Ventilatory response to carbon dioxide after intramuscular and epidural fentanyl

The authors compared the effects of administration of fentanyl 200 micrograms on the ventilatory response to carbon dioxide in two groups of nine healthy unpremedicated subjects: one group received fentanyl as an intramuscular injection; in the other group, fentanyl was injected into the epidural space. In the intramuscular group, the slope of the ventilatory response to CO2 did not decrease significantly. In the epidural group, the slope of the ventilatory response to CO2 decreased significantly from 2.48 +/- 1.05 to 1.77 +/- 0.7, 1.74 +/- 0.7, and 2.07 +/- 0.74 L X min-1 X mm Hg-1 at 30, 60, and 120 min after injection (chi +/- SD, P less than or equal to 0.05), respectively. At each time of the study, plasma fentanyl levels were significantly lower in the epidural group than in the intramuscular group (P less than or equal to 0.05). These results suggest that epidural fentanyl induces a nonsystemic ventilatory depression that may be due to the rostral spread of the drug.