Background and Objectives. It is commonly thought that pain stimulates respiration. The goal of this study was to determine any effect on ventilation caused by acute pain in patients with upper extremity injuries by measuring CO ₂ -response curves before and after regional anesthesia.

Methods. Four patient volunteers refrained from taking analgesic medications during the study period. Visual analogue scale (VAS) pain scores (10 cm scale), resting ventilation and P _ET CO ₂ , and steady-state, CO ₂ -response curves (4% and 7% inspired CO ₂ ) were determined just before and after axillary block anesthesia.

Results. Axillary block anesthesia reduced VAS pain scores from 7.1 ± 1.1 to 0.7 ± 0.5 ( p < 0.05). Resting ventilation and P _ET CO ₂ were unchanged. CO ₂ -response curve slopes decreased from 1.84 ± 0.54 to 1.40 ± 0.58 l/min ⁻¹ .mm Hg ⁻¹ ( p < 0.05), and the curves were displaced 3.2 ± 1.2 mm Hg to the right at 25 l/min ⁻¹ V _E ( p < 0.05).

Conclusions. Other causes for the respiratory changes observed are discussed. It is concluded that acute pain stimulates respiration as measured by the steady-state, CO ₂ -response method and that relief of acute pain results in a decrease in CO ₂ -mediated ventilatory drive. More research is needed using both experimental pain models and patients with chronic pain.