What is First-Degree AV Block?

Mary McMahon

First-degree atrioventricular (AV) block is a variation in the normal rhythm of the heart where the electrical signal between the atria and ventricles is slightly delayed. This condition usually does not cause any symptoms, and may be an incidental finding during a routine examination for an unrelated medical problem. Treatment is generally not required unless there are concerns that the first-degree AV block may progress into a more serious heart block, which can be an issue if a patient has an underlying medical problem.

First-degree AV block causes a delay in signals between the atria at the top of the heart and the ventricles below.
First-degree AV block causes a delay in signals between the atria at the top of the heart and the ventricles below.

In a patient with first-degree AV block, the PR interval, which is the amount of time needed for a signal to travel from the atria to the ventricles, is greater than 0.2 seconds. In healthy individuals, this interval is smaller than 0.2 seconds. The PR interval can be clearly seen on an electrocardiograph, allowing a doctor to diagnose the patient with first-degree AV block.

Someone with first-degree AV block might be asked to undergo regular ECGs to detect any changes to the heart.
Someone with first-degree AV block might be asked to undergo regular ECGs to detect any changes to the heart.

Athletes commonly exhibit this pattern in the electrical signals of their hearts as a result of increase in vagal tone, which means that they are experiencing a higher rate of impulses from the vagus nerve, causing inhibition, or straining, of the heartbeat. People also can develop this condition in association with a variety of infectious diseases and problems with the heart valves. The patient generally does not experience any problems as a result of the slightly elongated PR interval. If this condition is identified, the patient may be evaluated for other risk factors to see if monitoring is needed in the future to check for changes.

One concern with first-degree AV block is the risk that it may proceed to a complete AV block, where the signal doesn't reach the ventricles at all. If a patient has another heart problem or is experiencing a heart attack, there is a risk of escalation in the severity of the condition. Medications may be provided and it is possible for the patient to need a mechanical pacemaker in the future to control the heart rhythm and keep the heart functioning normally.

Many people live with a first-degree AV block and are unaware of the condition. Others have it diagnosed during a routine medical examination and may need to take no additional action to address the abnormal heart rhythm. In cases where it is a cause for concern, a doctor can discuss how to proceed in the case and provide information about treatment options for the patient. Treatment can vary from simply asking a patient to undergo regular electrocardiograms to check for signs of changes to recommending artificial pacemaking to control the patient's heartbeat.

Abnormalities in a patient's PR interval can be seen clearly on an electrocardiogram.
Abnormalities in a patient's PR interval can be seen clearly on an electrocardiogram.

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