ecgsweep

Blocked (Non-Conducted) PAC

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

A blocked PAC after every sinus beat, halving the ventricular rate and imitating sinus bradycardia.

Updated

On the trace

Try each step, then check it.
  1. Read the rate first, and be suspicious of it.
    Show 42 bpm, regular. Everything about this says sinus bradycardia.
  2. Now look at the T waves, every one of them.
    Show Each carries a second hump on its tail. That is a premature P wave, and it is there after every single conducted beat, not occasionally.
  3. Check what follows those P waves.
    Show Nothing. No QRS after any of them.
  4. Count the atrial events, not the ventricular ones.
    Show The atria are firing at about 82 a minute, twice the ventricular rate. The atrium is not slow at all.
  5. Draw the conclusion the rate hid.
    Show This is not bradycardia. It is a normal sinus rate with every second atrial beat blocked, and the block is appropriate because those beats are far too early to conduct.

How to recognise it

FeatureValueNotes
Ventricular rateSlow and regular, 42 bpm hereThe appearance that starts the misdiagnosis.
Atrial rateAbout twice it, 82 hereNormal. Count the P waves, including the hidden ones.
The clueA premature P on every TNot on some. Every conducted beat carries one.
ConductionAlternate beats blockedThe blocked impulses arrive during the node's refractory period.
QRSNarrow and normalNothing is wrong below the atria.

The strip

Thirty seconds of blocked atrial bigeminy in lead II, drawn by the simulator. Work through the steps above on it: take the rate, then look closely at every T wave.

Mechanism

An ectopic focus fires early after each sinus beat, at a fixed coupling interval, exactly as in ordinary atrial bigeminy. The difference is only the timing: these ectopics arrive early enough that the AV node is still refractory, so none of them conducts. Half the atrial beats therefore produce no ventricular beat, and the ventricular rate is half the atrial one.

The regularity is what makes it convincing. Because the coupling is fixed, the conducted beats are evenly spaced, and an evenly spaced slow rhythm reads as sinus bradycardia rather than as anything dropped.

Clinical impact

An ectopic fires after every sinus beat, too early to conduct
Only the sinus beats reach the ventricles, halving the rate
The sinus node and the AV node are both behaving normallyBenign
Read as sinus bradycardia or AV block, it has led to pacemaker implantationOvertreatment

A patient may genuinely be symptomatic from a rate in the forties. That is an argument for suppressing the ectopy or removing its trigger, not for pacing a healthy conduction system.

Management

1

Count the P waves before treating the rate

If the atrial rate is normal and only alternate beats conduct, the bradycardia is apparent rather than real. This single step is what the case reports exist to encourage.

2

Remove what is driving the ectopy

Stimulants, alcohol, electrolyte disturbance, thyroid disease. The rate follows the ectopic burden.

3

Withhold the pacemaker

Pacing treats a conduction system that has failed. Here it has not. Ablation of the ectopic focus has been used successfully in refractory symptomatic cases.

Differential

Sinus Bradycardia

The rhythm this imitates. In true sinus bradycardia the T waves are ordinary and the atrial rate is genuinely slow. Here the T waves are deformed and the atria are going twice as fast as the ventricles.

2:1 AV Block

Also halves the ventricular rate, and also hides a P in a T. The difference is timing: in 2:1 block the non-conducted P arrives on schedule, whereas the blocked PAC is premature.

Atrial Bigeminy

The same pattern with ectopics late enough to conduct. The rate stays normal because nothing is dropped.

The full pictureBlocked (Non-Conducted) PACHow a blocked PAC works, and how to find the premature P hiding in the T, are covered in the base rhythm.

References

  1. Blocked Atrial Bigeminy as an Unusual Cause of Bradycardia: A Case Report — International Medical Case Reports Journal, 2025
  2. 2019 ESC Guidelines for the management of patients with supraventricular tachycardia — European Heart Journal, 2020