Short QT syndrome pathophysiology
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]Sumanth Khadke, MD[2]
Overview
Short QT syndrome types 1-3 are due to increased activity of outward potassium currents in phases 2 and 3 of the cardiac action potential. This causes a shortening of the plateau phase of the action potential (phase 2), causing a shortening of the overall action potential, leading to an overall shortening of refractory periods and the QT interval. In the families afflicted by short QT syndrome, two different missense mutations have been described in the human ether-a-go-go gene (HERG). These mutations result in the expression of the same amino acid change in the cardiac IKr ion channel. This mutated IKr has increased activity compared to the normal ion channel, and would theoretically explain the above hypothesis. Short QT syndrome types 4 and 5 are due to abnormalities in the calcium channel.
Pathophysiology
Genetics
In the families afflicted by short QT syndrome, mutations have been described in three genes, KvLQT1, the human ether-a-go-go gene (HERG), and KCNJ2. Mutations in the KCNH2, KCNJ2, and KCNQ1 genes cause short QT syndrome. These genes provide instructions for making proteins that act as channels across the cell membrane. These channels transport positively charged atoms (ions) of potassium into and out of cells. In cardiac muscle, these ion channels play critical roles in maintaining the heart's normal rhythm. Mutations in the KCNH2, KCNJ2, or KCNQ1 gene increase the activity of the channels, which changes the flow of potassium ions between cells. This disruption in ion transport alters the way the heartbeats, leading to the abnormal heart rhythm characteristic of short QT syndrome. Short QT syndrome appears to have an autosomal dominant pattern of inheritance.
Due to the autosomal dominant inheritance pattern, individuals may have family members with a history of unexplained or sudden death at a young age (even in infancy), palpitations, or atrial fibrillation. The penetrance of symptoms is high in affected family members.
Pathogenesis
Mechanism of arrhythmia generation
In general, the action potential duration with normal repolarization depends on a delicate balance between the repolarizing currents acting on the myocardium. A decrease in inward depolarizing current like INa or ICa and an increase in the outward repolarizing current like Ito, IK1, IK-ATP, IACh, IKr, or IKs will lead to early repolarization of cardiac ventricles. This decreases the action potential duration and hence QT interval. The genetic studies reveal the cause of arrhythmia to be due to transmural dispersion of repolarization. Dispersion of repolarization and loss of refractoriness which serves as a substrate to initiate re-entry by promoting unidirectional block. Abrupt shortening of wavelength (calculated as a product of the refractory period and conduction velocity) of cardiac impulse helps in the maintenance of reentry. Present data on SQTS suggests shortening of action potentials is heterogeneous in epicardium or endocardial cells in comparison to subendocardial M cells. This eventually translates as an increase in transmural dispersion of repolarization and tall, positive T waves on ECG.
Associated Conditions
Conditions associated with [disease name] include:
- [Condition 1]
- [Condition 2]
- [Condition 3]
Gross Pathology
On gross pathology, [feature1], [feature2], and [feature3] are characteristic findings of [disease name].
Microscopic Pathology
On microscopic histopathological analysis, [feature1], [feature2], and [feature3] are characteristic findings of [disease name].