First-degree AV block is a delay in the electrical signal that travels from the upper chambers of your heart to the lower chambers. Despite the name, it’s not actually a “block” at all. Every heartbeat still gets through, just slightly slower than normal. It’s diagnosed on an ECG when the PR interval, the time it takes for the signal to travel from the atria to the ventricles, exceeds 200 milliseconds. About 4% of the general population has it, and most people never know unless it shows up on a routine test.
Why It’s Called a “Block” (and Why That’s Misleading)
The American Heart Association’s 2018 guidelines explicitly call first-degree AV block a “misnomer.” In true heart block, some or all electrical signals fail to reach the lower chambers entirely. In first-degree AV block, every signal makes it through. The delay is real, but the block isn’t. A more accurate name would be “first-degree AV delay,” and many cardiologists use that term instead.
On an ECG, the PR interval measures the gap between the electrical activity starting in the atria (the P wave) and the signal arriving in the ventricles (the QRS complex). A normal PR interval falls between 120 and 200 milliseconds. Anything above 200 ms qualifies as first-degree AV block. A typical example might show a PR interval of 240 ms. When the PR interval stretches beyond 300 ms, it’s considered “marked” first-degree AV block, and that’s when symptoms become more likely.
What Happens Inside the Heart
The delay occurs at a tiny structure called the AV node, a cluster of specialized cells that sits between the upper and lower chambers. The AV node normally acts as a gatekeeper, briefly pausing each electrical signal before passing it along. This pause gives the atria time to finish contracting and push blood into the ventricles before they squeeze.
In first-degree AV block, the signal takes longer than usual to cross through this area. The slowdown happens for a few reasons. The cells in the AV node are small, surrounded by connective tissue, and connected to each other through low-conductance junctions that resist electrical flow. Research shows that the biggest chunk of the delay actually occurs right at the entrance to the node, where atrial cells hand off the signal to nodal cells. Think of it as a bottleneck at a doorway rather than slow walking speed down a hallway, though both factors play a role.
Common Causes
Many cases have no worrisome cause at all. High vagal tone, the state of a well-tuned “rest and digest” nervous system, naturally slows conduction through the AV node. This is especially common in athletes and younger, physically fit people. For years, athletic bradycardia and AV conduction delays were chalked up entirely to vagal tone, though newer research suggests actual structural remodeling of the AV node also plays a part in trained athletes.
Medications are another frequent culprit. Beta-blockers, calcium channel blockers, and digoxin all slow AV conduction as part of how they work. So do certain antiarrhythmics, antidepressants, antipsychotics, and sedatives. If you started a new medication and your next ECG shows a prolonged PR interval, the drug is often the explanation.
Beyond fitness and medications, several medical conditions can cause or contribute to the delay:
- Coronary artery disease or other structural heart problems
- Infections like Lyme disease, which can inflame the heart’s conduction tissue
- Thyroid disease
- Autoimmune conditions like sarcoidosis or rheumatic heart disease
- Obstructive sleep apnea
- Congenital heart abnormalities present from birth
- Electrolyte imbalances or toxin exposure
PR Intervals Change With Age
What counts as “normal” for the PR interval shifts throughout life. In a newborn, the average PR interval is around 108 to 110 ms. By age 4 or 5, it’s about 123 ms. In teenagers and young adults, the average reaches 138 to 147 ms, and by the 20s and 30s, a normal PR interval can sit anywhere from 147 to 150 ms. The 200 ms threshold for diagnosing first-degree AV block in adults doesn’t apply the same way in children, where a PR interval of 160 ms might already be significantly prolonged for a toddler.
This gradual lengthening tracks with the natural decline in resting heart rate as children grow. Pediatric ECGs are interpreted using age-specific reference ranges rather than a single adult cutoff.
Symptoms (or Lack of Them)
Most people with first-degree AV block feel nothing. The delay is too small to affect how the heart pumps blood. The AHA guidelines note that it is “typically asymptomatic” and not concerning for progression to a more serious type of heart block.
The exception is marked first-degree AV block, where the PR interval exceeds 300 ms. When the delay gets this long, the timing between atrial and ventricular contractions falls out of sync. The atria may contract while the valves between the chambers are still closed from the previous beat. This drives up pressure in the lungs and reduces the amount of blood the heart pumps with each beat. The result is a condition sometimes called “pseudo-pacemaker syndrome,” which can cause fatigue, shortness of breath, dizziness, and difficulty exercising.
Long-Term Health Risks
First-degree AV block was long considered completely harmless, but population studies have added nuance to that picture. A Danish study of more than 288,000 people found that those with prolonged PR intervals had a 26% increased risk of developing atrial fibrillation compared to people with mid-range PR intervals. Another analysis found a 41% increased risk of atrial fibrillation for every standard-deviation increase in PR interval length.
In people who already have heart failure, the picture is more concerning. First-degree AV block is present in 15% to 51% of heart failure patients, a rate far higher than the general population. Studies of cardiac resynchronization therapy have linked first-degree AV block in heart failure patients to higher rates of hospitalization and mortality. The delay can worsen mitral valve leakage and impair the heart’s pumping efficiency.
Not all data points in the same direction. A large Finnish study followed over 10,000 participants for up to 41 years and found no increase in mortality, hospitalizations, or atrial fibrillation among those with prolonged PR intervals. The risk likely depends on context: a mildly prolonged PR interval in an otherwise healthy person carries a different meaning than the same finding in someone with existing heart disease.
When Treatment Is Needed
For the vast majority of people, first-degree AV block requires no treatment. The AHA guidelines recommend monitoring and follow-up rather than intervention for asymptomatic patients. If a medication is causing the delay and it’s not causing symptoms, there’s generally no reason to change it.
Treatment becomes relevant in two scenarios. First, if a reversible cause exists, addressing it can resolve the delay. Stopping or adjusting a medication, treating Lyme disease, or correcting a thyroid problem may bring the PR interval back to normal. Second, if marked first-degree AV block is causing clear symptoms like fatigue, breathlessness, or exercise intolerance, a pacemaker may be considered. The guidelines stress that symptoms must clearly correlate with the conduction delay before a pacemaker is warranted, given the procedural risks and long-term maintenance involved.
First-Degree AV Block in Athletes
Finding first-degree AV block on a sports physical or pre-participation ECG is common and usually reassuring. The trained heart adapts to endurance exercise by slowing its resting electrical conduction, and a PR interval just above 200 ms in a fit young person is a normal physiological response rather than a sign of disease.
There is one caveat worth noting. Research suggests that the electrical remodeling from years of intense training can, from roughly the fifth decade of life onward, sometimes evolve into symptomatic slow heart rhythms that require a pacemaker. This doesn’t mean every athlete with first-degree AV block will develop problems later, but it is a reason lifelong athletes should stay aware of changes in their exercise tolerance or new symptoms like lightheadedness as they age.