Variables of patient-controlled analgesia. 3: Test of an infusion-demand system using alfentanil.

Patient-controlled infusion-demand analgesia was studied using alfentanil. We were unable to identify an optimal dose and administration rate; doses required range from 100 to 900 micrograms alfentanil. The mean concentration of alfentanil in blood associated with return of pain (i.e. immediately before demand) was 58 ng/ml on day 1 and 37 ng/ml on day 2. This difference was despite similar drug consumption on both days.