Sinus rhythms start at the sinus node and spread normally throughout the heart via the atria, AV node, Bundle of His, bundle branches and Purkinje fibres.

Sinus rhythms feature normal P waves, PR intervals and QRS complexes.

Normal sinus rhythm has a rate of 60-100 beats per minute in adults.

Sinus tachycardia has a rate above 100 beats per minute in adults.

Sinus bradycardia has a rate below 60 beats per minute in adults.

A sinus arrhythmia appears the same as a normal sinus rhythm except its rate gets gradually faster then slower with each breath.

The more uncommon variations on sinus tachycardia can help to illustrate the key differences between re-entry and automaticity in arrhythmias.

An inappropriate sinus tachycardia may have a gradual onset and offset, and the baseline rate may be higher than normal. Its proposed mechanisms include increased automaticity.

A sinus node re-entrant tachycardia will have a sudden onset because it has a re-entry circuit.

Two common types of arrhythmias include fibrillation and flutter.

Fibrillation is chaotic, with the affected part of the heart all trying to depolarise from many different origins. This results in a quivering part of the heart. Fibrillation can either affect the atria or the ventricles.

• Atrial Fibrillation is irregular with absent P waves. It is the most common sustained arrhythmia

Flutter is a bit more organised than fibrillation, with regular flutter waves spreading out from the affected part of the heart. Flutter waves are usually very fast at about 300 beats / min.

• Atrial Flutter has regular flutter waves instead of P waves, which can make the baseline look like a sawtooth. Because the AV node normally can't conduct at this extremely fast rate, it usually only lets some of these impulses through to the ventricles.

Atrial fibrillation can be described as coarse or fine depending on the size of the fibrillation waves (e.g. coarse if height ≥1 mm), but this does not have any particular clinical significance.

Atrial fibrillation can also be described by the ventricular rate:

• A fast ventricular rate can be described as AF with Rapid Ventricular Rate (RVR). Sometimes this is called Fast AF, but the distinction should be made that the rate of fibrillation in the atria is the same, only the ventricular response is faster.
• A slow ventricular rate can be described as Slow AF
• A normal ventricular rate can be described as Rate Controlled AF

Another uncommon variation of AF is Regularised AF, which features absent P waves and a fibrillating baseline but regular QRS complexes. It occurs when there is a complete conduction block at the AV node so the fibrillation impulses do not reach the ventricles, and either the AV junction or ventricles themselves take over with an escape rhythm. It is classically associated with digoxin toxicity.

Atrial flutter can be described by the ratio of flutter waves to ventricular QRS complexes. The most common is 2:1 atrial flutter, where the atrial rate is ~300 bpm and the ventricular rate is ~150 bpm. Other ratios include 3:1, 4:1 or even higher grades of block, which can make for a slow ventricular response in the extreme. The AV node is usually unable to conduct fast enough to permit 1:1 conduction, so 1:1 atrial flutter is rare. If there is a variable block, the ratio of flutter waves: QRS complexes will vary and the QRS complexes will be irregularly spaced apart.

Ectopic atrial rhythms originate outside of the SA node, somewhere else within the atria. Atrial rhythms usually have P waves that are a different shape to normal, or multiple different shapes. The PR interval may be normal or shortened, depending on how close the ectopic focus is to the AV node.

An Atrial Escape rhythm occurs when the SA node fails, e.g. after a long pause. It will have a rate >60 bpm and regular but abnormally shaped P waves.

Atrial tachycardia can be focal or multifocal. If the P waves are all the same and the QRS complexes are regularly spaced apart, it is likely a Focal Atrial Tachycardia. If there are 3 or more different P wave shapes it can be called a Multifocal Atrial Tachycardia.

If the rhythm has 3 or more different P wave shapes but is not a tachycardia, it can be called a Wandering Atrial Pacemaker.

A Coronary sinus rhythm is an unusual variation where the P waves are negative in II, III and aVF and the PR interval is normal.

A SupraVentricular Tachycardia (SVT) can technically be any rhythm that originates above the ventricles, including sinus, atrial or junctional rhythms. Commonly the term SVT is used to represent a specific type of junctional re-entry rhythm known as Atrioventricular Node Re-entrant Tachycardia (AVNRT). In people with abnormal accessory conduction pathways another form of SVT is Atrioventricular Re-entry Tachycardia (AVRT).

