Autonomic tone attenuates drug-induced QT prolongation.
BACKGROUND
The QT interval is a predictor of sudden death. Many drugs prolong the QT, primarily through I(Kr) block. Autonomic tone directly affects heart rate and ventricular repolarization, but its effects in the setting of I(Kr) block are unknown.


OBJECTIVE
Determine the effects of autonomic tone on heart rate and QT interval after I(Kr) block.


RESULTS
Healthy adults (n = 9) were administered ibutilide with ECGs obtained at regular intervals. On a separate day, subjects were administered intravenous propranolol and atropine to induce autonomic block, resulting in intrinsic heart rate and QT interval; ibutilide was administered again, with serial ECGs obtained at regular intervals. No differences in baseline RR, QT, or QTc intervals were seen between the two study days. Ibutilide in the setting of intact autonomic tone prolonged QTc by 43 +/- 8 msec (P = 0.001) and prolonged RR interval by 93 +/- 39 msec (P = 0.04). Autonomic block alone on the second day prolonged QTc by 16.7 +/- 9.4 msec (P = 0.02). An additional 68 +/- 5 msec prolongation of QTc was seen with ibutilide in the setting of double autonomic block (P = 0.036). There was no effect of ibutilide on intrinsic heart rate (P = 0.125).


CONCLUSIONS
Autonomic block results in an exaggeration of drug-induced QT prolongation by ibutilide. I(Kr) block has no effect on intrinsic heart rate. QT-prolonging drugs may have more deleterious effects in those with underlying autonomic dysfunction, thus increasing the risk for sudden death.
Division of Pediatric Cardiology, Vanderbilt Children's Hospital, Nashville, Tennessee 37232, USA. Andrew.Smith@vanderbilt.edu
Andrew H
AH
Smith
Kris J
KJ
Norris
Dan M
DM
Roden
Prince J
PJ
Kannankeril
D000889:Anti-Arrhythmia Agents / Q000008:administration & dosage* / Q000009:adverse effects; D001285:Atropine / Q000008:administration & dosage / Q000009:adverse effects; D001340:Autonomic Nerve Block / Q000379:methods; D001341:Autonomic Nervous System / Q000187:drug effects* / Q000503:physiopathology*; D004305:Dose-Response Relationship, Drug; D004338:Drug Combinations; D004562:Electrocardiography / Q000187:drug effects*; D005260:Female; D006329:Heart Conduction System / Q000187:drug effects* / Q000503:physiopathology*; D006339:Heart Rate / Q000187:drug effects*; D006801:Humans; D008133:Long QT Syndrome / Q000139:chemically induced / Q000503:physiopathology; D008297:Male; D008875:Middle Aged; D011433:Propranolol / Q000008:administration & dosage / Q000009:adverse effects; D013449:Sulfonamides / Q000008:administration & dosage / Q000009:adverse effects
D000889:Anti-Arrhythmia Agents; D004338:Drug Combinations; D013449:Sulfonamides; C067192:ibutilide; D001285:Atropine; D011433:Propranolol