A cardiac arrhythmia, also called cardiac dysrhythmia, is a disturbance in the regular rhythm of the heartbeat. Several forms of cardiac arrhythmia are life-threatening and considered medical emergencies.

In order to diagnose the type of arrhythmia present an electrocardiogram (abbreviated ECG or EKG) should be ordered.

Types of arrhythmias

Premature complexes

Premature complexes. Such complexes represent the most common interruption of normal sinus rhythm, most frequently arising from the ventricles and less often from the atria and the AV node.

Bradycardia (brady-arrhythmias)

In bradycardia the heart beat is less than 60 beats/min. Brady-arrhythmias usually do not pose a diagnostic dilemma and there are relatively few treatment options (atropine, pacing).

Tachycardia (tachy-arrhythmias)

In tachycardia the heart beat is more than 100 beats/min. Tachy-arrhythmias are usually not life threatening if short in duration.

Though the underlying mechanism of the tachycardia critically determines both prognosis and therapy, initial investigation may allow only for characterization of the tachycardia as either narrow complex (QRS duration <120 ms) or wide complex (QRS duration >120 ms) as read in the EKG.

I. Narrow complex tachycardia (QRS < 120 ms)

Narrow complex tachy-arrhythmias originate from impulses in the atrium and thus do not affect the width of the QRS wave, hence the name narrow (QRS < 120 ms by EKG). It can be further classified according to its rhythm as either regular or irregular.

a. Regular narrow complex tachycardia

Sinus tachycardia
Paroxysmal supraventricular tachycardia: These are paroxysmal; i.e., characterized by an abrupt onset and abrupt termination. They are caused by an accessory pathway in the conduction system between atria and ventricles. This maybe dual AV node (AVNRT) which is present in many people or an accessory pathway (AVRT e.g. Wolf-Parkinson-White syndrome).
- AV nodal re-entrant tachycardia (AVNRT): This is initiated by an ectopic atrial impulse that travels down a dual AV node pathway.
- Atrio-ventricular reciprocating tachycardia (AVRT): Here the ectopic impulse bypasses the AV node into the ventricle via an accessory pathway. These may be orthodromic, which are retrograde and present with a paroxysmal "narrow complex" tachycardia. Or they may be antidromic, which are antegrade and present as a "wide complex" tachycardia). Wolf-Parkinson-White syndrome is an example of AVRT.
Atrial tachycardia: P wave rate < 250/min. Other characteristics include a long RP interval.
Atrial flutter: Characterized by very rapid P waves known as flutter waves. In the most common form the P waves are twice as fast as the ventricular rate (i.e. only half the impulses from the atrium are being conducted to the ventricle, also known as a 2:1 conduction

b. Irregular narrow complex tachycardia

Atrial fibrillation (AF)
Multifocal atrial tachycardia (MAT)
Atrial flutter with variable heart block
Frequent premature atrial complexes (PACs)

II. Wide complex tachycardia (QRS > 120 ms)

Wide complex tachy-arrhythmias originate from impulses in the ventricles and hence affect the width of the QRS wave, hence the name wide (QRS > 120 ms by EKG). It can be further classified according to its rhythm as either regular or irregular.

a. Regular wide complex tachycardia