The heart rate is the number of times the heart beats per unit of time. There are several abnormalities of this rate and rhythm. 

Bradycardia

This is a term that describes different conditions in which the heart beats at an unusually slow rate.

When the heart beats normally the impulse for the beat originates at the sinoatrial node (SA node). In patients with bradycardia the impulse for the beats are sent by the SA node at a slow rate.

The heart rate could also be slowed if the impulses are delayed while they travel through the conduction system of the heart causing it to beat.

• Sinus bradycardia is a term given to unusually slow heartbeat that occurs in normal persons especially in athletes or during a state of deep relaxation. This can also be seen in response to medications like beta blockers.
• Bradycardia caused due to impairment of the electrical conduction system of the heart may cause heart block. Atrioventricular block or AV block occurs when electrical impulses are impeded as they travel from the atrium to the ventricles via the AV node (atrioventricular nodes). Heart block may be first–degree heart block or second–degree heart block. Third–degree heart block indicates a complete heart block that occurs when no electrical impulses reach the ventricles from the atrium.
• Sick sinus syndrome is a condition where the heart’s pacemaker, the SA node, fails and this produces an irregular heartbeat. Patients with sick sinus syndrome may have both a slow and a rapid heart rate.
• Many types of bradycardia do not cause any symptoms. Some however may cause dizziness, weakness or fainting (called syncope).
• Abnormal rate and rhythm of the heart is usually diagnosed using an electrocardiogram. A Holter monitor is a 24 hour monitoring of the ECG that shows changes in the heart rhythm over the course of a 24-hour period.

Tachycardias

Tachycardias indicate an abnormally raised heart rate even at rest. A heart rate of over 100 beats per minute at rest indicates tachycardias.

• Supraventricular tachycardia – this may occur in young and healthy persons. In this an electrical impulse does not fade out even after completion of the heart beat and tends to over stimulate the heart muscles by continuing to move in a rapid circle within the conduction system. This could occur due to an extra electrical pathway that forms a short circuit within the heart’s conduction system.
• Atrial fibrillation is one of the common types of tachyarrythmias. This occurs in the atrium of the heart. In a normal heart the impulse originates in the SA node, whereas in atrial fibrillation, many electrical impulses are fired at a rapid rate randomly throughout the atria down to the ventricles. This results in very rapid and irregular heart rates.
• Atrial flutter – this is a condition where the electrical impulses fire rapidly but it results in regular and organized rhythm.
• Ventricular tachycardia – this is a condition where electrical impulses in the heart arise from the ventricles instead of the SA node.
• Ventricular  fibrillation results when electrical impulses are fired from many sites in the ventricle that leads the heart to quiver instead of pumping blood. Left untreated it may result in death.