The influence of cardiac autonomic activity on the QT-variability index in able-bodied and incomplete spinal cord injured individuals.
OBJECTIVE
To investigate, via autonomic blockade, if the QT-variability index (QTVI) is a measure of cardiac autonomic regulation in able-bodied (AB) and incomplete spinal cord injured (SCI) individuals.


METHODS
Four SCI (41.6±13.4years; C4-C7, AIS B-D, 13.4±13.4years post injury) and 4 AB (33.0±7.8years) individuals were tested. QTVI was determined from electrocardiographic readings obtained during supine rest and cardiovascular (CV) stress, with and without autonomic blockade. CV stress was induced by 40° head-up tilt, the hand submerged in 10°C water and the jaw clenched. Autonomic blockade was achieved with metoprolol (β-blockade) and atropine (cholinergic blockade).


RESULTS
There was no group×condition interaction for QTVI, although there was a significant main effect for condition. After collapsing across groups, QTVI increased with CV stress (p=0.01) and decreased with subsequent β-blockade (p=0.04), suggesting that during CV stress, QTVI is reflective of cardiac sympathetic activity. During supine rest, β-blockade did not change QTVI (p=0.24), however, cholinergic blockade increased QTVI (p<0.001), suggesting that during rest, QTVI is inversely related to cardiac parasympathetic regulation.


CONCLUSIONS
During times of CV stress, QTVI reflects cardiac sympathetic activity, while during resting conditions, QTVI is inversely related to cardiac parasympathetic activity. These relationships persist after autonomically incomplete SCI.
Department of Kinesiology, Brock University, 500 Glenridge Avenue, St. Catharines, Ontario ON L2S 3A1, Canada.
Hisham
H
Sharif
Department of Kinesiology, Brock University, 500 Glenridge Avenue, St. Catharines, Ontario ON L2S 3A1, Canada.
Lisa M
LM
Cotie
Department of Physical Therapy, Seton Hall University, South Orange, NJ, USA; VA RR&D National Center of Excellence for the Medical Consequences of Spinal Cord Injury, Bronx, NY, USA.
Michael F
MF
La Fountaine
Department of Kinesiology, Brock University, 500 Glenridge Avenue, St. Catharines, Ontario ON L2S 3A1, Canada. Electronic address: dditor@brocku.ca.
David S
DS
Ditor
D058671:Adrenergic beta-1 Receptor Antagonists; D000328:Adult; D000889:Anti-Arrhythmia Agents; D001285:Atropine; D001341:Autonomic Nervous System / Q000187:drug effects / Q000503:physiopathology*; D003430:Cross-Sectional Studies; D004562:Electrocardiography; D005260:Female; D006339:Heart Rate / Q000187:drug effects / Q000502:physiology*; D006801:Humans; D008297:Male; D008790:Metoprolol; D012146:Rest / Q000502:physiology; D013119:Spinal Cord Injuries / Q000503:physiopathology*; D013312:Stress, Physiological / Q000187:drug effects / Q000502:physiology; D016683:Supine Position / Q000502:physiology; D014630:Vagus Nerve / Q000187:drug effects / Q000503:physiopathology
D058671:Adrenergic beta-1 Receptor Antagonists; D000889:Anti-Arrhythmia Agents; D001285:Atropine; D008790:Metoprolol
Autonomic blockade; QTVI; Spinal cord injury