AVNRT variations are described by their RP interval, or the time difference between speeds of the R wave and retrograde P wave.

Most AVNRT is Slow-Fast, which means that the ventricles are slower to activate than the atria. This will produce a retrograde P wave that is either hidden in the QRS or is just at the end of the QRS (also known as a pseudo R' or S wave).

In Fast-Slow AVNRT, the atria take longer to activate so there is a longer interval between the start of the R wave and the start of the retrograde P wave. The P wave is more likely to be visible after the QRS instead of being hidden within it.

Occasionally there may be Slow-Slow AVNRT, where the P wave and QRS are both delayed. This can cause a retrograde P wave to be visible before the QRS complex.

AVRT variations depend on the direction of conduction through the AV node.

Orthodromic AVRT is conducted anterograde through the AV node. The QRS complexes can be narrow.

Antidromic AVRT is conducted retrograde through the AV node. The QRS complexes will be wide.

Junctional ectopic rhythms are classified based on their rate.

A Junctional Escape rhythm is 40-60 bpm.

A Junctional Bradycardia is < 40 bpm.

An Accelerated Junctional Rhythm is 60-100 bpm.

A Junctional Tachycardia rhythm is >100 bpm.

Two common types of arrhythmias include fibrillation and flutter.

Fibrillation is chaotic, with the affected part of the heart all trying to depolarise from many different origins. Ventricular Fibrillation is totally chaotic.

Flutter is a bit more organised than fibrillation, with regular flutter waves spreading out from the affected part of the heart. Flutter waves are usually very fast at about 300 beats / min.

Ventricular Flutter has giant sawtooth beats but is rare to see, as it usually denegerates into fibrillation quite quickly.

Ventricular escape is a wide complex rhythm at 20-40 bpm. It is also known as an Idioventricular rhythm.

An Accelerated Idioventricular Rhythm (AIVR) has a rate of 50-120 bpm.

Ventricular tachycardia has a rate > 120 bpm.

Types of Ventricular Tachycardia depend on the QRS morphology. If they are all the same, it can be called Monomorphic VT. If they are many different shapes changing from beat to beat, it can be called Polymorphic VT. If there are multiple QRS shapes occurring in runs of each (not changing beat to beat) it can be named Pleomorphic VT. A special variation is Bidirectional VT, which is classically associated with digoxin toxicity.

VT can also be classified by its duration. Sustained VT lasts >30 sec. Nonsustained VT is at least 3 beats in duration but <30 sec.

Idiopathic VT occurs in the absence of any structural heart disease. Most Idiopathic VT originates in the right ventricle, e.g. Right Ventricular Outflow Tract Tachycardia (RVOT) , which is a variation of VT with a LBBB morphology and an inferior or right axis deviation.

Left ventricular idiopathic VT is most commonly caused by Fascicular VT, which usually has a RBBB pattern and left axis deviation.

Premature beats arrive sooner than the next beat is expected.

Premature atrial complexes (PACs) may have an abnormal P wave shape because they start outside of the SA node. The QRS can be narrow.

Premature Junctional Complexes (PJCs) have a narrow QRS and either no P wave or a retrograde P wave that occurs after the QRS.

Premature Ventricular Complexes (PVCs) have a wide QRS.

Escape beats commence later than expected. 3 or more escape beats in a row can be called an escape rhythm.

Atrial escape beats or rhythms can have abnormally shaped P waves and a narrow QRS.

Junctional escape beats or rhythms can have hidden or retrograde P waves and a narrow QRS.

Ventricular escape beats or rhythms will be wide.

Extra beats can occur in repeating patterns.

Bigeminy is a rhythm where every second beat is ectopic. If the ectopics are atrial in origin, it can be called Atrial Bigeminy, or Ventricular Bigeminy if they are ventricular in origin.

Similarly, Trigeminy has an ectopic every third beat, and Quadrigeminy has an ectopic every 4th beat.

If the ectopic beats are of varying shapes, they can be called Multifocal ectopics.