Premature Atrial Complex (PAC) • LITFL • ECG Library Diagnosis

Atrial Ectopic and Premature Atrial Complex (PAC)

These arise from ectopic pacemaking tissue within the atria. There is an abnormal P wave, usually followed by a normal QRS complex.

AKA: Atrial ectopics, atrial extrasystoles, atrial premature beats, atrial premature depolarisations.

Groups of pacemaker cells throughout the conducting system are capable of spontaneous depolarisation.
The rate of depolarisation decreases from top to bottom: fastest at the sinoatrial node; slowest within the ventricles.
Ectopic impulses from subsidiary pacemakers are normally suppressed by more rapid impulses from above.
However, if an ectopic focus depolarises early enough — before the arrival of the next sinus impulse — it may “capture” the ventricles, producing a premature contraction.
Premature contractions (“ectopics”) are classified by their origin — atrial (PAC), junctional (PJC) or ventricular (PVC).

ECG Features of Premature Atrial Complex (PAC)

PACs usually have the following features:

An abnormal (non-sinus) P wave is followed by a QRS complex.
The P wave typically has a different morphology and axis to the sinus P waves.
The abnormal P wave may be hidden in the preceding T wave, producing a “peaked” or “camel hump” appearance — if this is not appreciated the PAC may be mistaken for a PJC.
PACS arising close to the AV node (“low atrial” ectopics) activate the atria retrogradely, producing an inverted P wave with a relatively short PR interval ≥ 120 ms (PR interval < 120 ms is classified as a PJC).
PACs that reach the SA node may depolarise it, causing the SA node to “reset” — this results in a longer-than-normal interval before the next sinus beat arrives (“post-extrasystolic pause”). Unlike with PVCs, this pause is not equal to double the preceding RR interval (i.e. not a “full compensatory pause”).
PACs arriving early in the cycle may be conducted aberrantly, usually with a RBBB morphology (as the right bundle branch has a longer refractory period than the left). They can be differentiated from PVCs by the presence of a preceding P wave.
Similarly, PACs arriving very early in the cycle may not be conducted to the ventricles at all. In this case, you will see an abnormal P wave that is not followed by a QRS complex (“blocked PAC”). It is usually followed by a compensatory pause as the sinus node resets.

Classification of Premature Atrial Complex (PAC)

PACs may be either:

Unifocal – Arising from a single ectopic focus; each PAC is identical.
Multifocal – Arising from two or more ectopic foci; multiple P-wave morphologies.

PACs often occur in repeating patterns:

Clinical Significance of Premature Atrial Complex (PAC)

PACs are a normal electrophysiological phenomenon not usually requiring investigation or treatment.
Frequent PACs may cause palpitations and a sense of the heart “skipping a beat”.
In patients with underlying predispositions (e.g. left atrial enlargement, ischaemic heart disease, WPW), a PAC may be the trigger for the onset of a re-entrant tachydysrhythmia — e.g. Atrial fibrillation, atrial flutter, AVNRT, AVRT.

Causes of Premature Atrial Complex (PAC)

Frequent or symptomatic PACs may occur due to:

ECG Examples

Example 1

This rhythm strip displays the typical pattern of frequent PACs (arrows) separated by post-extrasystolic pauses.

Example 2

Blocked PAC:

This hidden PAC gives a peaked appearance to the T wave (circled).
The PAC is not not followed by a QRS complex, indicating that it has not been conducted to the ventricles (“blocked PAC”).
It is followed by a compensatory pause.

Example 3

Normally and aberrantly-conducted PACs:

There is an aberrantly conducted PAC, best seen in aVL and aVF (circled).
This could be mistaken for a ventricular ectopic — however, it is clearly preceded by an abnormal P wave.
A normally-conducted PAC is also present on the rhythm strip (circled).

NB. The rhythm strip is not recorded simultaneously.

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