How does the pacemaker work?

Karthik Gurumurthy

Thanks to the research of  physician/scientist Luigi Galvani that we learned about how the muscle responds to electrical stimulation.

The fibers of the middle muscular layer of the heart wall (also known as myocardium) are excitable and able to conduct impulses from one fiber to another. The speed of the impulses received, hence the rhythm of the heart, depends to some extent on the involuntary nervous system. 

Both the atrial and the ventricular myocardia have this quality, but the rate of the former’s contractions is actually somewhat faster than the rate of the latter’s. To initiate and maintain one steady rhythm, the heart has its own natural pacemaker; the sinoatrial node. Lying in the right atrium at the recess of the vena cava superior, this node beats the fastest of all myocardial muscles.

If the sinoatrial node fails, the atrioventricular node takes over, causing the heart to beat about 50 to 60 times per minute rather than 70 to 80.

Issues such as ventricular arrest, atrioventricular block  may cause an insufficient or irregular heartbeat, which physicians are able to correct with an artificial pacemaker. A pacemaker can start a ventricular contraction by transmitting evenly spaced repetitive current pulses to the heart from an outside electrical source.

Different types of pacemakers are available to correct different problems, but basically the machine consists of electrodes, conducting wires, and a pulse generator. The electrodes, made of gold or platinum, are good conductors. They may be unipolar or bipolar, but the latter are more efficient.

If the electrodes are in direct contact with the heart, less voltage is required than if they do not actually touch the cardiac tissue.

The conducting wires are made up of stainless steel coils. The generator controls the rate and amplitude of impulses, which may be fixed or may change according to the needs of the heart. If the heart is weak, for example, but needs stimulus only from time to time, the pacemaker shuts off when the heart is working well so as not to compete or interfere with it. This is the high level or the gist of how pace makers work.