An irregular pulse at a pre-employment medical
The tracing
A 34-year-old man is seen for a pre-employment medical. He has no complaints, takes nothing, and cycles about 200 km a week. He has never blacked out, and has never noticed his heartbeat.
The nurse records a pulse of 60 and notes that it is irregular. He is otherwise well: blood pressure 118/70, heart sounds normal, chest clear.
What is going on here?
Answer
Show the answer and explanation
Second-degree atrioventricular block, Mobitz type I — Wenckebach conduction, here in a 4:3 ratio.
- March out the P waves. They are regular at about 78 a minute, and they stay regular through the pauses. The atria are doing nothing unusual.
- Now measure the PR interval of each beat in turn. It lengthens: roughly 135 ms, then about 200, then about 270.
- Then a P wave arrives and is followed by nothing at all.
- After the pause, the next PR is short again and the sequence starts over. Four P waves, three conducted beats, repeating.
- The QRS is 79 ms and the axis is +54. Both normal.
Two consequences of that pattern are worth pointing out, because they are what let you recognise it across a room. The first is group beating: the complexes cluster, and the clusters repeat. An irregular pulse that repeats itself is not a random irregularity. The second is that the pause is shorter than two P–P intervals, because the increment in PR shrinks even as the PR itself grows — so the R–R intervals shorten across the group before the drop.
This tracing was generated, not recorded. It opens in the studio with every dial live.
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