Author's note - yes, I know the image links are broken! Sorry about that, I will fix them when I get the time...

An abnormality of the cardiac rhythm is called a cardiac arrhythmia – of which there are two main types:
Bradycardia, where the heart rate is slow (<60 b.p.m.)
Tachycardia, where the heart rate is fast (>100 b.p.m.)


  Sinus denotes that the rhythm of the heart is still being generated by the sinuatrial node, so the P-wave and QRS complex are generally normal.

  Sinus Bradycardia:Slower than 60bpm, otherwise normal.
Causes: Hypothermia, hypothyroidism, drugs ( b -blokers, digitalis and other antiarrhythmic drugs), acute ischaemia and infarction of the sinus node.

Sinus tachycardia: Faster than 60bpm, otherwise normal.
Causes: Fever, exercise, emotion, pregnancy, anaemia, thyrotoxicosis.

  Sinus arrhythmia: During inspiration, parasympathetic tone falls and the heart rate quickens, and on expiration the heart rate falls. This variation is normal, particularly in children and young adults. There is still one P wave per QRS complex and a constant P-R interval, but a progressive beat to beat change in R-R.


  Atrioventricular Blocks

These are problems in conducting between the Sinuatrial node and the Atrioventricular node.

  First Degree AV Block: Where the PR interval > 0.22 sec but there is still one P-wave per QRS.


Second degree AV Block: The Mobitz blocks are types of Second degreee AV block
Mobitz I Block (Wenckebach phenomenon):
Here there is progressive PR interval prolongation until a P wave fails to conduct.

Mobitz II Block: This is a special type of second degree block that occurs when an absent QRS complex is not preceded by progressive PR interval prolongation - i.e. PR remains constant. Look for a P-wave not followed by a QRS.

2:1 or 3:1 Block: Occurs when every second or third P wave conducts to the ventricles. P-R interval remains normal in the conducted beats.

Third degree (complete) AV Block: Occurs when no P waves conduct to the ventricles. There will be no relationship between P-wave (atrial) rate and QRS complex (ventricular) rate. QRS complexes tend to be abnormally shaped due to abnormal spreading of depolarisation across the ventricles.


  Atrial tachyarrhythmias

Here, ‘atrial' denotes that the electrical activity begins in the atria (i.e with a P-wave).

Atrial ectopic beats (extrasystoles): Appears as early and abnormal P waves, followed by normal QRS complexes.

Atrial tachycardia: High rate of atrial depolarisation (e.g. around 150/min). P-waves can be seen superimposed ontop of T-waves.

Atrial Flutter: Atrial rate around 300 b.p.m. regular sawtooth-like P-waves (known as F waves) between QRS complexes (the ventricular rate becomes independent of the atrial rate and remains much slower).

Atrial Fibrillation: Continuous rapid >400 b.p.m. activation of atria, irregular QRS complexes.

Junctional (Nodal) Tachyarrhythmias

Here, the depolarisation originates from around the AV node.

  Junctional ectopic beats (Extrasystoles): Appears as early normal QRS complex, without a preceeding P wave, followed by a compensatory pause.

Junctional (nodal) tachycardia: No visible P waves, normal QRS complexes, rhythm is rapid 140-280 b.p.m.

Ventricular Tachyarrythmias

Note that the depolarisation originates in the ventricles here. Consequently, the QRS is much wider, as it takes longer for the electrical activity to spread.

Ventricular ectopic beats (extrasystoles): No P wave, broad (>0.12 sec) QRS complex

Ventricular tachycardia: Rapid ventricular rhythm with broad (>0.14 sec) QRS complexes at a rate of 120 b.p.m. or more

Ventricular Fibrillation: Very rapid and irregular ventricular activation with no mechanical effect.