1. Introduction

A premature atrial complex (PAC), also called a premature atrial contraction, atrial premature beat, atrial ectopic, or atrial extrasystole, is an early depolarization originating from an ectopic focus in the atrial myocardium rather than from the sinoatrial (SA) node. On the surface ECG, the key clue is an early P wave whose morphology differs from the patient's sinus P waves.

Ventricular activation usually proceeds through the normal atrioventricular (AV) conduction system, producing a QRS complex that is commonly narrow. Depending on the timing of the impulse and the refractory state of the AV node and His–Purkinje system, a PAC may conduct with bundle-branch aberrancy or fail to conduct to the ventricles.

2. Electrophysiology and mechanism

Pacemaker activity is not exclusive to the SA node. Atrial myocardial cells and subsidiary pacemaker tissue can generate spontaneous impulses. Under ordinary conditions, the faster sinus rhythm suppresses competing subsidiary activity. An ectopic atrial focus that fires before the next expected sinus impulse can depolarize the atria prematurely.

What happens after the ectopic impulse?

1. Atrial activation: The ectopic focus activates the atria along a pathway that differs from normal sinus activation. The surface P wave therefore has a different shape, axis, or polarity.
2. AV nodal conduction: If the AV node has recovered sufficiently, the impulse reaches the ventricles. The QRS is usually narrow when ventricular activation uses the His–Purkinje system normally.
3. Aberrant conduction: If part of the His–Purkinje system remains refractory, ventricular activation may be delayed and the QRS becomes wide, often with a right bundle branch block (RBBB)-type pattern.
4. Non-conduction: If the premature impulse reaches the AV node while it is refractory, it may not produce a QRS. The premature P wave can be concealed in the preceding T wave.
5. SA-node resetting: A PAC that penetrates and depolarizes the SA node may reset its timing. The next sinus beat is delayed, producing a pause that is commonly non-compensatory.

3. Characteristic ECG features

Interpret the premature beat by comparing it with the surrounding sinus beats. The most useful diagnostic information is the timing and morphology of atrial activity—not QRS width in isolation.

How to assess the P wave

• Identify the baseline sinus P-wave morphology in multiple beats and, where possible, multiple leads.
• Look for an early atrial deflection before the expected sinus P wave. It may be superimposed on the preceding T wave.
• Compare polarity and shape with sinus P waves. A different morphology supports an ectopic atrial origin.
• Assess the PR interval of the conducted PAC. It may differ from the sinus PR interval because the ectopic impulse enters the AV node at a different time and location.
• Do not use a single PR threshold as the sole discriminator between a PAC and a premature junctional complex (PJC). Interpret the atrial activation sequence and the relationship of P to QRS.

QRS morphology and conduction

A narrow QRS is typical when the premature atrial impulse conducts normally. A wide QRS does not automatically mean a premature ventricular complex (PVC): an early PAC can encounter a refractory bundle branch and conduct aberrantly. A visible premature P wave preceding the wide QRS is a key clue to supraventricular origin.

When the premature P wave is not followed by a QRS, consider a blocked PAC. Inspect the preceding T wave carefully; the ectopic P wave may be hidden within it.

4. A systematic approach to identifying a PAC

1. Confirm recording quality: Check calibration, paper speed, artifact, and whether the rhythm strip is long enough to compare several sinus cycles.
2. Establish the underlying rhythm: Identify baseline sinus P waves and determine the usual P–P and R–R intervals.
3. Find the early event: Look for an atrial deflection occurring before the next expected sinus P wave.
4. Compare P-wave morphology: Determine whether the premature P wave differs from the sinus P wave. Search the T wave if no separate P wave is obvious.
5. Follow the impulse: Is there a QRS after the premature P wave? If yes, assess QRS width and morphology. If no, consider a blocked PAC.
6. Assess the pause: Determine whether the next sinus beat is delayed and whether the sinus node may have been reset. Do not assume a fixed compensatory pause.
7. Classify the pattern: Describe isolated, frequent, unifocal, multifocal, bigeminal, trigeminal, couplet, or short-run ectopy as appropriate.
8. Correlate clinically: Relate ectopy to symptoms, medication and stimulant exposure, electrolyte status, ischemia, structural heart disease, and documented atrial arrhythmias.

5. Diagnostic criteria and classification

There is no universally required numerical cutoff for diagnosing an individual PAC on a standard ECG. Diagnosis is based on the premature timing and ectopic atrial morphology, together with the subsequent conduction pattern.

Unifocal and multifocal PACs

• Unifocal: PACs have a similar P-wave morphology, suggesting a common ectopic focus.
• Multifocal: PACs show more than one distinct P-wave morphology, supporting multiple atrial ectopic sites.

Common patterns

6. Differential diagnosis

Blocked PAC or AV block? A practical distinction

Do not diagnose second-degree AV block simply because a P wave is not followed by a QRS. First establish whether that P wave is on time. A premature, morphologically different P wave—often partly hidden in the T wave—strongly suggests a blocked PAC. In contrast, a non-conducted sinus P wave occurs at the expected sinus timing and has the usual sinus morphology. Review the full rhythm strip and clinical context before concluding that intrinsic AV conduction disease is present.

7. Aberrantly conducted PACs

A PAC may reach the ventricles while one bundle branch is still refractory from the preceding beat. The resulting intraventricular conduction delay produces a wide QRS. The source article notes that aberrancy commonly has an RBBB-type morphology, related to the relatively longer refractory period of the right bundle branch.

