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Answer to the Rhythm Puzzle
J. L. R. M Smeets
The ECG (electrocardiography) shows a sinus
bradycardia of 42 beats/min, a horizontal electrical heart axis and
a narrow QRS complex. The second to fourth QRS
complex is preceded by a P wave with a variation in the
PQ time. The PQ time in the second QRS complex is
220 ms, in the third 200 ms and the fourth 140 ms. No P
wave precedes the fifth QRS complex. In the sixth QRS
complex, the P wave is incorporated in the S wave of the
QRS complex. This pattern is suggestive for isorhythmic
The junctional rhythm (such as the fifth and sixth
complex in this ECG) is temporarily faster than the sinus
rhythm resulting in a pseudo-shortening of the PQ time.
Since both the junctional and the sinus rate are nearly
identical, there seems to be dissociation. However, during
prolonged registration of the ECG 1:1 AV conduction,
during a slightly faster sinus rhythm is present. This
rhythm should not be confused with complete AV block
when there is no relation between atrial and ventricular
Levy et al. have shown that a baroreceptor-initiated
feedback mechanism is operative in isorhythmic
dissociation. Elevated systolic pressure is the trigger of
baroreceptor discharge, which produces sinoatrial slowing .
In our patient with symptomatic bradycardia and
isorhythmic dissociation, we implanted a DDD pacemaker.
After implantation, he had no further complaints of
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