Science vs supposition: the case of target-controlled propofol infusion.

Editor—I continue to be confounded by the use of target-controlled propofol infusion (TCI) dosing paradigm as an accepted basis for presenting the scientific study in anaesthesia journals without specific documentation of instantaneous and total dose infusion rates, in conjunction with body weight and duration of infusion at the study point. Although TCI may be utilitarian in the clinical administration of drug for clinical anaesthesia, the recent study by Yufune and colleagues 1 clearly documented a real and important departure from scientific measured blood levels using the 'black box' technology of TCI. This non-clinical study was also undertaken in patients removed from surgical stimulation: surgery imparts typically significant sympathetic responses which change haemodynamics and, expectantly, drug kinetics. Although bispectral index (BIS) ranged between 25 and 50 with the average BIS near 35–40, in spite of TCI set for induction at 6 s then at 90 s reduced for the study period to 4 mg ml 21 , this BIS indicates significant overdose, as the 'Target/desired' BIS level for minimizing drug administration in clinical anaesthesia is the Target of 50–60. TCI infusion rates were apparently chosen to reflect surgical needs, but were based on a paradigm for children per reference and resulted in measured median blood levels of 2.5 mg ml 21 , instead of the set 4 mg ml 21 by study documented measurements. 2 To state that the BIS was significantly decreased in the remifentanil 0 mg kg 21 min 21 group is further an odd statement, as no drug was administered and this logically, simply reflects only the natural course over time of the non-stimulated anaesthetic state on BIS, as the haemodynamic effects of preparatory (i.e. 'surgical') stimuli of intubation, etc. spontaneously decline, something quite commonly noted clinically. BIS is known to vary directly to the surgical stimulation, exhibit extinction/regression phenomenon over time after termination of surgical stimulation , and is not specific to the depth of anaesthesia in all instances. The utility of narcotic administration during clinical anaesthesia is to reduce hypnotic doses, inhibit spinal reflexes, and to offset haemodynamic effects of surgical stimulation. The effects on anaesthetic depth in the absence of surgery would be expected to be quite different than in this non-surgical setting. The authors also presumed to study if: 'remifentanil may decrease the blood flow at pro-pofol clearance sites and increase the Cp during constant propofol infusion', but TCI by definition is not a …