Pediatric emergency medicine trisk 1505 1505

when the patient is at rest is a key to the diagnosis. The upper limits of the
corrected QT interval (QTc ) (using Bazett formula: QTc = (QT)/√RR interval)
are <450 msec, and a QTc >480 msec may be suggestive of LQTS in a patient
presenting with syncope. There are congenital and acquired causes for a
prolonged QT interval.
The genetic arrhythmia syndromes consist of three subtypes: LQT1, LQT2, and
LQT3. The disease occurs due to mutations in genes that encode for cardiac ion
channels important in ventricular repolarization. Syncope in these patients is
related to polymorphic ventricular tachycardia (torsades de pointes) and death is
due to ventricular fibrillation. One form (LQT3) can also be associated with
bradycardia and slow heart rates that may cause syncope. Most cases are
associated with an autosomal dominant form of the syndrome (i.e., Romano–
Ward syndrome), which shows variable penetrance. LQT1 is the most common
genetic subtype, and the triggers for syncope or sudden death in affected patients
include emotional or physical stress, such as diving and swimming. One form of
LQT1, the Jervell and Lange-Nielsen syndrome, is autosomal recessive and
associated with a high risk of sudden death and congenital deafness. In LQT2,
syncope or sudden death can occur with stress or at rest. Cardiac events triggered
by sudden loud noises, such as the ringing of an alarm clock, are virtually
pathognomonic for this condition. Acquired causes for a prolonged QTc interval
include hypocalcemia, hypokalemia, hypomagnesemia, hypothyroidism, eating
disorders, and the use of drugs that prolong QT interval (e.g., haloperidol,
methadone, pentamidine, and sotalol).