In an adult the heart beats between 60-100 bpm. Each heart beat is initiated by an electric impulse arising spontaneously from the normal pacemaker of the heart called the sinoatrial node (SA node) . The SA node is located at the upper part of the right atrium where the superior vena cava enters it. For a normal heart beat to occur the electric impulse starts at the SA node , travels through specialized conduction tissue to the atrioventricular node (AV node). On the way it activates the right and left atrium. The AV node is situated at the junction of the atria and the ventricles. The AV node regulates the conduction of electrical impulses in to the ventricles. From the AV node the electrical impulse traverses the bundle of His, then the left and right bundle and through the purkinje fibers stimulates the myocardium (heart muscle).
On an average the heart beats 100.000 times a day, pumping almost 7500 liters of blood through its chambers to the rest of the body and then back to the heart. Over 70 years that adds up to more than 2.5 billion heart beats. Abnormalities in the heart could result in a slow heart rate (bradycardia) or in a fast heart rate (tachycardia). They are collectively called cardiac arrhythmias. Today most arrhythmias are treatable. This was not the case 50 years ago. Over the last 50 years we have been able to understand arrhythmias better and newer technologies have given us life saving treatments for many of these life threatening conditions. Resuscitation a patient from cardiac arrest due to an abnormally fast heart rhythm simply and effectively with an electrical shock using an external defibrillator was one of the most significant advances.
The SA node and AV node can malfunction and result in an abnormally slow heart rate. At other times any part of the heart can take over as the heart’s pacemaker giving rise to an abnormal heart rate.
Common types of arrhythmias:
A common arrhythmia, affects a large number of people. As an isolated abnormality they are generally benign and asymptomatic. Occasionally patients may complain that they feel the heart has stopped beating momentarily. Caffeine and stress increase the occurrence of premature beats.
Atrial fibrillation – incidence increases with age. Develops when a disturbance in the electrical signals causes the individual muscle fibers in the two atria to contract haphazardly and not in unison. As a result not all blood is forced out of these chambers and the blood that remains there could form clots. At times these clots could get dislodged travel through the body and block blood vessels in the brain and result in a stroke.
Bradycardia – a slow heart beat or bradycardia causes a person to feel fatigued, dizzy and lightheaded and may result in fainting spells.
