Cardiovascular responses to an orthostatic challenge and electrical-stimulation-induced leg muscle contractions in individuals with paraplegia

Abstract

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 (f _c), stroke volume (SV), cardiac output (Q˙ _c), mean arterial pressure (MAP), total peripheral resistance (TPR), limb volumes and indices of neural modulation of f _c, [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 f _c (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.