Atrial premature complexes (APCs)
Early beats that start somewhere in the atria other than the SA node and then reach the ventricles by the usual route.
Key ECG features
- ✓Beat arrives early
- ✓Premature P wave, often abnormally shaped
- •QRS resembles the patient's normal complexes
- ±Pause that is not fully compensatory
- ±Premature P hidden in the preceding T wave
- ±Non-conducted if very early: a P with no QRS
✓ required · • typical · ± may be present
ECG characteristics
A premature, often abnormally shaped P wave precedes an early QRS that resembles the patient's normal complexes - the giveaway is timing, not shape. The premature P may be hidden in the preceding T wave, and the beat usually resets the sinus node, giving a pause that is not fully compensatory. A very early one can find the AV node refractory and fail to conduct at all, leaving a P with no QRS.
This illustration is schematic. Dashed line is a normal rhythm. The grid is not drawn to scale.
Common causes
Atrial enlargement or stretch from structural heart disease is the usual substrate. They also occur with atrial inflammation, systemic illness, and high sympathetic tone.
Why it matters
Because the QRS looks normal they are subtler than ventricular premature complexes and easy to miss. Frequent ones can mark atrial disease and may foreshadow atrial fibrillation.
Differentiate from
- Ventricular premature complexesBoth are premature. This one is preceded by a premature P wave and its QRS looks like the patient's normal complexes; a ventricular premature complex is wide with no preceding P.
- Atrial fibrillationFrequent atrial ectopy can look irregularly irregular; the discrete premature P wave in front of each early beat is the difference.
- Sinus pause & arrestA very early atrial beat can fail to conduct and leave a pause. Look inside the preceding T wave for the hidden premature P - a sinus pause has no P at all.