[The perimedullary vascular channel does not play a particularly important role in causing respiratory depression...

The authors studied the role of the perimedullary vascular channel on long-lasting respiratory depression after the epidural administration of fentanyl 0.2 mg. At the first step, we compared the blood concentration of fentanyl in the azygos vein after thoracic epidural and that after caudal administration. After thoracic epidural administration, the peak concentration usually occurred at 3 min post-injection and averaged 7.6 +/- 1.6 ng.ml-1. After caudal administration, fentanyl concentrations were always below the limit of detection (1 ng.ml-1) in all patients throughout the course. At the second step, based on the results of the first step, we compared the respiratory effects of thoracic epidural fentanyl with those of caudal fentanyl. Th11-12 epidural fentanyl depressed ventilatory response to CO2 in the same fashion and to the same degree as caudal fentanyl. All these results suggest that the perimedullary vascular channel does not play a particularly important role in causing respiratory depression after epidural fentanyl.