Fig. 2. Time course of calculated alfentanil plasma concentrations (broken line) caused by infusion rates shown as area under the curve (male subject; age, 30 yr; weight, 75 kg) (A ). Infusion of alfentanil resulted in a significant decrease in oxygen saturation (Spo2;P < 0.01, analysis of variance), whereas mean arterial pressure (MAP) and heart rate (HR) remained unchanged (nonsignificant, analysis of variance) (B ). Pain ratings (C ) as well as areas of pin-prick hyperalgesia (D ) and touch-evoked allodynia (E ) were significantly reduced by alfentanil (P < 0.01, analysis of variance). The flare reaction, determined via laser Doppler imaging, was slightly diminished by the medication (P < 0.05, analysis of variance) (F ). Data are expressed as mean ± SEM (n = 12); *P < 0.05, planned comparisons corrected with the Bonferroni procedure. NRS = numeric rating scale.