Effects of autonomic blockade on nonlinear heart rate dynamics

Analysis of nonlinear heart rate (HR) dynamics may provide greater insight into neurocardiac influences during exercise and disease than traditional HR variability. However, the physiological basis of nonlinear HR dynamics has not been investigated in individuals with spinal cord injury (SCI). The purpose of this study was to compare the effects of autonomic blockade in SCI and able-bodied participants. Five participants (42 +/- A 13 years) with SCI (C4-C7, AIS B-D, 13 +/- A 13 years post-injury) and four able-bodied controls (33 +/- A 8 years) underwent beta(1)-adrenergic and vagal blockade in the supine and cardiovascular stress positions. Cardiovascular stress consisted of 40A degrees tilt plus sustained isometric jaw contraction and cold water submersion of the right hand. In both SCI and able-bodied participants, vagal blockade significantly increased HR (p < 0.05) and resulted in significant reductions in sample entropy and correlation dimension in the supine and cardiovascular stress positions (p < 0.05). During the cardiovascular stress position, baseline sample entropy (p < 0.05) and correlation dimension (p < 0.05) were lower in participants with SCI. Nonlinear measures were also significantly correlated with HR (p < 0.05). The results suggest that vagal modulations are a primary modulator of nonlinear HR signals in both SCI and able-bodied participants, while the role of the beta(1)-adrenergic system remains less defined. Further study is required to elucidate the role of the autonomic nervous system in nonlinear HR dynamics in both SCI and able-bodied populations.