Sex differences in the effect of dyspnea on thermal pain threshold in young healthy subjects.

BACKGROUND Previous study has demonstrated that dyspnea exerts inhibitory influence on pain, and empirical research supports the existence of sex differences in pain. To test the hypothesis that the inhibitory influence of dyspnea on the pain sensation may be less in females than in males, the authors investigated the sex differences in the responses of thermal pain threshold to dyspnea in healthy young subjects. METHODS The authors measured changes in thermal pain threshold in 30 female subjects and 30 male subjects before and during dyspnea produced by a combination of hypercapnia and elastic loading, and compared the difference between males and females. RESULTS The thermal pain threshold significantly increased during loaded breathing in male subjects (46.0 degrees +/- 1.3 degrees vs. 47.2 degrees +/- 1.2 degrees C; P < 0.01, baseline vs. loaded breathing), whereas no change was observed in female subjects (46.1 degrees +/- 1.3 degrees vs. 46.0 degrees +/- 1.4 degrees C; P > 0.1). No significant correlation was observed between the values of dyspneic visual analog scale and changes in thermal pain threshold. Comparison of the different phases of the menstrual cycle in female subjects also showed that there was no consistent effect of the particular phase on thermal pain threshold (45.7 degrees +/- 1.0 degrees vs. 46.1 degrees +/- 1.4 degrees C; P > 0.1, follicular phase vs. luteal phase during baseline; and 45.9 degrees +/- 1.1 degrees vs. 46.0 degrees +/- 1.7 degrees C; P > 0.1, follicular phase vs. luteal phase during loaded breathing). CONCLUSION The inhibitory influence of dyspnea on the pain sensation is less in females than in males, but the sex difference may not be explained by female reproductive hormones alone.