Experimental pain augments experimental dyspnea, but not vice versa in human volunteers

Background: Pain and dyspnea frequently coexist in many clinical situations . However. whether the two different symptoms interact with each other has not been elucidated. To elucidate the interaction between pain and dyspneic sensations, the authors investigated separately the effects of pain on dys pnea and the effects of dyspnea on pain in 15 healthy subjects. Methods: Subjects were asked to rate their sensation of pain or dyspnea usi ng a visual analog scale (VAS) during pain stimulation produced by tourniqu et inflation (inflation cuff pressure: 350 mmHg) around the calf, and/or th e respiratory loading consisted of a combination of resistive load (77 cm H 2O.l(-1).s(-1)) and hypercapnia induced by extra mechanical dead space (255 ml). In addition to changes in VAS scores, changes in ventilatory airflow and airway pressure were continuously measured. Results: Pain stimulation and loaded breathing increased VAS scores, ventil ation, and occlusion pressure (P-0.1). The addition of a pain stimulus duri ng loaded breathing increased the dyspneic VAS score (median 56 [interquart ile range 50-62] vs. 64 [55-77]: before vs. after addition of pain stimulus , P < 0.05) with concomitant increases in minute ventilation (10.8 [10.1-13 .3] vs. 12.4 [11.0-14.8] l/min, P < 0.05) and P-0.1 (5.5 [4.9-7.2] vs. 6.8 [5.8-9.0] cm H2O, P < 0.05). The addition of respiratory loading during pai n stimulation did not cause a significant change in pain VAS score (40 [33- 55] vs. 31 [30-44]: before vs. after addition of respiratory loading), alth ough both additional burdens increased further minute ventilation (10.0 [8. 8-10.9] vs. 12.0 [10.6-13.2] l/min, P < 0.05) and P-0.1 (2.5 [2.0-3.0] vs. 6.2 [4.9-7.0] cm H2O, P < 0.05). Conclusion: The authors' findings suggest that pain intensifies the dyspnei c sensation, presumably by increasing the respiratory drive, whereas dyspne a may not intensify the pain sensation.