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Blocked (Non-Conducted) PAC

normal
Lead II
25 mm/s10 mm/mV
HR
72
bpm
RR
833
ms
QT
358
ms
Type

Blocked Atrial Bigeminy

A repeating pattern where every sinus beat is followed by a premature atrial contraction that is blocked at the AV node. Because only the sinus beats conduct, the ventricular rate falls to roughly half — producing a deceptively regular, slow rhythm.

Recognition· 4
  • Regular slow rhythm (looks like sinus brady)
  • A premature P on every conducted T (bifid T)
  • Those P waves are not conducted (no QRS)
  • Atrial rate is ~twice the ventricular rate
Mechanism

Every sinus beat triggers an early PAC that is blocked — so only half the atrial beats conduct

An atrial ectopic focus fires early after each sinus beat, landing on the T wave while the AV node is still refractory. These PACs are blocked, so no QRS follows. Because the pattern repeats every beat, the effective ventricular rate is about half the atrial rate, mimicking a bradyarrhythmia.

Findings· 3
  • Each conducted beat's T wave carries a superimposed premature P (bifid/notched T) that is not followed by a QRS.
  • The result is a regular, slow ventricular rhythm — easily mistaken for sinus bradycardia, SA exit block, or 2:1 AV block.
  • The clue is the extra P deforming every other T.
Clinical Impact
Mistaken for sinus bradycardia / SA exit block / 2:1 AV block
↓
Unnecessary pacemaker implantation
↓
Symptoms from the slow ventricular rateRISK

Benign bradycardia mimic — don't rush to a pacemaker

Management· 2
1

Primary Treatment

No specific treatment for the ectopy.

2

Additional Options

Recognise the blocked P waves; address triggers (stimulants, electrolytes) if frequent.