Spontaneous indices correlate with baroreflex gain only in adults with spinal cord injury.

Purpose: Spontaneous indices have been widely used to assess baroreflex gain despite their numerous limitations and concerns regarding their validity, reliability, and reproducibility. In this retrospective study, we investigated whether spontaneous baroreflex indices reflect cardiovagal baroreflex gain assessed by the neck-chamber technique in those with spinal cord injury (SCI) and in uninjured individuals. SCI represents a model of preserved cardiovagal baroreflex control coupled with impaired sympathetic effects on the vasculature.

Methods: We derived three spontaneous indices of baroreflex sensitivity (sequence method, low-frequency (LF), and high-frequency (HF) transfer function) and compared them with baroreflex gain obtained via the neck-chamber technique in adults with SCI (n = 29; neurological level C1-T10, ≤ 2 years since injury) and uninjured adults (n = 14).

Results: In both groups, spontaneous indices were highly correlated with each other (all p < 0.01). In uninjured participants, neck suction baroreflex gain did not relate to any spontaneous index. In individuals with SCI, neck-chamber gain correlated significantly with spontaneous indices (all r > 0.43, p < 0.05); these relationships were significantly stronger in individuals with neurologically complete injuries (sequence: r = 0.67, p < 0.01; LF: r = 0.79, p < 0.001; HF: r = 0.76, p < 0.001). However, Bland-Altman analysis revealed a strong proportional bias, with spontaneous indices consistently and progressively overestimating neck-chamber gain (all r > 0.91, p < 0.001).

Conclusions: These results suggest that sympathetic activity is largely responsible for the lack of correspondence between spontaneous and neck-chamber baroreflex gains. However, even in individuals with a neurologically complete SCI, where sympathetic influences are minimal, spontaneous indices may not consistently reflect baroreflex gain derived from other methods.