Hypertension-related hypoalgesia, autonomic function and spontaneous baroreflex sensitivity

Objective: The mechanisms involved in the relationship between pain perception and hypertension are poorly understood. This study has sought to investigate whether the spontaneous baroreflex sensitivity and the autonomic nervous system balance are related to hypertension-associated hypoalgesia. Methods: In the morning, 73 untreated male subjects (45 hypertensives, 28 normotensives) were submitted to a simultaneous recording of electrocardiographic and blood pressure signals in resting condition. The tracings were analysed off-line to evaluate the spectral components of the low frequency (LF) and high frequency (HF) powers (autoregressive algorithm; LF/HF ratio used in subsequent analysis as an index of sympathovagal balance), and the alpha_LF (α_LF), an index of baroreflex sensitivity. After the rest period, the subjects underwent dental pain perception evaluation (pulpar tester: test current increasing from 0 to 0.03 mA, expressed in relative Units) to determine the dental pain threshold and tolerance. Afterwards, a 24-h ambulatory blood pressure monitoring was performed. Results: A significant relationship was observed between α_LF and pain threshold (r=-0.34; p=0.003). When a multivariate analysis was computed to control for age, 24-h systolic pressure and LF/HF ratio, α_LF was a predictive independent factor associated with pain threshold (model p=0.019; r=-0.31; p=0.025). Moreover, the 24-h systolic pressure was independently associated with pain threshold (model p=0.019; r=0.30, p=0.031). The relationship between α_LF and relative tolerance was not statistically significant. When the association between the LF/HF ratio and pain sensitivity was assessed as a secondary endpoint, no significant relationship was observed. Since no significant interaction was found, the effect of α_LF and LF/HF ratio on pain perception was assumed to be similar in normotensive and hypertensive subjects. Conclusions: The relationship found between unstimulated baroreflex sensitivity and pain threshold suggests a modulation of pain perception by baroreflex pathways in hypertension-associated hypoalgesia. In a baseline condition, the autonomic nervous system balance does not seem to influence pain sensitivity.