Blocked (Non-Conducted) PAC
Blocked (Non-Conducted) PAC
An atrial ectopic too early to conduct: a premature P hidden in the preceding T, no QRS, and an unexplained pause.
On the trace
Read the strip in this order.
- Find the pause.
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The rhythm runs at 833 ms between beats, and then one gap is 1334 ms. Something interrupted an otherwise regular sinus rhythm. - Do not look at the pause. Look at the T wave before it.
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That is where the answer is. The T of the beat preceding the pause is not the same shape as the others: it carries a second, later hump, peaking about 300 ms after the R. - Compare it with any other T on the strip.
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The ordinary T is a single smooth curve. This one is deformed, notched, taller at its tail. The extra deflection is a premature P wave superimposed on it. - Confirm that nothing follows it.
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No QRS. The atria depolarised and the impulse got no further. - Time the return.
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The next conducted beat comes 1334 ms after the last one, which is well under two sinus cycles. The blocked ectopic still reset the sinus node even though it never reached the ventricles.
How to recognise it
| Feature | Value | Notes |
|---|---|---|
| The pause | Unexpected, 1334 ms here | Interrupting an otherwise regular rhythm. |
| The clue | A premature P on the preceding T | Deforming it into a notched or bifid shape. Compare T waves across the strip. |
| Conduction | None | No QRS after that P. |
| Return cycle | Non-compensatory | The ectopic reset the sinus node on its way through the atria. |
| Everything else | Normal | Narrow QRS, normal PR, normal rate between events. |
The strip
Mechanism
The ectopic fires very early, earlier than the PACs that do conduct. The atria have recovered and depolarise, giving the premature P. The AV node has not: it is still absolutely refractory from the preceding sinus beat, so the impulse dies there. Nothing reaches the ventricles and no QRS is drawn.
The pause that follows is not a failure of anything. The sinus node was reset by the ectopic, like any PAC, and simply restarts its cycle. What makes the strip confusing is that the beat responsible is nearly invisible: it lands on the T wave of the beat before, where it is read as part of the T rather than as a P of its own.
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Why this is the most misdiagnosed pause in cardiology
Every alternative explanation for a sudden pause is more serious than the right one. Sinus arrest, sinoatrial exit block and second-degree AV block all imply disease of the conduction system; a blocked PAC implies nothing at all. The differences are visible only if you examine the T wave before the pause, which is not where the eye naturally goes: the eye goes to the empty space.
The practical rule is to make the comparison explicit. Put the T before the pause next to a T from a beat in the middle of the run. If they differ, the extra deflection is a P and the diagnosis is benign.
Why the P is so hard to see
A P wave is small, roughly 0.1 to 0.2 mV, and a T wave is several times larger. Superimposing the two produces a composite that still reads as a T, just a slightly wrong one. The earlier the ectopic, the deeper into the T it lands and the better it hides. The ectopics that are easiest to see are the ones late enough to conduct, which is to say the ones that are not blocked.
The pattern that becomes a bradycardia
When blocked PACs occur sporadically they produce isolated pauses. When they occur after every sinus beat, the ventricular rate halves and the strip reads as a regular bradycardia. That variant has its own article and its own literature, because it has led to pacemakers being implanted in patients whose conduction system was entirely normal.
What it is not
It is not a sign of AV nodal disease. The node refused an impulse that arrived during its absolute refractory period, which is the node working correctly. A node that drops impulses arriving at normal times is a different matter and a different diagnosis.
Clinical impact
The harm from a blocked PAC is almost entirely iatrogenic. The beat is benign; the misreading is not.
Management
Make the diagnosis on the T wave
Compare the T before the pause with the others. A notched or taller-tailed T carrying a premature P settles it without further investigation.
Do not pace a normal conduction system
A pause explained by a blocked PAC is not an indication for pacing, however long it looks. Exclude the mimics by finding the P, not by assuming it is absent.
Treat triggers only if the ectopy is frequent
Caffeine, alcohol, stimulants, electrolyte disturbance and thyroid disease, as for any atrial ectopy.
Reassure
No treatment is needed for the beat itself. Most patients only need to be told that the skipped beat they can feel is harmless.
Differential
The same beat after every sinus complex, which halves the ventricular rate and turns isolated pauses into an apparent bradycardia.
A true failure of the sinus node to fire. There is no premature P anywhere, and the T before the pause is an ordinary T.
A P wave that arrives on time and is not conducted. The blocked PAC's P is early and sits on the previous T; the Mobitz II P is on schedule and sits on the baseline.
The same ectopic arriving slightly later, when the node has recovered. It conducts, so there is a QRS and an early beat rather than a pause.
References
- Blocked Atrial Bigeminy as an Unusual Cause of Bradycardia: A Case Report — International Medical Case Reports Journal, 2025
- 2019 ESC Guidelines for the management of patients with supraventricular tachycardia — European Heart Journal, 2020