Hisham Sharif1, Lisa M Cotie1, Michael F La Fountaine2, David S Ditor3. 1. Department of Kinesiology, Brock University, 500 Glenridge Avenue, St. Catharines, Ontario ON L2S 3A1, Canada. 2. 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. 3. Department of Kinesiology, Brock University, 500 Glenridge Avenue, St. Catharines, Ontario ON L2S 3A1, Canada. Electronic address: dditor@brocku.ca.
Abstract
OBJECTIVES: 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. CONCLUSION: 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.
OBJECTIVES: 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. CONCLUSION: 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.