the generation and conduction of electrical impulses. These impulses cause the heart muscle to contract. The continuous cycle of contraction followed by relaxation causes blood to be pumped around your body.

The speed and force of your hearts contractions are influenced by exercise, temperature, and hormonal activity.

First, the sinoatrial (SA) node (also referred to as the pacemaker of the heart) fires, generating impulses that travel throughout the muscular atrial wall. This causes both the right and left atria to contract. The SA node is located in the upper wall of the right atrium. It is made of nodal tissue that behaves like both muscle and nervous tissue.

Next the atrioventricular (AV) node, that is located near the bottom of the right atrium, is triggered by the impulse from the SA node. The first role of the AV node is to delay the signal for about a tenth of a second. This delay allows the atria to contract and push blood into the ventricles. The AV node then sends an impulse to the Ventricles.

The impulse is sent through the atrioventricular bundle. This bundle of nerve fibres then splits into the right and left bundle branches. These carry the impulse down the center of the heart and then to the left and right ventricles.

At the base of the heart the bundle branches start to divide further into Purkinje fibres.

When the impulse reach these fibres they trigger the muscle fibres in the ventricles to contract. The right ventricle sends blood to the lungs through the pulmonary artery. The left ventricle pumps blood to the aorta.

The Cardiac Cycle is the sequence of events that occurs every time the heart beats. The diastole phase of the cardiac cycle is when the atria and ventricles are relaxed and blood flows freely into the heart. During the systole phase, the ventricles contract sending blood to the lungs and the rest of the body.

Disorders of the heart's conduction system can cause problems by either blocking, diverting, slowing or accelerating the impulses. Some conditions can cause no symptoms, others can cause palpitations, pain, lightheadedness or shortness of breath and others can be life threatening.

Arrhythmia may be classified by rate (fast=tachycardia, slow=bradycardia), mechanism (spontaneous, re-entry, triggered) or duration (isolated, couplets, runs, non-sustained, sustained).

It is also appropriate to classify by origin site:

Sinus bradycardia
Premature atrial contractions (PACs)
Wandering atrial pacemaker
Atrial tachycardia
Multifocal atrial tachycardia
Supraventricular tachycardia (SVT)
Atrial flutter
Atrial fibrillation (Afib)
Junctional arrhythmias
AV nodal reentrant tachycardia
Junctional rhythm
Junctional tachycardia
Premature junctional contraction

Premature ventricular contractions (PVCs), sometimes called ventricular extra beats (VEBs)
Premature ventricular beats occurring after every normal beat are termed "ventricular bigeminy"
PVCs that occur at intervals of 2 normal beats to 1 PVC are termed "PVCs in trigeminy"
Three premature ventricular grouped together is termed a "run of PVCs" in general, runs lasting longer than three beats are referred to as ventricular tachycardia
Accelerated idioventricular rhythm
Monomorphic ventricular tachycardia
Polymorphic ventricular tachycardia
Ventricular fibrillation
Torsades de pointes

Heart blocks
also known as AV blocks, because the vast majority of them arise from pathology at the atrioventricular node. They commonly cause bradycardia:

First degree heart block, (long PR interval)
Second degree heart block
Type 1 Second degree heart block, also known as Mobitz I or Wenckebach
Type 2 Second degree heart block, also known as Mobitz II
Third degree heart block, also known as complete heart block.

Sudden arrhythmic death syndrome
Sudden arrhythmic death syndrome (SADS), is a term used as part of sudden unexpected death syndrome to describe sudden death due to cardiac arrest brought on by an arrhythmia.

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