| Total dose |
Increases plasma leels and side effects |
| Lipid solubility |
Increasing lipid solubility increases vascular uptake rather than rostral spread, eg. fentanyl/pethidine versus morphine. CSF (rather than plasma) concentration parallels analgesia. |
| Opiate receptor affinity |
High for lofentanil & sufentanil (les vascular absorption; shorter duration of action) |