Anti-nociceptive properties of cardiopulmonary baroreceptors in patients with chronic back pain.

Abstract

Introduction: Reduced pain perception following a persistent noxious stimulus during a study session (short-term habituation) is believed to be partially mediated by descending inhibitory mechanisms, although these mechanisms have not been fully elucidated. We examined the hypothesis that cardiopulmonary baroreceptor would significantly increase short-term habituation in chronic back pain (CBP) patients.

Methods: A short-term habituation protocol was utilized that involved 1-sec pulses (×10) at 105% heat pain threshold on the anterior forearm at 0.5 Hz. Cardiopulmonary baroreceptor unloading was performed via lower body negative pressure (LBNP) that reduces central venous pressure to elicit a reflex increase in sympathetic nerve activity.

Results: Short-term habituation was observed in young, healthy participants (n = 11), as indicated by a reduction in subjective pain ratings across the 10 repetitive heat pulses (-42% ± 29, P < 0.01, n = 11). Short-term habituation was also observed in CBP patients (-32% ± 30, P < 0.01, n = 12). Cardiopulmonary baroreceptor unloading via LBNP significantly reduced pain ratings across the 10 repetitive heat pulses in CBP patients compared with supine control (patient positioned in LBNP chamber but without a reduction in pressure) and upright sitting (chair), as indicated by a more negative area under the curve index (LBNP: -16.3 ± 4.1; Control: -14.4 ± 2.6; Upright sitting: -15.1 ± 4.1, P = 0.02). However, LBNP-mediated reductions in pain ratings were selective to CBP patients with more severe symptoms, i.e., neuropathic pain (LBNP: -14.7 ± 2.1; Control: -12.8 ± 1.4; Upright sitting: -12.1 ± 1.2, P = 0.04), whereas no effect of LBNP was observed in young, healthy participants (P = 0.83). In support, CBP patients with neuropathic pain exhibited significantly elevated mechanical pressure pain threshold during LBNP (P = 0.04).

Conclusions: Together, these findings demonstrate an association between cardiopulmonary baroreceptor unloading and a reduction in pain perception during repetitive noxious stimuli in CBP patients, particularly among CBP patients with greater pain severity.