First-Degree AV Block on an ECG: What Does It Mean and Is It Serious?

First-degree AV block on an ECG means the electrical signal from the heart's upper chambers to the lower chambers is slowed, but every signal still gets through. On the tracing this shows as a PR interval longer than 200 milliseconds (0.20 seconds). It is the mildest form of heart block and often needs no treatment.

AV stands for atrioventricular. The AV node is the relay point between the atria (upper chambers) and the ventricles (lower chambers). The PR interval is the time from the start of the atrial signal to the start of the ventricular signal. A cardiology review notes that, despite the name, first-degree AV block is not a true block: each beat is conducted, just with a delay.

Reports may write it as "1st degree AV block", "first-degree heart block" or "prolonged PR interval". This guide explains the label. It cannot tell you whether your own result matters; that depends on symptoms, medicines, other ECG findings and a clinician's assessment.

First-degree AV block by itself rarely causes symptoms, so new symptoms deserve attention rather than being blamed on the label. Call your local emergency number if you have chest pain, severe difficulty breathing, fainting with no quick recovery, or light-headedness with sweating or sickness. Do not drive yourself to hospital.

Get urgent advice the same day if you feel dizzy all the time or it keeps coming back, you have fainted, or breathlessness is getting worse with normal activity or when lying down. These can be signs of a slower or more advanced rhythm problem that a resting ECG may not have captured.

Usually not. Cleveland Clinic describes it as the mildest type of heart block and says it usually needs no treatment and does not usually get in the way of normal activities. The NHS also states that first-degree heart block does not usually need treatment. Many people only find out because of a routine ECG.

There are some caveats. The cardiology review notes that a markedly long PR interval, above 0.3 seconds, can cause symptoms, particularly during exercise. The finding is also read differently when it appears with other changes, such as a bundle branch block, a very slow rate or symptoms. NHLBI notes that conduction problems become more common with age.

Often, yes. Regular endurance training strengthens the vagus nerve's slowing effect on the heart. A review of ECGs in athletes lists a PR interval over 200 ms among training-related changes that are physiological adaptations to exercise and do not need further evaluation in an athlete without symptoms.

Being fit does not explain everything. An athlete with fainting, chest pain, unusual breathlessness, palpitations or a drop in performance should still be assessed, as should anyone with a family history of heart problems. Tell the clinician what type of training you do, how often and for how long.

Several medicines can lengthen the PR interval. Sources list beta-blockers, calcium channel blockers such as verapamil and diltiazem, digoxin and antiarrhythmic medicines. Cleveland Clinic also lists diuretics, muscle relaxants, sedatives, antidepressants and antipsychotics among risk factors for heart block.

Other possible contributors include an overactive vagus nerve, sleep apnoea, electrolyte imbalances, thyroid problems, autoimmune conditions, coronary artery disease or a previous heart attack, cardiomyopathy, heart valve disease, heart inflammation or infection, and heart surgery. Sometimes no cause is found.

Do not stop or change a prescribed medicine because of an ECG report. Bring a full list of what you take, including non-prescription products, and ask the prescriber or a pharmacist whether any could be contributing.

Follow-up is more likely to be suggested when the finding is new, the PR interval is very long, it appears alongside other ECG changes, you take medicines that slow conduction, or you have symptoms or a known heart condition. Depending on the situation, a clinician may consider:

• A repeat ECG to compare the PR interval over time.
• Blood tests, such as electrolytes or thyroid function.
• A wearable heart monitor, if symptoms come and go.
• An exercise test, which the review notes can help unmask symptoms with exertion.
• A review of medicines that slow the heart's conduction.

In most people, no. Treatment usually means managing a cause, such as reviewing a medicine or treating a thyroid or electrolyte problem. NHLBI notes that people with very mild or no symptoms may not need any treatment.

For people without symptoms, the 2018 US bradycardia guideline, as cited in the review, recommends against a permanent pacemaker for first-degree AV block. Pacemakers are considered in other situations, such as higher-degree heart block or symptoms clearly linked to a slow rhythm.

Take a copy of the ECG report and any earlier ECGs to the appointment. It also helps to note when the ECG was done and why, for example a routine check, a pre-operative test or a sports screening, and whether you had recently started or changed any medicine.

If you have had symptoms, write down what they felt like, when they happened, how long they lasted and what you were doing. A short, factual record is easier to discuss than memory alone.

• What is my PR interval, and has it changed compared with earlier ECGs?
• Does the ECG show anything else, such as a slow rate or bundle branch block?
• Could any of my medicines or supplements be lengthening the PR interval?
• Do I need a repeat ECG, blood tests, a heart monitor or an exercise test?
• Which symptoms should make me seek help sooner?

On SameCase, patients and verified doctors share de-identified cases. Seeing how others recorded their PR interval, medicines and follow-up can help you think of questions for your own appointment. Similar cases cannot diagnose you or predict what will happen to you; use them to prepare for the conversation with your clinician.