Heart block shows up on an ECG as a disruption in the timing between the P wave (representing the atria firing) and the QRS complex (representing the ventricles responding). What that disruption looks like depends on the degree of block, ranging from a slightly prolonged pause between beats to a complete disconnect where the atria and ventricles march along at entirely different rates.
How a Normal ECG Sets the Baseline
To spot heart block, you first need to know what normal looks like. On a healthy ECG, every P wave is followed by a QRS complex at a consistent interval. That gap between the start of the P wave and the start of the QRS is called the PR interval, and it normally runs between 120 and 200 milliseconds (three to five small squares on standard ECG paper). Each P wave pairs neatly with one QRS complex, and the spacing stays steady beat after beat. Heart block, in all its forms, breaks one or more of these rules.
First-Degree Heart Block
First-degree block is the subtlest pattern. Every P wave still conducts to the ventricles, and every QRS complex still appears on schedule. The only abnormality is that the PR interval stretches beyond 200 milliseconds. On the ECG strip, you see a wider-than-normal gap between each P wave and its QRS, but the rhythm is regular and no beats are dropped. It looks almost normal unless you measure carefully.
When the PR interval exceeds 300 milliseconds, the block is considered “marked.” At that point the P wave may start to crowd into or even overlap with the preceding T wave, making the tracing look unusual even at a glance. Most people with first-degree block have no symptoms at all, though marked cases can cause lightheadedness or fatigue because the atria and ventricles lose their ideal timing.
Second-Degree Heart Block: Mobitz Type I (Wenckebach)
Mobitz type I creates one of the most recognizable patterns in all of electrocardiography. The PR interval gets a little longer with each successive beat until one P wave fails to conduct entirely and no QRS follows it. After that dropped beat, the cycle resets: the next conducted beat has the shortest PR interval in the sequence, and the gradual lengthening begins again.
On the ECG strip, you see a group of beats where the PR intervals visibly grow, then a lone P wave sitting by itself with no QRS after it, then the pattern repeats. The grouped beating gives the rhythm a distinctive “footprint” that’s often recognizable before you even start measuring intervals. The PR interval right after the dropped beat is always the shortest in the group. In some atypical versions the prolongation isn’t perfectly smooth (the increments may be uneven or the PR may even shorten slightly just before the dropped beat), but the core signature of progressive lengthening followed by a dropped QRS remains.
Second-Degree Heart Block: Mobitz Type II
Mobitz type II looks very different from type I. Here, the PR interval stays constant from beat to beat, with no gradual lengthening at all. Then, without warning, a P wave simply fails to conduct and the expected QRS never appears. On the next conducted beat, the PR interval is unchanged.
The key features to look for are at least two consecutive conducted P waves with identical PR intervals, a single dropped QRS, and then a return to the same PR interval afterward. The P-P interval (the spacing between consecutive P waves) also stays constant through the dropped beat, confirming that the atria are firing regularly and the problem lies further down in the conduction system. Because the block occurs below the level of the AV node, the QRS complexes in Mobitz type II are often wider than normal, reflecting sluggish conduction through the ventricles even on the beats that do get through.
This distinction matters clinically. Mobitz type II carries a higher risk of progressing to complete heart block, which is why identifying it correctly on the ECG is important.
2:1 AV Block: The In-Between Pattern
Sometimes the ECG shows every other P wave failing to conduct, producing a neat alternating pattern: conducted beat, dropped beat, conducted beat, dropped beat. This 2:1 ratio is its own category because you can only see one PR interval between each dropped beat, making it impossible to tell whether the PR would have lengthened (type I) or stayed constant (type II). The strip shows twice as many P waves as QRS complexes, spaced evenly, with the conducted beats all sharing the same PR interval. Additional clues like QRS width or changes in the ratio over time help determine which type of block is driving it.
Third-Degree (Complete) Heart Block
Complete heart block produces the most dramatic ECG pattern. The atria and ventricles are electrically disconnected. P waves appear at their own regular rate (typically 60 to 100 beats per minute), and QRS complexes appear at a much slower rate (often 30 to 40 beats per minute), but there is no consistent relationship between them. P waves march across the strip at one rhythm while QRS complexes march at another. Sometimes a P wave lands just before a QRS and might look like it conducted, but on the next beat the timing is completely different.
The hallmarks on the ECG are: more P waves than QRS complexes, no repeating PR interval, a slow ventricular rate, and often a wide QRS complex because the ventricles are being driven by a backup pacemaker lower in the conduction system rather than the normal pathway. The atrial rate and the ventricular rate are each regular on their own, but they have nothing to do with each other. This “AV dissociation” is the defining feature.
Because the ventricular rate drops so low, third-degree block typically causes noticeable symptoms: fainting, severe fatigue, shortness of breath, and dizziness. It’s usually apparent on the ECG within seconds because the slow, wide QRS complexes stand out immediately.
Quick Visual Comparison
- First-degree: Every P wave conducts. PR interval longer than 200 ms. Rhythm looks normal but the gap is too wide.
- Mobitz I (Wenckebach): PR interval progressively lengthens until one beat drops. Grouped beating pattern on the strip.
- Mobitz II: PR interval stays fixed. A beat drops without warning. QRS is often wide.
- 2:1 block: Every other P wave fails to conduct. Cannot classify as type I or II from the ratio alone.
- Third-degree: P waves and QRS complexes are completely independent. Ventricular rate is slow (30 to 40 bpm). No consistent PR interval exists.
What Causes These Patterns
Heart block can result from age-related wear on the conduction system, coronary artery disease, heart surgery, or inflammation of the heart muscle. Several common medications also slow conduction through the AV node. Beta-blockers are the most frequent culprits, involved in roughly two-thirds of drug-related cases. Certain calcium channel blockers (the types used for heart rate control, not blood pressure), digoxin, and some antiarrhythmic drugs can also produce or worsen AV block. High potassium levels amplify the effect, sometimes pushing a borderline conduction delay into a clinically significant block.
The location of the block in the conduction system tends to predict what the ECG looks like. Blocks at the AV node (higher up) usually produce first-degree block or Mobitz type I with narrow QRS complexes. Blocks below the AV node, in the bundle branches, are more likely to show Mobitz type II or complete heart block with wide QRS complexes. That QRS width is one of the most useful clues when you’re trying to judge the severity of what you see on the tracing.