title: Differences in heart rate variability between cardioinhibitory and vasodepressor responses to head-up tilt table testing.
abstract: BACKGROUND
  Patients with syncope show different responses to head-up tilt (HUT) test, which may be due to different pathophysiological mechanisms.
  
  
  METHODS
  HUT (70 degrees) was performed in 24 patients who experienced recurrent syncope. Nine patients had a cardioinhibitory (CI) response, 7 patients had a vasodepressor (VD) response, and 8 patients had a mixed (MX) response. Heart rate variability was analyzed at 60-sec periods during HUT.
  
  
  RESULTS
  Total spectrum (TS) was greater at rest and 1 min after syncope in the CI and MX groups as compared to the VD group. Low frequency spectrum (LF) was significantly greater during rest and the first minute after syncope in the CI groups as compared with the VD group. After the rest period, the CI and MX groups showed more elevated high frequency spectrum (HF) values than the VD group (p < 0.01). One minute after syncope, the HF increased in the CI and MX groups but not in the VD group (p < 0.01). The VD group showed higher LF/HF ratio from the beginning of rest (3.9 +/- 4.1) as compared to the CI and MX groups (p < 0.01). This difference was most significant 2 min before syncope occurred. The CI and MX groups showed greater pNN50 and rMSSD as compared to the VD group.
  
  
  CONCLUSIONS
  Our results suggest that vagal tone is higher in subjects showing cardioinhibitory and mixed responses to HUT. In contrast, patients with a vasodepressor response showed predominantly sympathetic activity. These findings suggest that there are different pathophysiological mechanisms underlying syncope.
authors.affiliation: Departamento de Electrofisiología, Instituto Nacional de Cardiología Ignacio Chávez, México, D.F., Mexico.
authors.forename: C E
authors.initials: CE
authors.lastname: Guzmán
authors.forename: G M
authors.initials: GM
authors.lastname: Sánchez
authors.forename: M F
authors.initials: MF
authors.lastname: Márquez
authors.forename: A G
authors.initials: AG
authors.lastname: Hermosillo
authors.forename: M
authors.initials: M
authors.lastname: Cárdenas
mesh_terms: D000293:Adolescent; D000328:Adult; D001341:Autonomic Nervous System / Q000503:physiopathology*; D002648:Child; D005260:Female; D006321:Heart / Q000294:innervation*; D006339:Heart Rate / Q000502:physiology*; D006801:Humans; D007022:Hypotension / Q000503:physiopathology; D008297:Male; D008875:Middle Aged; D013575:Syncope / Q000209:etiology*; D018667:Tilt-Table Test; D014664:Vasodilation / Q000502:physiology*