Short PR-interval and normal QRS-complex

ECG: Sinus rhythm with a rate of 78 bpm; fixed and short (115 ms) PR-interval; normal QRS pattern;

Comments: This patient presents an ECG combining a short PR-interval (<120 ms) and a QRS-complex of normal duration and morphology (with no delta wave). In 1952, Lown, Ganong and Levine described a syndrome coupling this electrocardiographic pattern with the occurrence of tachycardia. Ensuing studies did not allow identifying a unique cause for this type of presentation but rather found various pathophysiological mechanisms potentially explaining the morphology of a short PR without any changes in QRS-complex:

• Accelerated atrioventricular conduction accelerated, with physiological properties of decremental conduction remaining preserved; electrophysiological study reveals a short AH interval (explaining the short PR) but also a normal and physiological Wenckebach point (preserved decremental conduction and progressive prolongation of the PR-interval with increase in rate); this type of presentation is observed in infants or children but also in approximately 2% of adults; it is therefore a variant of the norm;
• Accelerated nodal conduction with loss of decremental conduction properties: in this instance, atrioventricular conduction is very rapid during an atrial arrhythmia episode with an increased risk that the rhythm degenerates into ventricular fibrillation; electrophysiological study reveals a short AH interval and a very high Wenckebach point (the PR remains very short for high rates, which explains the possibility of very rapid conduction during atrial arrhythmia);
• Presence of specific fibers which shunt the nodal filter (short PR) with no direct connection between atrial fibers and ventricular myocardial fibers (the QRS-complex remains normal, ventricular depolarization is achieved exclusively via the His-Purkinje system); this may involve atrionodal fibers called James fibers (direct connection between atrium and lower aspect of the atrioventricular node) or atrio-Hisian fibers i.e. Brechenmacher fibers (direct connection between atrium and bundle of His);
•  While the mechanisms explaining the short PR pattern are varied, this is also the case for the various types of tachycardias explaining the palpitations observed in these patients with evidence of episodes of junctional tachycardias, atrial arrhythmias and rare ventricular tachycardias, depending on the patients;

Take-home message: Various mechanisms can explain the electrocardiographic pattern of a short PR-interval with normal QRS (without pre-excitation): accelerated nodal conduction with or without presence of specific fibers that originate from the atrium, shunt the nodal filter and terminate at the atrioventricular node or bundle of His.