Our findings have implications for studies of the causes of cognitive decline and, in clinical practice, for the information given to patients before surgery.

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Twenty-nine patients (age range 14-81 yr) undergoing orthopaedic surgery received alfentanil 100 micrograms kg-1 given as two i.v. boluses followed by a fixed rate infusion of 1 micrograms kg-1 min-1 for 44-445 min. Additional 1-mg bolus doses of alfentanil were administered as required. Plasma samples were assayed for alfentanil using radioimmunoassay. Pharmacokinetic parameters were estimated by a model-independent approach and by curve-fitting. Regression analysis showed no statistical relationship between T 1/2 beta, Cl or Vd and the duration of the infusion, total dose or body weight. We found no significant correlation between age and T 1/2 beta of alfentanil for patients younger than 40 yr. For patients older than 40 yr, T 1/2 beta increased linearly with age. There was no significant decrease in Cl with age, although the lower values for Cl (100-200 ml min-1) were generally found in subjects older than 60 yr. The present study demonstrated that a 100-micrograms kg-1 loading dose and a 1-micrograms kg-1 min-1 infusion may be appropriate for analgesia in general surgical procedures.

The effect of nitrous oxide (N2O) inhalation on the pattern EP with the onset and disappearance of checkerboard patterns was studied in four healthy volunteers. Checkerboards with checks ranging from 2 to 55 min of arc were presented to subjects sedated at various levels. Under the highest level introduced, 60% N2O, fixation of the pattern was still adequate. The major consistent changes in the pattern onset EP under N2O inhalation are (a) preferential attenuation of the response to small checks and (b) an increase of the response amplitude to large checks. The change in the amplitude versus check-size curve under N2O inhalation restricts the assessment of visual acuity on the basis of the pattern EP. In patients sedated with N2O, EP methods can be used only as a gross indicant of visual acuity.