Is first degree heart block dangerous? Cardiology Basics

Is first degree heart block dangerous? Cardiology Basics

PR interval is measured from the onset of P wave to the onset of QRS complex. It includes the P wave and the PR segment. Normal PR interval is from 120 to 200 ms. If it is prolonged, it is called as first degree atrioventricular (AV) block.

PR interval is the time taken for activation of the atria and conduction of the impulses down to the ventricles and initiate activation of the ventricles. This includes the physiological delay in conduction at the AV node. The delay allows complete emptying of the atria before the ventricles start contracting. The atrial help to ventricular filling acts like an atrial booster at the end of passive diastolic ventricular filling.

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. But this does not produce any functional abnormality.

First degree AV block can sometimes be a manifestation of rheumatic fever. First degree heart block can also occur in inferior wall myocardial infarction. In inferior wall infarction, it can progress to second degree or third degree AV block. Third degree AV block is also known as complete AV block and may need installation of a temporary pacemaker to support in case of undue bradycardia.

Other causes are myocarditis, Lyme disease – a spirochaetal infection, and some medications which slow the electrical conduction of the heart, typically antiarrhythmic agents and digoxin. Sometimes it can be due to age related degeneration in the conduction system of the heart. Some of these can sometimes progress to complete heart block and necessitate temporary or permanent pacing.

Calcification of the aortic valve in degenerative aortic valve disease can spread to the conduction system and produce first degree AV block. This is because aortic valve is located very near to the proximal part of the atrioventricular conduction system. Similarly, aortic valve endocarditis can also cause first degree AV block.

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, aortic valve endocarditis causing first degree AV block can progress rapidly to complete heart block and need early surgery. This progression usually indicates the formation of a valve ring abscess in aortic valve endocarditis. Valve ring abscess is a risky condition, which will need excision of the infected material and implantation of an artificial valve in addition to a prolonged course of appropriate antibiotic therapy.