Sympathetic Neural and Hemodynamic Responses to Painful Stimuli are Related to Perception of Pain

Abstract

Painful stimuli cause activation of the sympathetic nervous system, increasing efferent muscle sympathetic nerve activity (MSNA) and altering downstream hemodynamic responses. However, there is considerable inter-individual variability in the perception of pain to a given painful stimulus; how this magnitude of perceived pain relates to sympathetic neural and hemodynamic responses remains to be fully elucidated. PURPOSE: Thus, the purpose of this study was to evaluate the relationship between the perception of pain and the corresponding sympathetic neural and hemodynamic responses to a painful stimulus in healthy normotensive men and women. METHODS: Heart rate (HR), MSNA, and blood pressure (BP) were measured at baseline (supine, rest) and during a two-minute cold pressor test (CPT) in 15 subjects (6 female, 9 male; age: 31.5 ± 7.5 years, body mass index (BMI): 25.1 ± 3.4 kg/m2, mean ± SD). Stroke volume (SV), cardiac output (Qc), and total peripheral resistance (TPR) were calculated using the model flow method. Immediately following the CPT subjects rated their pain on a verbal descriptor scale (Numerical Rating Scale, range 0-10). Statistical significance was set at p < 0.05. RESULTS: Subjects were grouped according to pain ratings given following the CPT (Pain ≥7, i.e., “severe” pain, n=9; and Pain ≤ 6, i.e. “none” to “mild-to-moderate” pain, n=6). The two groups were similar with regards to gender, age, and BMI. Subjects who rated their Pain >6 had significantly larger increases during the CPT in MSNA total activity (1208 ± 631 vs. 173 ± 87 a.u./min), burst frequency (24 ± 15 vs. 9 ± 6 bursts/min), burst incidence (38 ± 18 vs. 13 ± 5 bursts/100 heartbeats,), TPR (223 ± 178 vs.79 ± 107 dynes/s/cm-5), Qc (1.26 ± 0.63 vs. 0.54 ± 0.39 L/min) and mean BP (15 ± 6 vs. 6 ± 7 mmHg) compared to individuals who rated their Pain ≤6 (n=6). Changes in HR and SV in response to the CPT were not related to pain ratings. CONCLUSION: In healthy, normotensive men and women, the sympathetic neural, BP, & vasoconstrictor responses to a fixed painful stimulus are positively related to the magnitude of pain perception. These findings may have important clinical implications, as hypoalgesia (i.e., decreased sensitivity to pain) is a common characteristic of many cardiovascular disease states.