Cardiovascular responses to an orthostatic challenge and electrical-stimulation-induced leg muscle contractions in individuals with paraplegia
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The purpose of this study was to investigate the cardiovascular and haemodynamic responses that occur during moderate orthostatic challenge in people with paraplegia, and the effect of electrical stimulation (ES)-induced leg muscle contractions on their responses to orthostatic challenge. Eight males with complete spinal lesions between the 5th and 12th thoracic vertebrae (PARA) and eight able-bodied individuals (AB) volunteered for this study. Changes in heart rate (fc), stroke volume (SV), cardiac output (Q˙c), mean arterial pressure (MAP), total peripheral resistance (TPR), limb volumes and indices of neural modulation of fc, [parasympathetic (PNS) and sympathetic (SNS) nervous system indicators] were assessed during: (1) supine rest (REST), (2) REST with lower-body negative pressure at −30 torr (LBNP −30, where 1 torr = 133.32 N/m2), and (3) for PARA only, LBNP −30 with ES-induced leg muscle contractions (LBNP + ES). LBNP −30 elicited a decrease in SV (by 23% and 22%), Q˙c (by 15% and 18%) and the PNS indicator, but an increase in fc (by 10% and 9%), TPR (by 23% and 17%) and calf volume (by 1.51% and 4.04%) in both PARA and AB subjects, respectively. The SNS indicator was increased in the AB group only. Compared to LBNP −30, LBNP + ES increased SV (by 20%) and Q˙c (by 16%), and decreased TPR (by 12%) in the PARA group. MAP was unchanged from REST during all trials, for both groups. The orthostatic challenge induced by LBNP −30 elicited similar cardiovascular adaptations in PARA and AB subjects. ES-induced muscle contractions during LBNP −30 augmented the cardiovascular responses exhibited by the PARA group, probably via reactivation of the skeletal muscle pump and improved venous return.
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