title: Impaired baroreflex gain in patients with inappropriate sinus tachycardia.
abstract: BACKGROUND
  The aim of this study was to determine the characteristics of heart rate variability (HRV), blood pressure variability (BPV), and baroreflex gain (BRG) at rest and during orthostatic stress in patients with clinical criteria of inappropriate sinus tachycardia (IST).
  
  
  RESULTS
  Beat-to-beat HRV and BPV, measured by time- and frequency-domain methods, and noninvasive BRG, calculated by cross-spectral analysis, were obtained during 10 minutes both at rest and during the stabilization phase (5-15 min) of orthostatic stress at 60 degrees in 8 patients with clinical criteria of IST and 9 healthy volunteers (CON). IST patients had a higher resting mean heart rate (78.8 +/- 5.3 vs 58.5 +/- 4.2 beats/min, P=0.01) and mean blood pressure (90.4 +/- 2.4 vs 72.0 +/- 4.2 mmHg; P=0.002). RMSSD, pNN50m, and BRG were significantly reduced in IST patients at rest. BRG during orthostatic stress (7.2 +/- 0.8 vs 20.3 (2.4 ms/mmHg, P <0.01) was significantly reduced in IST patients. Delta BRG (-16.9%+/- 11 vs -50.1%+/- 5, P=0.02) was markedly blunted during orthostatic stress in IST patients.
  
  
  CONCLUSIONS
  BRG was markedly impaired both at rest and during orthostatic stress in IST patients. This alteration may be responsible for the higher resting heart rate and mean blood pressures seen at rest and may facilitate tachycardia during orthostatic stress. A primary alteration in sinus node automaticity coupled with impaired BRG determines heart rate response to orthostatic stress in patients with IST.
authors.affiliation: Autonomic Physiology Laboratory, Research Institute, Fundacion Cardiovascular de Colombia, Bucaramanga, Santander, Colombia.
authors.forename: Hernando
authors.initials: H
authors.lastname: Leon
authors.forename: Juan Camilo
authors.initials: JC
authors.lastname: Guzman
authors.forename: Tom
authors.initials: T
authors.lastname: Kuusela
authors.forename: Rejane
authors.initials: R
authors.lastname: Dillenburg
authors.forename: Mark
authors.initials: M
authors.lastname: Kamath
authors.forename: Carlos A
authors.initials: CA
authors.lastname: Morillo
mesh_terms: D000222:Adaptation, Physiological; D000328:Adult; D017704:Baroreflex; D001794:Blood Pressure; D004244:Dizziness / Q000150:complications / Q000175:diagnosis / Q000503:physiopathology; D005080:Exercise Test / Q000379:methods; D005260:Female; D006339:Heart Rate; D006801:Humans; D008297:Male; D011187:Posture; D013616:Tachycardia, Sinus / Q000150:complications / Q000175:diagnosis / Q000503:physiopathology*