The influence of cardiac autonomic activity on the QT-variability index in able-bodied and incomplete spinal cord injured individuals
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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.
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