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Premature Atrial Contractions (PAC)

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

Every other beat is a premature atrial contraction, at a fixed coupling interval.

Updated

On the trace

The beat itself is an ordinary PAC: early, with an abnormal inverted P and a narrow QRS. What this strip adds is the pattern.

Try each step, then check it.
  1. Count in twos.
    Show Every second beat is the ectopic. Sinus, PAC, sinus, PAC, without a run of two sinus beats anywhere.
  2. Measure the coupling.
    Show 497 ms from each sinus beat to the PAC that follows it, the same every time. A fixed coupling interval is what a single focus firing off each sinus beat looks like.
  3. Measure the cycle before each ectopic.
    Show 1108 ms, because it contains the previous PAC's pause. The rhythm alternates long and short, which is why the pulse feels irregular even though nothing about it is random.
  4. Check the rate.
    Show About 75 bpm here. Every ectopic still conducts, so bigeminy does not slow the ventricles. That is what separates it from the blocked kind.

How to recognise it

FeatureValueNotes
PatternStrict alternationOne sinus beat, one PAC, repeating.
CouplingFixed, 497 ms hereConstant from beat to beat, unlike the variable coupling of multifocal ectopy.
Ventricular rateUnchanged, 75 bpm hereEvery beat conducts, so the count is normal even at a 50% ectopic burden.
P wavesAbnormal on every second beatInverted here, different from the sinus P.
QRSNarrow throughoutBoth the sinus and the ectopic beats.

The strip

Thirty seconds of atrial bigeminy in lead II, drawn by the simulator. Work through the steps above on it: count the beats in twos, then compare the two intervals that alternate.

Mechanism

One ectopic focus is firing in a fixed relationship to each sinus beat, so every sinus depolarisation is followed by an ectopic one at the same interval. Why a focus locks into this pattern rather than firing at random is not well settled, but the fixed coupling says the two are linked rather than independent.

Clinical impact

Half of all beats come from an ectopic atrial focus
Every one conducts, so the ventricular rate is normal
Palpitations, often described as a constant flutter rather than isolated skipsSymptoms
A sustained 50% ectopic burden is high, and burden is what carries the riskBurden

Management

1

Look for a reversible driver

Sustained bigeminy more often has a cause than isolated ectopy does: stimulants, alcohol, thyroid disease, electrolyte disturbance, hypoxia.

2

Quantify the burden

Ambulatory monitoring, because a rhythm strip cannot say whether this lasts minutes or all day, and the answer changes what follows.

3

Treat symptoms if they warrant it

A beta-blocker for troublesome palpitations. There is no indication to suppress the ectopy itself.

Differential

Atrial Trigeminy

The same thing in threes: two sinus beats to each ectopic, so a third of the beats rather than half.

Blocked Atrial Bigeminy

The dangerous mimic. If the bigeminal PACs are not conducted, the ventricular rate halves and the rhythm reads as bradycardia.

Ventricular Bigeminy

The ventricular version. No P before the ectopic, wide QRS, and a fully compensatory pause.

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