title: Sympathoexcitation increases the QT/RR slope in healthy men: differential effects of hypoxia, dobutamine, and phenylephrine.
abstract: BACKGROUND
  Dynamic ventricular repolarization assessed by QT/RR slopes studies the effects of modifications in cardiac repolarization independently of variations in RR interval (RR). The effects of changes in sympathetic and vagal activity on the QT/RR slope are controversial. We tested the hypothesis that sympathoexcitation is an important determinant of the QT/RR slope.
  
  
  RESULTS
  We compared the effects of a reflex sympathetic activation in response to hypoxia, to the direct effects of the infusion of the beta-adrenergic agent dobutamine, on the QTa (apex) and QTe (end)/RR slopes. Dobutamine was titrated to obtain similar increases in cardiac output than with hypoxia. Cardiac vagal activity was estimated by rMSSD and pNN50. In a second group of healthy subjects, we assessed the effect of a reflex cardiac vagal activation in response to phenylephrine infusion on the same variables. We observed a similar increase in QTa and QTe slopes during hypoxia and dobutamine (both P < 0.017 vs. normoxia), despite divergent changes in cardiac vagal activity, as rMSSD and pNN50 decreased with hypoxia compared to normoxia (P < 0.001) but increased during dobutamine infusion compared to hypoxia (P < 0.017). In contrast, these slopes did not change during the rises in rMSSD and pNN50 elicited by phenylephrine (P > 0.7).
  
  
  CONCLUSIONS
  Beta-adrenergic stimulation induces comparable increases in the QT/RR slopes than hypoxia, but in the presence of a larger cardiac vagal activity. Vagal cardiac activation by phenylephrine does not change the QT slopes. This reveals that the sympathetic system is an important determinant of QT/RR dynamicity in healthy men.
authors: Department of Cardiology, Erasme University Hospital, Brussels, Belgium. olivier.xhaet@Uclouvain.be
authors: Olivier
authors: O
authors: Xhaët
authors: Jean-Francois
authors: JF
authors: Argacha
authors: Atul
authors: A
authors: Pathak
authors: Marko
authors: M
authors: Gujic
authors: Anne
authors: A
authors: Houssiere
authors: Boutaïna
authors: B
authors: Najem
authors: Jean-Paul
authors: JP
authors: Degaute
authors: Philippe
authors: P
authors: Van de Borne
mesh_terms: D000328:Adult; D004280:Dobutamine / Q000494:pharmacology*; D006329:Heart Conduction System / Q000187:drug effects* / Q000502:physiology* / Q000503:physiopathology; D006801:Humans; D000860:Hypoxia / Q000503:physiopathology*; D008297:Male; D010656:Phenylephrine / Q000494:pharmacology*; D013565:Sympatholytics / Q000494:pharmacology*
chemical_list: D013565:Sympatholytics; D010656:Phenylephrine; D004280:Dobutamine