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The Normal Effects of Epinephrine and Isoproterenol on Heart Rate and QT Interval: Role in Unmasking the Long QT Syndrome

The Long QT syndrome (LQTS) describes a group of inherited or acquired abnormalities in cardiac repolarization associated with syncope and sudden death due to polymorphic ventricular tachycardia (torsade de pointes) and ventricular fibrillation.1 Recent studies have demonstrated mutations in cardiac ion channels that lead to functional changes in repolarization and risk of ventricular arrhythmias.2 The most common forms of inherited LQTS, LQT1, and LQT2 are related to mutations in the genes KVLQT1 coding for I Ks and HERG coding for I Kr, respectively, resulting in loss of function of a potassium channel. LQT3 results from a mutation in the gene SCN5A coding for the sodium channel (I Na), which results in a gain in function.3 The phenotypic expression of the ion channel dysfunction results in a prolonged QT interval (QTc > 460 msec) in approximately 64% of patients with LQT1, 81% of patients with LQT2, and 90% with LQT3.4 This phenotypic expression can be modulated by the autonomic nervous system such that changes in QT interval, T wave morphology, and symptoms may occur associated with exertion (LQT1), fright (LQT2), bradycardia (LQT2), or during sleep (LQT3).5 Holter monitors, exercise tests and, more recently, catecholamine infusions have been used by clinicians to support the diagnosis of LQTS, especially if the baseline ECG is normal. With these techniques, failure to shorten QT interval with exercise, development of pathologic QT intervals during 24-hour ECG recordings, and stimulation of excessive QT intervals or polymorphic ventricular tachycardia with catecholamine infusion have aided in the identification of patients and family members at risk.6-9 Genotype testing of individual patients can be done at special centers but is not widely available.10 In 30% of patients with clinical findings of LQTS, no responsible mutation is identified. In LQT1, nearly one third of patients with a positive genotype may have a normal resting ECG and QT intervals.4 There are smaller numbers of patients with latent ECG (normal QT interval) findings associated with the LQT2 and LQT3 genotypes. Several investigators have reported the clinical utility of graded epinephrine infusion studies in patients and families with LQTS. Initial reports by Ackerman et al.11 and later by