Bradycardia on an ECG shows a heart rate below 60 beats per minute, which you can spot by the wider-than-normal spacing between the tall QRS spikes. The specific pattern beyond that slow rate tells you what type of bradycardia is present, and some types look dramatically different from others. A simple sinus bradycardia looks like a perfectly normal ECG that’s just been stretched out, while heart blocks can show P waves wandering independently, QRS complexes dropping out entirely, or signals that never reach the ventricles at all.
How to Measure Heart Rate on a Strip
Before identifying bradycardia, you need to confirm the rate is actually slow. There are two quick methods. The first is to count the number of large squares between two consecutive R waves (the tallest peaks on the strip) and divide 300 by that number. If you count five large squares between R waves, that’s 300 divided by 5, giving you 60 bpm, right at the threshold. Six large squares would be 50 bpm, clearly bradycardic.
The second method is more precise: count the small squares between two R waves and divide 1,500 by that number. Each small square represents 0.04 seconds of time, so this gives you an exact rate. In bradycardia, you’ll see notably more space between each heartbeat cycle than on a normal-rate strip, and the overall tracing looks spread out.
Sinus Bradycardia: The Normal-Looking Slow Rhythm
Sinus bradycardia is the most common type, and it’s also the easiest to read. The ECG looks completely normal in every way except that the rate falls below 60 bpm. Every P wave is followed by a QRS complex. The rhythm is regular. The PR interval (the gap between the start of the P wave and the start of the QRS) falls within the normal range of 120 to 200 milliseconds. The QRS complexes are narrow and uniform. If you covered up the time markings, you might not even realize anything was different until you measured the R-to-R distance.
This pattern is extremely common in endurance athletes. A study of competitive endurance athletes found that 38% had resting heart rates at or below 40 bpm on continuous monitoring, with the bradycardic group averaging around 37 bpm. This is a physiological adaptation, not a disease, driven by changes in the heart’s pacemaker cells and increased activity of the nerve that naturally slows heart rate. Pauses of two to three seconds between beats were well tolerated in these athletes. The ACC/AHA guidelines actually use a threshold of below 50 bpm (not 60) when defining sinus bradycardia that may warrant clinical attention.
First-Degree AV Block: A Delayed Signal
First-degree AV block is technically more of a conduction delay than a true block, and its ECG signature is straightforward. Every P wave still conducts to a QRS complex, so no beats are dropped. The only abnormality is a PR interval longer than 200 milliseconds (one large square on the ECG paper). You’ll see a noticeable gap between the small rounded P wave and the tall QRS spike that follows it.
When the PR interval stretches beyond 300 milliseconds, it’s called a “marked” first-degree block. At that point, the delay between the atrial signal and the ventricular response becomes visually obvious even at a glance. On its own, first-degree block doesn’t necessarily cause a slow heart rate, but it often appears alongside sinus bradycardia and can progress to more significant blocks.
Second-Degree Block, Type I (Wenckebach)
This is where the ECG starts to tell a story you can follow beat by beat. In Wenckebach block, the PR interval gets progressively longer with each successive beat until one P wave fails to conduct entirely, meaning a QRS complex is “dropped.” Then the cycle resets and starts over.
Picture it as a repeating pattern: the first beat has a normal PR interval, the second beat’s PR is a little longer, the third is longer still, and then the fourth P wave sits alone with no QRS following it. The next beat resets to a shorter PR interval, and the whole sequence repeats. This creates a distinctive grouped beating pattern on the strip. The overall heart rate is slow because those periodically dropped beats reduce the number of ventricular contractions per minute. This type results from progressive fatigue of the cells in the AV node and is generally considered less dangerous than Type II.
Second-Degree Block, Type II (Mobitz II)
Mobitz II looks very different from Wenckebach and carries more clinical significance. Here, the PR interval stays exactly the same length in every conducted beat. There’s no gradual prolongation. Instead, P waves are blocked in an “all or nothing” fashion: the signal either gets through or it doesn’t, with no warning pattern beforehand.
On the ECG, you’ll see a series of normal-looking P-QRS cycles at a constant PR interval, then suddenly a P wave appears with no QRS after it. The blocked beats often follow a predictable ratio. In a 2:1 block, every other P wave is blocked, so you’ll see two P waves for every one QRS complex. In a 3:1 block, only one out of every three atrial signals reaches the ventricles. This pattern reflects a problem deeper in the conduction system, in the bundle of His or the Purkinje fibers, rather than in the AV node itself.
Third-Degree (Complete) Heart Block
Complete heart block produces one of the most distinctive ECG patterns in cardiology. The atria and ventricles beat completely independently of each other. P waves march along at their own rate (typically 60 to 100 bpm), and QRS complexes appear at a much slower rate (typically 30 to 40 bpm), with absolutely no relationship between the two.
If you try to measure the PR interval, you’ll find it constantly changing because the P waves and QRS complexes are drifting in and out of alignment by coincidence, not by conduction. Sometimes a P wave lands just before a QRS, sometimes it lands on top of one, sometimes it falls between them. There are always more P waves than QRS complexes on the strip. The ventricular rate depends on where the backup pacemaker kicks in. If it originates near the AV junction, the QRS complexes will be narrow and the rate will typically fall between 40 and 60 bpm. If the backup pacemaker sits lower in the ventricles, the QRS complexes will be wide and bizarre-looking, and the rate drops to 20 to 40 bpm.
Junctional and Ventricular Escape Rhythms
When the heart’s primary pacemaker fails or signals are blocked, backup pacemakers take over. These escape rhythms are common causes of bradycardia, and each has a recognizable ECG signature.
A junctional escape rhythm originates from the AV junction, producing a rate of 40 to 60 bpm. Its hallmark on the ECG is a narrow, normal-looking QRS complex with either absent P waves or inverted P waves (flipped upside down compared to normal). The P waves may appear just before, during, or just after the QRS, depending on whether the signal travels backward to the atria before, simultaneously with, or after activating the ventricles.
A ventricular escape rhythm comes from even lower in the heart and is significantly slower, typically 20 to 40 bpm. The QRS complexes are wide (longer than 120 milliseconds) and abnormally shaped because the electrical impulse spreads through the ventricles by an unusual route rather than the normal high-speed conduction pathways. These wide, slow complexes are easy to spot and indicate the heart is relying on its last-resort backup pacemaker.
Symptoms That Match the Strip
Not every slow heart rate on an ECG causes problems. Many people, especially athletes and younger adults, walk around with rates in the 40s or 50s and feel fine. When bradycardia does cause symptoms, the most common are dizziness, lightheadedness, fatigue, and fainting. Exercise intolerance and shortness of breath occur because the heart can’t increase its rate enough to meet demand during activity.
More concerning signs of poor blood flow include low blood pressure, cool or pale skin, confusion or mental status changes, and chest pain. In complete heart block or very slow escape rhythms, you may also notice visible pulsing in the neck veins, caused by the atria contracting against closed heart valves. The severity of symptoms generally correlates with how slow the rate is and which type of block is present: sinus bradycardia at 50 bpm rarely causes trouble, while complete heart block at 30 bpm with wide QRS complexes almost always does.