Respiratory effects of high-dose butorphanol

The effect of butorphanol on respiratory drive was assessed using a carbon dioxide response test (CRT). Eight male volunteers received 3 mg/70 kg of intravenous butorphanol every 30 min to a cumulative dose of 15 mg/70 kg (5 doses). Thirty minutes before the first butorphanol dose, each subject received normal saline to establish a baseline CRT. After each butorphanol dose, a CRT was repeated at 15 min to assess respiratory depression. Minute ventilation was plotted against PaCO2 to generate a regression line for saline and each dose. Slopes and intercepts for each line were calculated by least squares linear regression, and CRT displacement from saline was determined at each dose. The mean slope for each dose was not significantly different from the saline slope (p = 0.23-0.91). The mean displacement (+/- SEM) of the CRT from saline was greatest after the second dose (7.29 +/- 1.94 mm Hg) but not significantly different from the first or subsequent doses (p greater than 0.05). Butorphanol in doses of up to 15 mg/70 kg may have a 'ceiling effect' in respiratory depression.