Is first degree heart block dangerous?

Is first degree heart block dangerous?

First degree heart block is an abnormality in the electrical conduction of the heart noted on the ECG, the recording of the electrical activity of the heart. It is not a block in the blood vessels of the heart which we are more familiar with. The waves on an ECG are P, QRS complex and the T wave. Normal interval between the onset of the P wave and the onset of QRS complex is up to one fifth of a second (0.2 seconds). When this interval is increased, it is called first degree heart block or first degree atrioventricular or AV block to be specific. Atria are the upper chambers of the heart and ventricles the lower chambers.

This interval is known as the PR interval. PR interval is the time taken for the electrical activity to travel from the upper chambers to the lower chambers. Most of it is due to a delay produced in the atrioventricular or AV node at the junction between the upper and lower chambers. The delay allows the complete emptying of the upper chambers before the lower chambers start contracting. This gives a booster filling for the lower chambers apart from the passive filling which occurs when the lower chambers relax after a contraction.

Isolated first degree AV block or prolongation of the PR interval is not significant if it is only moderate prolongation. Significance is mainly for any associated abnormality of the heart which may progress. First degree AV block can occur in healthy athletes due to the enhanced activity of the vagus nerve arising from the lower part of the brain. But this does not produce any functional abnormality. First degree AV block can sometimes be a manifestation of diseases like rheumatic fever, a disease which affects predominantly the joints and the valves of the heart. First degree heart block can also occur in a heart attack involving the lower part of the heart known as inferior wall myocardial infarction.

Other causes are inflammation of the heart muscle known as myocarditis, Lyme disease caused by a bacterial infection, and some medications which slow the electrical conduction of the heart, typically given to control heart rhythm abnormalities. Sometimes it can be due to age related degeneration in the electrical conduction system of the heart. Calcium deposits in the aortic valve between the left ventricle, the lower left chamber and the aorta which receives the blood pumped out, can spread to the conduction system and produce first degree AV block. Any of these cases of first degree AV block can progress to second degree or third degree (complete heart block).

Complete heart block or third degree AV block is a dangerous condition in which the electrical signals from the upper chambers do not reach the lower chambers. The lower chambers then beat at a much slower rate of its own. This can lead to dizziness and loss of consciousness and sometimes even death if the lower chambers stop beating altogether. Complete heart block needs a temporary or permanent pacemaker depending on whether disease causing it is reversible or not. When it is caused by a heart attack involving the lower wall of the left ventricle it is usually reversible and needs only a temporary pacemaker.

Pacemaker is an electronic device which gives regular electrical pulses to the heart when its own electrical system is defective. Temporary pacemaker is kept outside the body and connected to the heart by lead wires introduced through the veins. Permanent pacemaker is implanted under the skin in the upper part of the chest and connected to the heart using leads introduced through the veins. There are also leadless pacemakers which are implanted directly in the right ventricle, the right lower chamber of the heart, through the vein in the groin using a delivery device.

First degree AV usually does not need any treatment by itself. There can be associated conditions which need treatment. Close observation is needed if first degree AV block is associated with other conditions as it can progress to complete heart block in that case. First degree AV block in older persons due to degeneration of the conduction system or aortic valve can also progress. In a very rare instance, infection of the aortic valve causing first degree AV block can progress rapidly to complete heart block and need early surgery. Aortic valve is situated very near the conduction system of the heart.