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

• Spinal cord injury (SCI) may be associated with cardiac autonomic dysfunctions.
• The influence of cardiac autonomics on the QT Variability Index (QTVI) was assessed.
• Autonomic blockade was administered during rest and cardiovascular stress (CVS).
• QTVI reflects cardiac sympathetic activity during CVS in autonomic incomplete SCI.
• QTVI inversely reflects cardiac vagal activity at rest in autonomic incomplete SCI.

ObjectivesTo 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.MethodsFour SCI (41.6 ± 13.4 years; C4–C7, AIS B–D, 13.4 ± 13.4 years post injury) and 4 AB (33.0 ± 7.8 years) 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).ResultsThere 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.ConclusionDuring 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.