Short QT

ECG: Sinus rhythm with sinus bradycardia (51 bpm) under beta-blocker; PR measurement of 180 ms; relatively short QT-interval (350 ms) with slight elevation of the J point in V1, V2; the biological work-up revealed a moderate hypercalcemia;

Comments: A QT-interval is defined as short if it is less than 350 or 320 ms (depending on the definitions). There appears to be an increased risk of syncope and sudden death when the QT-interval is less than 300 ms.

A pattern of short QT can be observed in various clinical situations:

•  A metabolic disorder: hyperkalemia (short QT, tall and peaked T-waves, wide QRS, conduction disorders), hypercalcemia (short QT, tall T-waves, prolongation of the PR-interval, possible tachycardia);
• A pharmacological cause: a digitalis impregnation may cause a slowing of the QT-interval associated with a coved depression primarily in the lateral leads and with flattened or negative T-waves;
• Acidosis;
• Hyperthermia (investigation for signs of hyperkalemia);
• Congenital short QT syndrome, very rare (a few dozen cases reported in the literature), of recent discovery, associated with an increased risk of atrial and ventricular fibrillation; it is a genetically-transmitted channelopathy; the patients are typically young with absence of structural cardiac disease, the first clinical sign being the occurrence of sudden death in adults or children, syncope or palpitations in about a third of cases; electrophysiological study allows highlighting very short atrial and ventricular refractory periods explaining the increased occurrence of arrhythmias in both stages and the short QT pattern; the electrocardiogram reveals 1) a short QT-interval < 320 ms or a corrected QT-interval < 340 ms; 2) there is a failure of the QT to adapt in response to heart rate, the QT-interval remains consistently short even for a bradycardia; it can appear normal on exertion; 3) tall, peaked and symmetrical T-waves in right and mid precordial leads; 4) onset of the T-wave immediately after the S wave;

Take-home message: A QT-interval can be considered short when its duration is less than 350 ms, a value less than 300 ms being clearly pathological and strongly evocative of a metabolic disorder and, exceptionally, of a congenital short QT syndrome.