Abstract
Persons with spinal cord injury (SCI) are subjected to few gravitational stressors during daily routines and have diminished autonomic cardiovascular control. PURPOSE: To identify cardiovascular autonomic responses to passive head-up tilt (HUT) in persons with central (tetraplegia) and peripheral (paraplegia) autonomic dysfunction compared with a non-SCI control group. METHODS: Subjects included 15 persons with chronic SCI, 7 with tetraplegia (T) and 8 with paraplegia (P), and 8 age- height- and weight-matched controls (non-SCI). Heart rate (HR) was continuously monitored and cardiovascular autonomic responses were assessed during two 5-minute periods at supine rest and at 45° HUT. Change from supine to 45° HUT was calculated for HR, high & low frequency HR variability (HF & LF) and the LF/HF ratio. Data are reported as mean±SD and statistical significance was set at p<0.05. RESULTS: Supine HR was comparable among the groups; however LF was reduced in the T compared with the non-SCI group. Controlling for supine HR, the magnitude of change in HR was reduced in the T compared to the P and non-SCI groups. There was a significant interaction effect among the groups for the relationship between the change in HR and change in HF; and the slope of this relationship was reduced in the T (−1.026: 95% CI: 0.78–2.41) compared with the P (−7.808: 95% CI: 2.27–7.01) and non-SCI (−6.985: 95% CI: 2.09–6.44) groups. The relationship between HR and LF/HF was similar between the T (0.312: 95% CI: 0.27–0.832) and the non-SCI (0.353: 95% CI: 0.345–1.06) but both were reduced compared to the P group (4.893: 95% CI: 1.397–4.31). CONCLUSION: Although the magnitude of vagal withdrawal was comparable among the three groups, the increase in heart rate was attenuated in the group with tetraplegia. This finding may reflect the independent influence of vagal withdrawal on heart rate in the absence of a catecholamine response to upright posture in those with tetraplegia.
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