A premature ventricular contraction (PVC) stands out on an ECG as a wide, bizarre-looking QRS complex that arrives earlier than the next expected heartbeat. The QRS duration is typically 120 milliseconds or longer, noticeably wider than the narrow complexes of normal sinus beats surrounding it. Once you know the handful of features that define a PVC, they become one of the easier abnormalities to spot on a rhythm strip.
The Core Features of a PVC
PVCs originate from somewhere in the ventricles rather than from the heart’s normal pacemaker in the sinus node. Because the electrical impulse takes an abnormal path through ventricular muscle instead of the fast-conducting pathway, the QRS complex is wide (at least 120 ms, or three small boxes on standard ECG paper) and looks dramatically different from the patient’s normal beats. The shape is often described as “bizarre” because it doesn’t resemble any typical bundle branch block pattern.
There is no normal P wave preceding the wide QRS. In a normal heartbeat, you see a P wave (atrial contraction) followed by a narrow QRS (ventricular contraction) in a predictable sequence. A PVC skips that sequence entirely. Sometimes a P wave from the sinus node happens to fall near or within the PVC, but it has no relationship to it and did not cause it.
The beat arrives early. This is the “premature” part. On the rhythm strip, you’ll see regular sinus beats marching along at a steady interval, then a wide complex that fires before the next sinus beat was due. That premature timing, combined with the wide and odd-looking QRS, is usually enough to make the identification.
T-Wave Direction and ST Changes
In a typical PVC, the T wave points in the opposite direction from the main deflection of the QRS complex. This is called T-wave discordance. If the QRS is predominantly upward (positive), the T wave will be downward (negative), and vice versa. This opposite-direction pattern is a normal repolarization response to the abnormal depolarization and helps confirm you’re looking at a PVC rather than something else. ST segment shifts in the same opposite direction are also expected and do not indicate ischemia by themselves.
The Compensatory Pause
After a PVC fires, there’s typically a longer-than-normal pause before the next sinus beat. This is called a full (or complete) compensatory pause. It happens because the PVC doesn’t reset the sinus node’s timing. The sinus node keeps firing on schedule, but the impulse right after the PVC can’t get through to the ventricles because they’re still recovering from the PVC. The next sinus beat then arrives right on time according to the original rhythm.
You can verify a full compensatory pause by measuring: the interval from the normal beat before the PVC to the normal beat after the PVC equals exactly two normal R-R intervals. This is a useful way to distinguish PVCs from premature atrial contractions (PACs), which typically produce an incomplete pause. A PAC resets the sinus node, so the next beat comes earlier than two full R-R intervals would predict.
Unifocal vs. Multifocal PVCs
If all the PVCs on a strip look identical to each other, they’re unifocal, meaning they originate from the same spot in the ventricles. Every PVC will have the same shape, width, and direction. Multifocal PVCs have more than one morphology, meaning they come from different locations and look different from each other on the strip. To determine this reliably, you often need a longer recording like a Holter monitor rather than a single 12-lead snapshot.
Multifocal PVCs are generally considered more clinically significant than unifocal ones because multiple irritable foci in the ventricles suggest more widespread electrical instability.
Common PVC Patterns
PVCs can appear as isolated events or in recognizable repeating patterns:
- Bigeminy: every other beat is a PVC, creating an alternating pattern of normal beat, PVC, normal beat, PVC.
- Trigeminy: every third beat is a PVC (two normal beats followed by one PVC, repeating).
- Quadrigeminy: every fourth beat is a PVC.
- Couplets: two PVCs in a row.
- Triplets: three PVCs in a row. Three or more consecutive PVCs at a rate above 100 beats per minute meets the definition of ventricular tachycardia. If it lasts fewer than 30 seconds, it’s classified as non-sustained ventricular tachycardia.
Distinguishing PVCs From PACs With Aberrant Conduction
The trickiest look-alike for a PVC is a premature atrial contraction (PAC) that conducts abnormally through the ventricles, producing a wide QRS. Both can appear as early, wide beats. A few features help sort them out.
Look for a P wave before the wide complex. A PAC with aberrant conduction will usually have a premature P wave just before or slightly buried in the preceding T wave. A PVC won’t have a related P wave. Next, check the pause. A full compensatory pause favors a PVC. An incomplete pause (the rhythm resets early) favors a PAC. Finally, if the wide beat has a right bundle branch block shape with a typical rsR’ pattern in V1, aberrant conduction of a PAC is more likely. PVCs tend to produce more unusual, less “clean” QRS morphologies.
The R-on-T Phenomenon
One PVC timing pattern deserves special attention. When a PVC lands directly on the T wave of the preceding beat, it’s called the R-on-T phenomenon. The T wave represents the vulnerable period of ventricular repolarization, and a PVC striking during this window can, in certain patients, trigger polymorphic ventricular tachycardia or ventricular fibrillation. On the ECG, you’ll see the PVC’s QRS complex superimposed on or immediately overlapping the prior T wave, with no gap between them.
R-on-T PVCs are not always dangerous in otherwise healthy hearts, and the phenomenon alone has not been shown to reliably predict dangerous arrhythmias in all patients. However, in the setting of a recent heart attack or existing heart disease, it is treated as a higher-risk finding.
Why PVC Burden Matters
If you’re reviewing a longer recording and counting PVCs, the total percentage of heartbeats that are PVCs is called the PVC burden. There is no single clean cutoff that separates safe from dangerous, but a burden above 10% is the most consistently reported threshold associated with PVC-induced cardiomyopathy, a condition where frequent PVCs gradually weaken the heart muscle. That said, studies have identified risk at thresholds as low as 4% and as high as 24%, and some larger studies without intervention have found associations with heart failure risk at burdens well below 1%. The clinical picture matters more than any single number.
Quick Identification Checklist
When you see an early beat on the rhythm strip and want to confirm it’s a PVC, run through these features:
- Timing: the beat arrives before the next expected sinus beat.
- QRS width: 120 ms or wider (three or more small boxes).
- QRS shape: bizarre, different from the patient’s normal complexes.
- P wave: absent or unrelated to the wide QRS.
- T-wave direction: opposite to the main QRS deflection.
- Pause: full compensatory pause follows the beat.
If all six line up, you’re looking at a PVC. If most but not all match, consider aberrantly conducted PACs or other wide-complex premature beats and look at additional leads for clarification.