• Identify the premature P wave before the wide QRS, even if the P wave is small or best seen in only some leads.
• Compare the wide complex with baseline QRS morphology and assess whether it has a bundle-branch-block pattern.
• Look for other normally conducted PACs in the same tracing; these may help establish the atrial ectopic origin.
• Do not label a premature wide complex as PVC solely because it is wide.

8. Clinical presentation and significance

PACs may be asymptomatic or perceived as skipped beats, fluttering, or brief palpitations. The sensation may occur after the premature beat or during the subsequent pause. Symptoms do not reliably quantify ectopic burden.

Clinical context and associated conditions

• Frequent PACs may occur with anxiety, sympathomimetic exposure, beta-agonists, excess caffeine, hypokalaemia, hypomagnesaemia, digoxin toxicity, or myocardial ischemia, as listed in the supplied reference.
• Consider underlying atrial or structural disease, including left atrial enlargement and ischemic heart disease, when ectopy is frequent or new.
• PACs can trigger re-entry or other supraventricular tachyarrhythmias in susceptible patients, including AF, atrial flutter, AV nodal re-entrant tachycardia (AVNRT), and AV re-entrant tachycardia (AVRT).
• Higher PAC burden is associated with increased likelihood of subsequent AF in observational data and consensus discussions. This association does not mean that every patient with PACs will develop AF.

9. Clinical evaluation and management principles

Management depends on symptoms, frequency, associated disease, and whether a clinically important arrhythmia is documented. A single incidental PAC in an otherwise well patient generally does not require a treatment protocol.

Initial assessment

1. Assess symptoms, onset, duration, frequency, triggers, and associated chest pain, dyspnoea, presyncope, or syncope.
2. Review medications and substances, including sympathomimetics, inhaled beta-agonists, stimulants, and digoxin where relevant.
3. Assess for underlying cardiac disease and signs of hemodynamic compromise.
4. Use ambulatory ECG monitoring when symptom–rhythm correlation or quantification of ectopic burden is clinically important.
5. Consider targeted laboratory testing (for example, electrolytes and thyroid function) or echocardiography when the clinical presentation suggests a reversible cause or structural heart disease. Testing should be individualized rather than automatic for every isolated PAC.

Management approach

• Asymptomatic, isolated PACs: Reassurance is often appropriate after relevant clinical assessment.
• Potential triggers: Address relevant stimulant or sympathomimetic exposure and correct documented electrolyte abnormalities; investigate suspected ischemia or drug toxicity according to the clinical presentation.
• Persistent troublesome symptoms: Consider specialist-guided symptom management. Beta-blockers may be considered in selected symptomatic patients after reversible causes and relevant contraindications have been reviewed.
• High ectopic burden or suspected atrial disease: Consider further rhythm assessment and evaluation for structural heart disease. Specialist input may be appropriate when symptoms are substantial, ectopy is very frequent, or sustained arrhythmia is suspected.
• Documented AF or another tachyarrhythmia: Manage that arrhythmia according to its own guideline-based pathway; do not extrapolate AF treatment to isolated PACs.

Evidence note: The 2023 ACC/AHA/ACCP/HRS AF guideline describes PACs as potential triggers of AF and notes an association between PAC burden and AF development. The 2025 EHRA antiarrhythmic drug consensus discusses selected pharmacological options for symptomatic PACs; this is not a blanket recommendation to medicate all PACs.

10. Common pitfalls and limitations

• Missing a P wave in the T wave: A blocked PAC may appear as a peaked or notched T wave. Inspect the contour and timing before diagnosing sinus pause or AV block.
• Calling every wide premature beat a PVC: Search for a preceding ectopic P wave and assess for aberrant conduction.
• Assuming a fully compensatory pause: PACs may reset the SA node; pause duration is variable.
• Over-interpreting PR duration: PR timing alone does not reliably distinguish all PACs from PJCs. Analyze atrial activation and P–QRS sequence.
• Confusing blocked PACs with AV block: Establish whether the non-conducted P wave is premature and morphologically different from sinus P waves.
• Diagnosing AF from frequent ectopy: PACs can cause irregularity, but AF requires its own ECG criteria and visual confirmation.
• Relying only on automated interpretation: Automated systems may miss concealed P waves or misclassify aberrantly conducted PACs. Review the tracing directly.

11. ECG interpretation practice

The following are illustrative interpretation exercises based on the ECG patterns described in the supplied reference; they are not presented as independent patient cases.

Case 1 — Premature P wave hidden in the T wave

A rhythm strip shows a T wave with a new peaked contour. The next expected QRS does not occur, and a pause follows.

• First question: Is there an early atrial deflection superimposed on the T wave?
• Analysis: The early, non-sinus P wave is not followed by a QRS. The event is premature rather than an on-time sinus P wave.
• Interpretation: Blocked PAC, with a subsequent pause.
• Learning point: A concealed premature P wave can mimic a T-wave abnormality or second-degree AV block.

Case 2 — Premature wide QRS

A 12-lead tracing contains a premature wide complex. A small abnormal P wave can be identified before the complex in selected leads.

• Analysis: Establish that the atrial deflection is premature and precedes the wide QRS. Consider aberrant conduction through a partially refractory bundle branch.
• Interpretation: Aberrantly conducted PAC is favored over PVC when the preceding ectopic P wave is convincingly demonstrated.
• Learning point: QRS width alone does not establish ventricular origin.

Knowledge check

12. Quick reference and key takeaways

13. References