THE EFFECT OF LBNP ON HEART RATE AND SYMPATHOVAGAL BALANCE IN PEOPLE WITH TETRAPLEGIA 230

This study investigated alterations of heart rate (HR) and sympathovagal balance during lower body negative pressure (LBNP) in 7 males with tetraplegia(T; spinal cord lesions from C5-C7) and 7 able-bodied men (C). HR was continuously monitored during graded LBNP at -15, -30, -45 and -60 mmHg. Parasympathetic (PNS) and sympathetic (SNS) control of HR were derived using coarse grained spectral analysis. Compared to resting values, HR at -15 mmHg did not change significantly in either group. However, at -30, -45 and -60 mmHg LBNP, HR increased significantly (p<0.05) in T (by 18, 30, and 36 b·min^-1, respectively) and in C (by 7, 14, 26 b·min^-1, respectively), but markedly less in C compared to T(p<0.05). PNS was significantly reduced (p<0.05) in T at -15, -30 and-45 mmHg (by -0.01, -0.17, -0.20, respectively) and in C during -15, -45 and-60 mmHg (by -0.09,-0.10,-0.17, respectively). No significant differences in PNS were observed between T and C. SNS activity did not change significantly in T or C at any level of LBNP, except for a significant increase in C at -30 mmHg (p<0.05). This study demonstrates that exposure to high levels of orthostatic stress (above -15 mmHg LBNP) elicit a significantly greater cardioacceleration in individuals with tetraplegia compared to able-bodied subjects. Remarkably, these results suggest that in people with tetraplegia as well as in able-bodied cohorts the increase in HR during LBNP is primarily elicited by vagal withdrawal and not by increased sympathetic activity.