7.1 Premature ventricular contractions (PVCs)

Premature Ventricular Contractions (PVCs)

A premature ventricular contraction (PVC) is an early depolarization that originates from an ectopic focus below the bundle of His — somewhere in the ventricular myocardium or Purkinje network — rather than from the sinoatrial (SA) node. Because the impulse does not travel down the normal His-Purkinje highway, it spreads slowly cell to cell through ordinary muscle, a disorganized route. This produces the two hallmark features every CRAT candidate must recognize instantly: a wide QRS complex (≥0.12 second, often 0.14-0.16) and a bizarre, distorted morphology that looks nothing like the patient's normally conducted beats.

Core recognition criteria

Every PVC shares the same identification checklist:

• Premature — it arrives earlier than the next expected sinus beat.
• No preceding P wave — the impulse starts in the ventricle, so no atrial activity precedes the QRS. A retrograde P may occasionally follow the QRS.
• Wide, bizarre QRS — duration ≥0.12 s, with the T wave usually deflecting opposite to the main QRS direction.
• Full compensatory pause — the PVC usually does not reset the SA node, so the next sinus beat lands "on time."

That full compensatory pause is a favorite exam discriminator. A PVC typically produces a complete compensatory pause: the interval from the sinus beat before the PVC to the sinus beat after it equals two normal R-R intervals. A premature atrial contraction (PAC), by contrast, usually resets the SA node and produces an incomplete (non-compensatory) pause. On the strip, measure from the R wave before the ectopic beat to the R wave after it — if it spans exactly two sinus cycles, the pause is full.

Two variations reinforce that a PVC is ventricular in origin. An interpolated PVC squeezes between two normal sinus beats without disturbing them at all, so there is no compensatory pause and the underlying sinus rate marches through undisturbed. A fusion beat occurs when the ventricle is depolarized almost simultaneously by the descending sinus impulse and the ectopic ventricular focus, producing a QRS whose shape is a hybrid — part normal, part PVC.

Distinguishing PVCs from aberrantly conducted beats

Not every wide QRS is a PVC. A supraventricular impulse, such as a PAC, can conduct with aberrancy when part of the bundle-branch system is still refractory, producing a wide QRS that mimics a PVC. The distinguishing clue is the P wave: an aberrantly conducted PAC is preceded by a premature P wave, whereas a true PVC has no preceding P wave. When a wide, early beat appears, hunt for a hidden premature P wave buried in the preceding T wave before labeling it a PVC.

Unifocal versus multifocal

The site of origin determines PVC morphology. When every PVC on a strip looks identical, they arise from a single irritable focus — these are unifocal (uniform) PVCs. When PVCs show two or more distinctly different shapes, they arise from multiple foci or via different conduction pathways — multifocal (multiform) PVCs. Multifocal PVCs signal more widespread ventricular irritability and greater electrical instability, and they carry a higher risk of deteriorating into ventricular tachycardia or fibrillation. Recognizing "same shape versus different shapes" is a high-yield distinction.

Frequency and coupling patterns

CRAT questions frequently test the vocabulary of PVC patterns. Memorize these:

A couplet is two PVCs in a row; when three or more fire consecutively, the rhythm is by definition a run of ventricular tachycardia. This linkage — three in a row equals VT — bridges directly into the next section.

The R-on-T phenomenon

The R-on-T phenomenon occurs when a PVC falls on the T wave of the preceding beat. The peak and downslope of the T wave correspond to the relative refractory period — the "vulnerable period" of repolarization, when the ventricles are electrically heterogeneous and only partially recovered. A stimulus delivered here can trigger a chaotic re-entry circuit, precipitating ventricular tachycardia or ventricular fibrillation. R-on-T is therefore considered one of the most dangerous PVC patterns and is a classic exam red flag.

When are PVCs clinically concerning?

Isolated, unifocal PVCs are common and often benign, especially in structurally normal hearts. The features that make PVCs ominous — sometimes called the "warning arrhythmias" — include:

• Frequent PVCs (often cited as more than six per minute)
• Multifocal PVCs (varying shapes)
• Couplets or runs (pairs, triplets, salvos)
• R-on-T PVCs
• PVCs occurring during acute myocardial ischemia or infarction

Patients often perceive PVCs as a skipped beat, a flutter, or a forceful "thud," because the compensatory pause lets the ventricle fill more fully and the following sinus beat is more forceful. In the acute ischemic patient these patterns may herald deterioration into lethal ventricular arrhythmias and warrant close monitoring and physician notification. The CRAT technician's job is not to diagnose or treat, but to accurately identify and document PVC frequency (for example, the count per minute), morphology (unifocal or multifocal), and any coupling pattern, and — per protocol — to alert the clinical team when these warning patterns appear.