First-degree heart block is a minor delay in the electrical signals that tell your heart when to beat. It shows up on an ECG (electrocardiogram) as a PR interval longer than 200 milliseconds, meaning the signal from the upper chambers of your heart takes slightly longer than normal to reach the lower chambers. Despite the alarming name, it is generally considered benign and rarely causes symptoms on its own.
How Your Heart’s Electrical System Works
Each heartbeat starts with an electrical signal in the upper chambers (atria). That signal travels through a relay point called the AV node before reaching the lower chambers (ventricles), which do the heavy lifting of pumping blood. The time it takes for the signal to travel from the atria through the AV node to the ventricles is called the PR interval, and it normally falls between 120 and 200 milliseconds.
In first-degree heart block, the signal still makes the full journey every single time. No beats are skipped or dropped. The signal is just a little slow getting through the AV node. That’s why many cardiologists prefer the term “first-degree AV delay” rather than “block,” since nothing is actually blocked.
How It Differs From Other Heart Blocks
Heart block comes in three degrees, and the distinction matters. In second-degree heart block, some electrical signals fail to reach the lower chambers entirely, meaning occasional beats are dropped. In third-degree (complete) heart block, no signals get through at all, and the ventricles have to generate their own much slower rhythm. Both of these can cause serious symptoms and may require a pacemaker.
First-degree block is fundamentally different because every signal reaches the ventricles. The heart beats in a normal, regular rhythm. It simply takes a fraction of a second longer than usual for each signal to arrive.
Common Causes
In younger people, first-degree heart block is often caused by naturally high vagal tone, meaning the nerve that slows the heart is more active than average. This is especially common in athletes. The prevalence of first-degree AV block among athletes on a resting ECG is roughly 7.5%, and it’s generally considered a harmless adaptation to intense training rather than a sign of disease.
In older adults, the most common cause is gradual, age-related scarring of the heart’s conduction system. The same kind of wear and tear that stiffens joints can slow the electrical pathways in the heart over decades.
Other recognized causes include:
- Medications: Several heart rhythm drugs, blood pressure medications, and digoxin can slow conduction through the AV node as a side effect.
- Electrolyte imbalances: Low potassium or low magnesium levels can affect how electrical signals travel through heart tissue.
- Coronary artery disease and heart attacks: Reduced blood flow to the heart can damage the conduction system.
- Infections: Lyme disease, rheumatic fever, and certain other infections can inflame heart tissue and slow conduction.
- Inflammatory and autoimmune conditions: Lupus, rheumatoid arthritis, and sarcoidosis can all affect the heart’s electrical pathways.
Symptoms (or Lack of Them)
Most people with first-degree heart block have no symptoms at all. It’s typically discovered incidentally during a routine ECG or a pre-surgery workup. You can’t feel the extra few milliseconds of delay, and your heart pumps blood just as effectively.
In rare cases where the PR interval becomes very prolonged (sometimes called “marked” or “profound” first-degree block, with a PR interval above 300 or 400 milliseconds), the timing between the upper and lower chambers can become misaligned enough that some people notice fatigue, lightheadedness, or a sense that their heart isn’t pumping as strongly. This is uncommon and typically only relevant when the delay is extreme.
How It’s Diagnosed
Diagnosis is straightforward: a standard 12-lead ECG shows a PR interval longer than 200 milliseconds with a normal, regular rhythm and no dropped beats. In children, the threshold is age-dependent because younger hearts beat faster and naturally have shorter PR intervals. An infant’s PR interval normally tops out around 110 to 150 milliseconds, while a teenager’s upper limit of normal is about 180 milliseconds.
If you’re told you have first-degree heart block, your doctor may also check for reversible causes like medication side effects, electrolyte levels, or underlying infections, particularly if it’s a new finding.
First-Degree Block in Athletes
If you’re physically active and were told you have this finding, it’s worth knowing that first-degree heart block is one of the most common ECG variations seen in trained athletes. It reflects the strong influence of the parasympathetic nervous system (the “rest and digest” branch) that comes with cardiovascular fitness. In most cases, no additional testing or restrictions are needed.
The exception is when the PR interval is very prolonged, above 400 milliseconds. At that level, the delay could still be from high vagal tone, but it could also indicate a structural problem in the conduction pathway. Athletes with a PR interval that long typically undergo additional testing before being cleared for competition.
Does It Need Treatment?
Isolated first-degree heart block with no symptoms almost never requires treatment. There is no medication to take for it, and a pacemaker is not indicated. If a medication is causing the delay and it’s clinically significant, adjusting or switching that medication is usually sufficient to normalize the PR interval.
The main practical implication is awareness. If you know you have first-degree heart block, it’s useful information for any future healthcare encounters, particularly if you ever need anesthesia or are prescribed medications that further slow heart conduction. Having a baseline ECG on record helps doctors distinguish your normal from something new.
Long-Term Outlook and Monitoring
For most people, first-degree heart block remains stable for years or even a lifetime without progressing. However, some large population studies have found a modest association between first-degree block and a slightly higher long-term risk of atrial fibrillation and, in some cases, a higher likelihood of eventually needing a pacemaker compared to people with completely normal PR intervals. These risks are small in absolute terms and tend to matter more when the PR interval is significantly prolonged or when other heart conditions are present.
If you have an isolated first-degree block with no other cardiac issues, periodic follow-up with a standard ECG is generally all that’s recommended. The goal is simply to confirm the delay isn’t getting progressively longer, which could signal evolving conduction disease. For the vast majority of people, it doesn’t.