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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 (Non-Conducted) PAC

An atrial ectopic fires so early that the AV node is still refractory and cannot conduct it. The result is a premature P wave with no following QRS — the most common cause of an unexpected pause.

Recognition· 4
  • Unexpected pause
  • Premature P superimposed on the previous T (bifid T)
  • No QRS after that P
  • Non-compensatory pause
Mechanism

PAC arrives while the AV node is still refractory — the impulse is not conducted

An atrial ectopic focus fires very early in the cycle. The impulse reaches the AV node during its absolute refractory period, so it cannot conduct to the ventricles. The atria depolarise (premature P wave, often hidden in the preceding T) but no QRS follows. The PAC usually resets the SA node, producing a non-compensatory pause.

Findings· 2
  • Look for a premature P wave superimposed on the preceding T wave, deforming it into a bifid/notched shape — then NO QRS, then a (usually non-compensatory) pause.
  • The hidden P on the T is the key clue that separates this from sinus arrest, SA exit block, 2:1 AV block, and junctional/ventricular escape.
Clinical Impact
Misdiagnosis as sinus arrest, SA exit block, or 2nd-degree AV block
↓
Patterned (bigeminal) blocked PACs mimicking sinus bradycardia
↓
Unnecessary pacemaker implantationRISK

Benign — a common cause of a 'skipped beat' that is easily mistaken for a block or bradycardia

Management· 2
1

Primary Treatment

No treatment needed.

2

Additional Options

Recognise the buried premature P on the T to avoid over-investigation.