2:2Comprehensive Guide to EKG Premature Beats: PACs, PJCs, and PVCs

Premature beats are early depolarizations that interrupt the normal sinus rhythm of the heart.
These beats originate from various locations within the heart rather than from the primary pacemaker, the sinoatrial (SA) node.
While frequently benign in healthy individuals, their clinical significance depends on frequency, morphology, and the patient's specific clinical context.
Underlying pathology may be indicated if premature beats appear in specific patterns or high frequencies.
Normally, the SA node in the right atrium sets the heart's steady rhythm. A premature beat occurs when another part of the heart—the atria, the AV junction, or the ventricles—becomes "irritated" and fires an electrical signal before the SA node was scheduled to fire.
Analogy: A premature beat is essentially the heart "cutting in line," coming earlier than expected and causing a temporary irregularity in the rhythm.
Following a premature beat, the SA node usually resets itself, often resulting in a brief pause on the EKG before the next normal sinus beat appears.

PACs originate from an ectopic focus within the Atria, outside of the SA node.
On an EKG, a PAC appears earlier than the next expected sinus beat.
P Wave Morphology:
- A P wave is still present before the QRS complex because the impulse originates in the Atria.
- However, because it originates from an ectopic site rather than the SA node, the P wave will look different from a sinus P wave.
- Specific appearances may include peaked, flat, notched, or biphasic P waves.
QRS Complex:
- The QRS complex is usually narrow and normal-looking ( $< 0.12 \text{ s}$ or $120 \text{ ms}$ ).
- This occurs because once the impulse reaches the AV node, it follows the standard conduction pathway through the His-Purkinje system.
- This narrow appearance distinguishes PACs (and PJCs) from ventricular beats.
Non-compensatory Pause:
- PACs are characterized by a non-compensatory pause.
- The ectopic atrial impulse interrupts and resets the timing of the SA node.
- Because the SA node's internal clock is reset, the next sinus beat arrives earlier than it would have if the SA node had continued its original rhythm uninterrupted.
- This reset alters the overall timing of the rhythm, resulting in an R-to-R interval after the PAC that is longer than a normal sinus interval but does not perfectly compensate for the early beat.

PJCs are early beats originating in the Atrioventricular (AV) junction rather than the atria or ventricles.
P Wave Morphology:
- The P wave may be absent entirely if the impulse reaches the atria and ventricles simultaneously.
- The P wave may be inverted, particularly in leads $II$ , $III$ , and $aVF$ , due to retrograde depolarization of the atria.
- The P wave may appear before, during, or even after the QRS complex, depending on the exact location within the AV junction where the impulse originates.
QRS Complex:
- The QRS is typically narrow and normal ( $< 0.12 \text{ s}$ ).
- Conduction through the ventricles remains preserved via the His-Purkinje system once the impulse exits the AV junction.
Pause Distinction:
- PJCs are often followed by a non-compensatory pause, similar to PACs.
- The pause is generally shorter than the compensatory pause seen with PVCs.

PVCs are early beats originating in the ventricles, below the normal conduction system.
Timing: Like other premature beats, they occur sooner than the next expected sinus beat.
P Wave Morphology:
- There is typically no P wave before the QRS complex because the impulse does not start in the atria.
- Occasionally, a retrograde P wave may appear after the QRS complex if the impulse travels backward into the atria.
QRS Complex Hallmark:
- The QRS is wide, defined as greater than $120 \text{ ms}$ or $0.12 \text{ s}$ .
- The complexes often look bizarre, distorted, or wide and unusual compared to normal sinus beats.
T Wave Morphology:
- PVCs often exhibit discordant T waves, meaning the T wave points in the opposite direction of the main QRS deflection.
Compensatory Pause:
- PVCs are characterized by a compensatory pause.
- The PVC does not reset the SA node; the SA node keeps firing exactly on its original schedule.
- Because the SA node remains on schedule, the interval from the sinus beat preceding the PVC to the sinus beat following the PVC measures exactly two normal R-to-R intervals ( $2 \times R\text{-}R$ ).
- The rhythm "catches back up" and stays in step with the pre-PVC timing.

The following terms describe the frequency and patterns of premature beats, most commonly applied to PVCs:
- Bigeminy: Every other beat is a PVC (a repeating 1:1 ratio of normal beats to PVCs).
- Trigeminy: Every third beat is a PVC (two normal beats followed by one PVC in a repeating pattern).
- Quadrigeminy: Every fourth beat is a PVC (three normal beats followed by one PVC in a repeating pattern).
- Multifocal PVCs: These occur when PVCs on the same EKG tracing look different from one another. This indicates the impulses are originating from multiple irritated sites in the ventricles, which is clinically more significant and carries a higher risk profile.

While many PVCs are harmless, four specific patterns are considered "red flags" that warrant attention:
1. Frequent PVCs: When they occur often throughout a tracing or over a 24-hour period. This can increase the risk of sustained arrhythmias and may lead to cardiomyopathy over time.
2. Consecutive PVCs: Specifically, "runs" of three or more PVCs in a row. This is classified as non-sustained ventricular tachycardia (NSVT) and is significantly more worrisome than isolated beats.
3. Multifocal PVCs: Multiple points of origin in the ventricles indicate higher myocardial irritability.
4. R-on-T Phenomenon: The most dangerous scenario occurs when a PVC falls directly on the T wave of the preceding beat. Because the ventricles are still in the repolarization phase (a vulnerable period), this early impulse can precipitate life-threatening arrhythmias such as ventricular tachycardia or ventricular fibrillation.

Premature Atrial Contraction (PAC):
- Origin: Atria (Ectopic focus).
- P Wave: Present, abnormal shape.
- QRS: Narrow ( $< 0.12 \text{ s}$ ).
- Pause: Non-compensatory.
Premature Junctional Contraction (PJC):
- Origin: AV Junction.
- P Wave: Inverted, absent, or retrograde (before/during/after QRS).
- QRS: Narrow ( $< 0.12 \text{ s}$ ).
- Pause: Non-compensatory.
Premature Ventricular Contraction (PVC):
- Origin: Ventricles.
- P Wave: Absent (before QRS).
- QRS: Wide ( $> 0.12 \text{ s}$ ), bizarre appearance.
- Pause: Compensatory.