In clinical situations, such as with terminal cancer and chronic obstructive pulmonary disease, pain and dyspnea frequently coexist. To elucidate a possible association between pain and dyspneic sensations, Nishino et al. induced experimental pain and dyspnea during controlled conditions in 15 healthy volunteers (11 men and 4 women, aged 25–32 yr).
Volunteers were seated throughout the experiment and breathed through an apparatus that included a face mask, a pneumotachograph, and a one-way valve system. Dyspneic sensation was induced by a combination of inspiratory resistive loading and hypercapnia induced by extra dead space, while an orthopedic inflatable tourniquet was placed around the calf and inflated to 350 mmHg to induce experimental pain. Subjects were asked to rate their pain or dyspnea according to a visual analogue scale (VAS) from 0 to 100 during each experimental protocol.
In randomized order, volunteers were subjected to added respiratory load without pain for 9 min; pain stimulus for 9 min without additional respiratory load; addition of pain stimulus 4 min after the start of breathing with an added load; and addition of external respiratory load after 4 min of pain stimulation. In addition to recording of the VAS scores on a linear potentiometer, airflow, Pmask, partial pressure of end-tidal carbon dioxide (PETCO2), and P0.1 were all recorded on a thermal array recorder. Later analysis of the recorded data revealed that, with the start of respiratory loading, VI and PETCO2immediately increased, with a simultaneous increase in the dyspneic VAS score. These changes stabilized within 3 min and remained nearly steady for the duration of the protocol. The pain stimulus also elicited an immediate increase in pain VAS score, which tended to subside after 3 min.
When pain stimulus was added to loaded breathing component, subjects registered higher dyspneic VAS scores. However, adding respiratory loading during pain stimulation did not change the pain VAS score. The researchers point out that pain in these healthy subjects does not duplicate pain experienced in sick and dying patients, nor can tourniquet pain be generalized to other types of pain, such as visceral or neuropathic pain. However, given the results from this study, it is possible that attention to adequate pain management in sick patients may help to relieve their dyspnea.