Overview
- Originate from an ectopic pacemaker cell within the atria
- There is typically an early, abnormal P wave that is followed by a normal QRS complex
- Also referred to as: Premature atrial complex, atrial ectopic, atrial extrasystole, atrial premature beat, atrial premature depolarization
Why do ectopic beats come about?
- There are pacemaker cells throughout the conduction system capable of depolarizing spontaneously
- The depolarization rate of these pacemaker cells decreases from the sinus node (fastest) to the ventricles (slowest)
- Ectopic impulses from other pacemaking cells are suppressed by more rapid impulses
- If an ectopic focus depolarizes prior to the arrival of the next sinus impulse, it may “capture” the ventricles, thereby producing a premature contraction
- Premature contraction are classified by their origin: atrial (premature atrial contraction, PAC), junctional (premature junctional contraction, PJC), ventricular (premature ventricular contraction, PVC)
ECG Features
- Abnormal (non-sinus) P wave followed by a normal QRS complex
- Because this is a non-sinus P wave, the morphology and axis will often be different from sinus P waves
- Sometimes the abnormal P wave, which is occurring earlier than expected (hence, “premature”), may be buried in the preceding T wave, resulting in a “peaked” or “camel-hump” appearance; if unnoticed, a premature atrial contraction may be mistaken for a premature junctional contraction
- Premature atrial contractions originating near the AV node (ie, “low atrial” ectopics) activate the atria retrogradely, thereby producing an inverted P waves with relatively short PR intervals (≥120 ms; if <120 ms, it is classified as a premature junctional contraction)
- Premature atrial contractions may depolarize the sinus node, causing it to “reset” and a subsequent longer than normal interval prior to the next sinus beat (ie, “post-extrasystolic pause”)
- Unlike premature ventricular contractions, the pause after a premature atrial contraction is not double the proceeding RR interval (ie, non-compensatory pause)
- Premature atrial contractions arriving earlier in the cycle may be conducted aberrantly – often with a right bundle branch block morphologic pattern (due to the longer refractory period of the right bundle branch compared to the left bundle branch); aberrantly conducted premature atrial contractions can be differentiated from premature ventricular contractions by the presence of a preceding P wave
- Premature atrial contractions arriving very early in the cycle may not be conducted to the ventricles; hence, an abnormal P-wave is present without a subsequent QRS complex (ie, “blocked premature atrial contraction”); this is usually followed by a compensatory pause as the sinus node resets
Classification
- Unifocal: arise from a single ectopic focus with a similar P wave morphology
- Multifocal: arise from two or more ectopic foci with different P wave morphologies
- Bigeminy: every 2nd beat is a PAC
- Trigeminy: every 3rd beat is a PAC
- Quadrigeminy: every 4th beat is a PAC
- Couplet: 2 consecutive PACs
- Triplet: 3 consecutive PACs
Causes
- Cardiac: myocardial ischemia
- Other: anxiety, caffeine
- Electrolyte abnormalities: hypokalemia, hypomagnesemia
- Medications: sympathomimetics, beta-agonists, digoxin toxicity
Clinical Significance
- Premature atrial contractions are a normal electrophysiologic phenomenon often not requiring investigation or treatment
- If they are frequent, they can cause palpitations and a feeling like the heart is “skipping a beat”
- Premature atrial contractions in patients with underlying substrate (eg, left atrial enlargement, Wolff-Parkinson-White syndrome, ischemic heart disease) may trigger the onset of a reentrant tachycarrhythmia (eg, atrial fibrillation, atrial flutter, AV reentrant tachycardia AV nodal reentrant tachycardia)
Example ECG
Patient: 67-year-old female presents with a feeling like her “heart is skipping a beat”
Interpretation: Sinus bradycardia, premature atrial complexes, otherwise normal
– Ventricular rate: 58 BPM
– PR interval: 180 ms
– QRS duration: 84 ms
– QT/QTc interval: 402/394 ms
– P-R-T axes: 45 -4 28
Key Points from Example ECG
- P wave occurs earlier than expected with a non-compensatory pause
- P wave morphology and axis different from sinus P wave
- QRS complex similar to underlying rhythm
- PR interval varies in PAC
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