short QT syndrome

**Definition/ECG:** QTc ≤340 ms is diagnostic. QTc ≤360 ms plus a pathogenic mutation, family history of SQTS, family history of sudden death at age ≤40, or survival of a VT/VF episode also qualifies. The ECG often shows tall, peaked, symmetric T waves and a very short or absent ST segment. The QT interval does not adapt normally to changes in heart rate.

**Pathophysiology:** SQTS is caused by accelerated cardiac repolarization, which shortens the action potential duration and the effective refractory period. Short refractory periods and heterogeneous repolarization create a substrate for re-entry, causing AF and VT/VF. Inheritance is autosomal dominant. - Gain-of-function mutations in K⁺ channel genes are the most common cause: - SQT1: KCNH2 (hERG, I_Kr) - SQT2: KCNQ1 (I_Ks) - SQT3: KCNJ2 (I_K1) - Loss-of-function mutations in L-type Ca²⁺ channel genes (CACNA1C, CACNB2b) are rarer. They can produce an overlap with a Brugada-like ECG pattern.

**Clinical presentation:** Palpitations, syncope, and AF at a young age, including in infants. Sudden cardiac death can be the first manifestation, and SQTS is a possible cause of SIDS. The heart is structurally normal.

**Differential for a short QT (acquired causes):** - Hypercalcemia - Hyperkalemia - Acidosis - Hyperthermia - Digoxin effect (scooped ST segments with a shortened QT) - Increased catecholamines or sympathetic tone

Rule out these causes before diagnosing SQTS.

**Management:** - ICD is the treatment of choice for symptomatic patients and for survivors of cardiac arrest (secondary prevention). - Quinidine, a class Ia antiarrhythmic, blocks I_Kr and prolongs the QT. It is mainly effective in SQT1, and it is an option for patients who refuse or cannot have an ICD. - Sotalol and other class III agents are generally ineffective in SQT1 because the N588K hERG mutation reduces drug binding. - Screen first-degree relatives.

**Board pearls:** - Short QT is the opposite of long QT syndrome (loss-of-function K⁺ channel mutations, prolonged QT, torsades de pointes). - Think "short QT, young patient, AF or sudden death, normal heart" → gain-of-function K⁺ channelopathy → ICD. - Always exclude hypercalcemia in a patient with a short QT on the ECG.