A first degree heart block looks like a normal heart rhythm on an ECG with one key difference: the gap between the P wave and the QRS complex is too long. This gap, called the PR interval, stretches beyond 0.20 seconds (200 milliseconds), while a normal PR interval falls between 0.12 and 0.20 seconds. Every P wave is still followed by a QRS complex, and the heart rate is typically normal. That extra-long pause before each beat is the only thing that gives it away.
What You See on the ECG Strip
On a standard ECG tracing, each heartbeat produces a recognizable pattern: a small rounded bump (the P wave, representing the upper chambers contracting), a flat line, and then a taller, sharper spike (the QRS complex, representing the lower chambers contracting). In a healthy rhythm, the flat line between them is brief. In first degree heart block, that flat line is noticeably stretched out.
The critical detail is that every P wave still has a QRS complex following it. The electrical signal from the upper chambers always reaches the lower chambers. It just takes longer than it should to get there. The rhythm stays regular, the heart rate stays normal, and the ratio of P waves to QRS complexes remains 1:1. This is why some cardiologists prefer the term “AV delay” rather than “block,” since nothing is actually being blocked.
If you’re looking at an ECG printout, count the small squares between the start of the P wave and the start of the QRS complex. Each small square represents 0.04 seconds. More than five small squares (totaling more than 0.20 seconds) means the PR interval is prolonged. That single measurement is the entire diagnostic criterion.
How It Differs From More Serious Heart Blocks
The feature that separates first degree block from second and third degree blocks is straightforward: no beats are dropped. In second degree heart block (Mobitz type I), the PR interval gradually gets longer with each beat until one P wave fails to produce a QRS complex entirely, creating a “dropped beat.” In Mobitz type II, P waves occasionally fail to conduct without any warning pattern. In third degree (complete) heart block, the upper and lower chambers beat independently with no relationship between P waves and QRS complexes at all.
First degree block never drops a beat. If you see a P wave without a QRS following it, you’re looking at something more advanced.
Why It Shows Up
First degree heart block appears on ECGs for a variety of reasons, many of them harmless. In well-trained athletes, it shows up on roughly 7.5% of screening ECGs. The explanation is usually high vagal tone, the same nervous system adaptation that gives athletes their characteristically low resting heart rates. The conduction delay often disappears during exercise as the heart rate climbs and adrenaline speeds up the electrical pathway.
Certain medications that slow the heart’s electrical system can also produce it. Beta-blockers, calcium channel blockers, and other drugs that affect heart conduction are common culprits. Age-related changes to the heart’s electrical wiring, underlying heart disease, and electrolyte imbalances round out the list. In many cases, no specific cause is ever identified.
What It Feels Like (Usually Nothing)
First degree heart block is almost universally without symptoms. Most people discover it as an incidental finding on a routine ECG and have no idea anything was different about their heart rhythm. It causes no hemodynamic problems, meaning the heart still pumps blood effectively.
The exception is a more extreme form where the PR interval stretches beyond 0.30 seconds (300 milliseconds). At that point, the timing between the upper and lower chambers can become so misaligned that some people experience lightheadedness, shortness of breath, fatigue, chest discomfort, or even fainting. This is uncommon, but it’s the reason a very prolonged PR interval gets closer attention than a mildly prolonged one.
Does It Need Treatment?
Asymptomatic first degree heart block does not require treatment. It is considered a benign finding. If a medication is causing it, a doctor may adjust or discontinue that medication, but otherwise the standard approach is periodic monitoring. People with this finding can generally continue their usual activities without restriction.
The main reason for follow-up is that a small subset of patients with first degree block will eventually develop a higher-grade block over time. The progression rate is low, roughly 1 to 2% per year even in higher-risk groups. Routine ECGs at regular intervals can catch any changes early. The presence of additional conduction abnormalities on the ECG, like a bundle branch block pattern alongside the prolonged PR interval, raises the level of concern and may prompt closer monitoring or further testing.