What Are Cardiac Arrhythmias and How Are They Classified?

TL;DR

Cardiac arrhythmias are abnormalities in the heart’s rate, rhythm, sequence of conduction, or origin of conduction, classified primarily as tachyarrhythmias or bradyarrhythmias. The SA node normally sets a rate of 60–100 beats per minute; faster rhythms are further divided into supraventricular and ventricular types, while slower rhythms include sinus bradycardia and AV blocks. Read on to distinguish the major mechanisms and named rhythm disorders.

Transcript

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Key Insights

• An arrhythmia is an abnormality within rate, rhythm, sequence of conduction, or origin of conduction in the heart. The simpler working definition is abnormal electrical activity within the heart, but the four-part version captures cases where rate alone appears normal.
• The normal heart rate is set by the sinoatrial node and falls between 60 and 100 beats per minute. Anything above 100 beats per minute is a tachyarrhythmia and anything below 60 beats per minute is a bradyarrhythmia, so the normal band defines both abnormal categories.
• Supraventricular tachyarrhythmias are any tachyarrhythmias originating above the ventricles. They include sinus tachycardia from the SA node firing too fast, the atrial group of focal atrial tachycardia, multifocal atrial tachycardia, atrial fibrillation and atrial flutter, plus AVNRT and AVRT.
• Focal and multifocal atrial tachycardia differ only in the number of ectopic foci. One ectopic focus firing consistently in the atria produces focal atrial tachycardia, while roughly three abnormally firing ectopic areas produce multifocal atrial tachycardia, and many foci firing very fast produce atrial fibrillation.
• Atrial flutter is a supraventricular tachyarrhythmia occurring near the tricuspid valve that creates a reentrant circuit. It is grouped with the atrial tachyarrhythmias rather than with the nodal reentry rhythms, because its circuit sits in atrial tissue rather than within the AV node itself.
• PSVT, or paroxysmal supraventricular tachycardia, is a name given primarily to AVNRT and AVRT. AVNRT involves a reentrant circuit within the AV node, while AVRT involves a loop through an accessory pathway between the atria and the ventricles. Focal atrial tachycardia is sometimes included too.
• Monomorphic ventricular tachycardia comes from one irritable ventricular focus and is the most common type of V-tach. Polymorphic V-tach comes from multiple irritable ventricular areas and splits into a normal-QT form and a prolonged-QT form named torsades de pointes, meaning twisting of the points.
• Heart blocks are numbered in order of severity within the AV node. First degree is comparatively mild, second degree splits into Mobitz I (also called Wenckebach, featuring a dropped QRS) and Mobitz II, and third degree heart block can be very deadly if not treated quickly.

Questions & Answers

Q: What is a cardiac arrhythmia?

A cardiac arrhythmia is abnormal electrical activity within the heart. More fully, it is an abnormality in heart rate, rhythm, sequence of conduction, or origin of conduction.

Q: How are cardiac arrhythmias classified by heart rate?

The SA node normally sets the heart rate between 60 and 100 beats per minute. A rate above 100 beats per minute is classified as a tachyarrhythmia, while a rate below 60 beats per minute is classified as a bradyarrhythmia.

Q: What is the difference between supraventricular and ventricular tachyarrhythmias?

Supraventricular tachyarrhythmias originate above the ventricles and include sinus tachycardia, atrial tachyarrhythmias, AVNRT, and AVRT. Ventricular tachyarrhythmias arise from irritable areas within the ventricles and include ventricular tachycardia and ventricular fibrillation.

Q: How do focal atrial tachycardia, multifocal atrial tachycardia, and atrial fibrillation differ?

Focal atrial tachycardia comes from one ectopic atrial focus, while multifocal atrial tachycardia comes from multiple abnormal foci, such as three ectopic areas. Atrial fibrillation involves many ectopic areas firing even faster.

Q: What causes atrial flutter?

Atrial flutter is a supraventricular tachyarrhythmia caused by a reentrant circuit near the tricuspid valve. It is grouped with atrial tachyarrhythmias because the circuit occurs in atrial tissue.

Q: What is PSVT, and which arrhythmias does it include?

PSVT stands for paroxysmal supraventricular tachycardia and primarily refers to AVNRT and AVRT, although focal atrial tachycardia is sometimes included. AVNRT uses a reentrant circuit within the AV node, whereas AVRT uses an accessory pathway between the atria and ventricles.

Q: What is the difference between monomorphic and polymorphic ventricular tachycardia?

Monomorphic ventricular tachycardia arises from one irritable ventricular focus and is described as the most common type of ventricular tachycardia. Polymorphic ventricular tachycardia arises from multiple ventricular foci and can occur with either a normal or prolonged QT interval; the prolonged-QT form is called torsades de pointes.

Q: What are the main types of bradyarrhythmia and heart block?

Bradyarrhythmias include sinus bradycardia from an SA node firing too slowly and AV conduction blocks that disrupt electrical communication between the atria and ventricles. AV blocks are classified as first, second, or third degree; second-degree block divides into Mobitz I, also called Wenckebach, and Mobitz II, while untreated third-degree block is described as the most deadly.

Summary & Key Takeaways

• An arrhythmia is defined simply as abnormal electrical activity within the heart, but the fuller definition covers any abnormality in rate, rhythm, sequence of conduction, or origin of conduction. Rate is the easiest entry point: the SA node normally fires at 60 to 100 beats per minute, so faster is tachyarrhythmia and slower is bradyarrhythmia, with fibrillation reaching close to 350 beats per minute or more.

• Tachyarrhythmias divide by where they originate. Anything arising above the ventricles is supraventricular, including sinus tachycardia from a fast-firing SA node, atrial tachyarrhythmias such as focal atrial tachycardia, multifocal atrial tachycardia, and atrial fibrillation, plus atrial flutter from a reentrant circuit near the tricuspid valve, and the nodal reentry rhythms AVNRT and AVRT grouped under PSVT.

• Ventricular tachyarrhythmias arise from irritable areas inside the ventricles. Monomorphic ventricular tachycardia comes from a single irritable focus and is the most common type, while polymorphic V-tach comes from multiple foci and exists in normal-QT and prolonged-QT forms, the latter named torsades de pointes. Ventricular fibrillation involves multiple ectopic foci plus reentrant circuits and is described as a very dangerous rhythm.

• Bradyarrhythmias arise either from an SA node firing too slowly, producing sinus bradycardia, or from AV conduction blocks that break the electrical connection between atria and ventricles. Heart blocks are numbered first, second, and third degree, with second degree splitting into Mobitz I (Wenckebach, with a dropped QRS) and Mobitz II. Third degree block is the most deadly if not treated quickly.

• Sick sinus syndrome mixes fast and slow rhythms. Underlying SA node dysfunction, from causes such as fibrosis of the SA node or myocardial scarring, slows firing and produces sinus bradycardia. Because the SA node is dysfunctional, another atrial area compensates by taking over, and these patients develop an associated supraventricular tachycardia such as atrial flutter or atrial fibrillation.