title: Effect of isoproterenol on accessory pathways without overt retrograde conduction.
abstract: BACKGROUND
  Absence of overt retrograde accessory pathway conduction may be related to low resting sympathetic tone in patients with apparent unidirectional anterogradely conducting accessory pathways (UACAP).
  
  
  RESULTS
  To test this hypothesis, we studied the effect of isoproterenol on accessory pathway function and tachycardia induction in 18 patients (12 men and 6 women, ages 34 +/- 16 years [mean +/- SD]) with UACAP. After baseline study in the drug-free state, electrphysiologic testing was repeated during infusion of isoproterenol (0.5 to 1.5 micrograms/min, titrated to increase heart rate by 20%). Isoproterenol shortened the anterograde effective refractory period (398 +/- 117 vs 305 +/- 63 msec; P < 0.01; basic drive cycle length 600 msec) of the accessory pathway. However, retrograde accessory pathway conduction and atrioventricular reentrant tachycardia were exposed in only 3 (17%) patients by isoproterenol infusion. All 3 patients with retrograde accessory pathway revealed after isoproterenol had clinically documented tachycardia (supraventricular tachycardia in 2, atrial fibrillation in 1) during exercise, while none of the patients with persistent absence of retrograde accessory pathway conduction had this symptom.
  
  
  CONCLUSIONS
  We conclude that absence of overt retrograde conduction over accessory pathways may be related to low resting sympathetic tone in some individuals. Restoration of retrograde conduction with isoproterenol is unusual and most likely to be observed in patients with clinically documented paroxysmal supraventricular tachycardia related to exercise.
authors.affiliation: Department of Medicine, University of Western Ontario, London, Canada.
authors.forename: H G
authors.initials: HG
authors.lastname: Li
authors.forename: R
authors.initials: R
authors.lastname: Yee
authors.forename: G J
authors.initials: GJ
authors.lastname: Klein
mesh_terms: D000055:Accessory Nerve / Q000187:drug effects* / Q000502:physiology; D000293:Adolescent; D000328:Adult; D000368:Aged; D001283:Atrioventricular Node / Q000502:physiology; D002036:Bundle of His / Q000502:physiology; D004562:Electrocardiography; D015444:Exercise / Q000502:physiology; D005260:Female; D006801:Humans; D007545:Isoproterenol / Q000494:pharmacology*; D008297:Male; D008875:Middle Aged; D009431:Neural Conduction / Q000187:drug effects / Q000502:physiology*; D012183:Retrograde Degeneration / Q000502:physiology*; D013610:Tachycardia / Q000503:physiopathology; D013611:Tachycardia, Atrioventricular Nodal Reentry / Q000503:physiopathology; D017180:Tachycardia, Ventricular / Q000503:physiopathology
chemical_list: D007545:Isoproterenol