Shocks That Save The Heart 

Why a heart loses its rhythms and 
how the rhythms are restored, reports Biplab Das 

Cardiac arrhythmia is a condition in which the heartbeat loses its rhythm. An arrhythmic heart may beat too fast, too slow, skip a beat or have an extra beat. This affects the supply of blood to the body's organs, including the heart itself. In extreme cases death may occur due to heart failure. 

Dr. David Christini, a biomedical engineer from the Weill Medical College of Cornell University discussed the physics behind the rhythms of the heart, in his lecture on 'Physics of Cardiac Arrhythmia and its Relevance to Arrhythmia Prevention and Control' at Bethune College on January 21. The lecture was organised by Bethune College in association with the Calcutta chapter of the Indian Physics Association. 

Christini began with a brief account of heart functions. The heart is a muscular pump divided into four chambers - two atria located on the top and two ventricles located at the bottom. Each heartbeat starts in the right atrium. There, a specialised group of cells known as the sinoatrial node (SA node) sends electrical signals. The signal spreads throughout the atria to create an impulse in the lower chambers, or ventricles. To reach the ventricles, the impulse has to cross an insulated region called the atrioventricular node (AV node). As the electrical signal travels through the heart, it contracts. First the atria contract, sending blood to the ventricles. After a few seconds the ventricles contract, pumping blood throughout the body. 

"It is a simple movement of sodium and potassium ions that generates the initial impulse in the SA node," said Christini. Simultaneous influx of sodium ions and outflow of potassium ions from the cell generate a potential difference across the cell membrane and create the impulse at the SA node, he explained. 

Any alteration in this normal process may lead to arrhythmia. When the insulated layer of the AV node becomes non-functional, cardiac impulses rush to the ventricles, breaking the normal rhythm. This is electrical arrhythmia, a sort of electrical short circuit, said Christini. 

Arrhythmia may be caused by formation of scar tissue inside blood vessels. Christini compared scar tissues with boulders placed in a river. "Just as boulders obstruct the flow of water, scar tissue changes the direction and rate of blood flow in a blood vessel," said the speaker. Scar tissue even causes backflow of blood and breaks the rhythm. In this condition the heart rate becomes faster. It goes to 200 beats a second from the normal 60 to 80 beats. 

In the second part of his lecture, he discussed the role of physics in treating arrhythmia. Pacemakers are implanted in the patient's body to administer brief and mild electric shocks. The shock keeps the heart beating normally. 

Christini described a procedure in which a catheter with four or five electrical points is inserted into the femoral vein in the thigh. Through the veinous system it reaches the right ventricle, where it synchronises the electrical impulse. 

The defibrillator is a lifesaving device used in emergency rooms and intensive care rooms. In fibrillation, the conduction system goes absolutely haywire, the heart muscle contracts without any coordination and it is incapable of pumping blood. The defibrillator administers a massive shock directly through the chest wall, for a fraction of second. The shock stops the heart completely, and when it resumes beating, it is in the normal rhythm. 

Christini added that before arrhythmia occurs, the heart generates large waves interspersed with short waves. This abnormal pattern can be detected by computer programs. According to him, the development of physics has contributed a lot towards reducing sudden death from cardiac arrhythmia.