ecgsweep

Premature Atrial Contractions (PAC)

normal
Lead II
25 mm/s10 mm/mV
HR72bpm
RR833ms
QT358ms
Type

Every third beat is a premature atrial contraction: two sinus beats, then an ectopic, repeating.

Updated

On the trace

Try each step, then check it.
  1. Count in threes.
    Show Two sinus beats, then one ectopic, then the group repeats. The pattern holds across the whole strip.
  2. Measure the coupling.
    Show 497 ms from the second sinus beat to the PAC, fixed every time, exactly as in bigeminy. The pattern differs; the beat does not.
  3. Find the group boundary.
    Show The long cycle is the one containing the pause after each ectopic. Two ordinary sinus cycles then follow, which is what gives the pulse its repeating three-beat feel.
  4. Check the rate.
    Show About 74 bpm. Every ectopic conducts, so the ventricular count is normal.

How to recognise it

FeatureValueNotes
PatternRepeating groups of threeTwo sinus beats to each PAC.
Ectopic burdenOne beat in threeHalf that of bigeminy.
CouplingFixed, 497 ms hereConstant, as in bigeminy.
Ventricular rateUnchanged, 74 bpm hereEvery beat conducts.
QRSNarrow throughoutThe ectopic conducts by the normal route.

The strip

Thirty seconds of atrial trigeminy in lead II, drawn by the simulator. Work through the steps above on it: count the beats in threes, then measure the coupling and the pause.

Mechanism

The same single focus as in bigeminy, locked to the sinus rhythm at a longer interval: it fires once for every two sinus beats rather than once for every one. Whether a patient shows bigeminy or trigeminy at a given moment often shifts with autonomic tone and rate, and the two commonly alternate on the same recording.

Clinical impact

An ectopic atrial focus fires once every three beats
Every beat conducts, so the ventricular rate is normal
Often unnoticed, or felt as a regular stumbleSymptoms
A third of beats is still a substantial ectopic load if it is sustainedBurden

Management

1

Look for a reversible driver

The same list as any frequent atrial ectopy: stimulants, alcohol, thyroid disease, electrolytes, hypoxia.

2

Quantify the burden

Ambulatory monitoring, since a strip cannot say how much of the day this occupies.

3

Reassure, and treat only symptoms

No indication to suppress the ectopy. A beta-blocker if palpitations are genuinely troublesome.

Differential

Atrial Bigeminy

The same beat every second cycle rather than every third, so twice the burden.

Ventricular Trigeminy

The ventricular version: no P in front of the ectopic, wide QRS, compensatory pause.

Blocked (Non-Conducted) PAC

If the ectopic is not conducted there is no QRS at all, and the rhythm gains a pause instead of an early beat.

The full picturePremature Atrial Contractions (PAC)The beat itself, the non-compensatory pause and what the burden means are covered in the base rhythm.

References

  1. Frequent premature atrial complexes as a predictor of atrial fibrillation: Systematic review and meta-analysis — Journal of Electrocardiology, 2018
  2. 2024 ESC Guidelines for the management of atrial fibrillation developed in collaboration with the EACTS — European Heart Journal, 2024