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Patient
15-year-old girl admitted for palpitations;
Trace
In the initial part of the tracing, there is a sinus rhythm with a normal PR interval, a narrow QRS (presence of a septal q wave in V6) and aspecific repolarization disorders; following slowing of the heart rate, the appearance of ventricular pre-excitation with short PR, delta wave and repolarization disorders; the ST segment depression and the quasi-systematic inversion of the T wave in all leads relative to the initial tracing can be attributed to the modification of activation and to the appearance of pre-excitation;
Exergue
Conduction in a bundle of Kent is achieved according to the all-or-nothing principle. The danger of this type of accessory pathway resides in the presence of a short refractory period. An intermittent pattern in sinus rhythm (onset during a bradycardia) is indicative of a long refractory period and of its benign nature.
This young patient has an accessory pathway with intermittent anterograde conduction. Conduction in an accessory pathway operates on the all-or-nothing principle: the bundle is either in a refractory period or not, and the impulse is or is not transmitted to the ventricles.