Table 1

Congenital long QT syndrome

Locus of genetic defect	Molecular defect	Functional manifestation of defect	Possible treatment
LQT-1: chromosome 11		KvLQT1 mutation reduces I_Ks		Increased APD; EAD triggers activity	• Increase I_Ks
					• β Blockers
					• Increase outward or decrease inward currents
LQT-2: chromosome 7		HERG mutation reduces I_Kr		Increased APD; EAD triggers activity	• Increase I_Kr
					• β Blockers
					• Increase outward or decrease inward currents
LQT-3: chromosome 3		SCN5A mutation increases I_Na		Increased APD; EAD triggers activity	• Block late opening Na channels with mexilitine or lignocaine
					• β Blockers
					• Increase outward currents
LQT-5: chromosome 21	KCNE1 protein (min K) mutation (ancillary subunit for I_Ks channel complex)	Increased APD; EAD triggers activity	• As yet unknown
Chromosome 4	As yet unknown	–	–

APD, action potential duration; EAD, early after depolarisation; I_Ks, slowly activating outward K current; I_Kr, rapidly activating delayed rectifier current; LQT, long QT; SCN5A, gene for the cardiac Na channel.