British Journal of Anaesthesia, Vol 77, Issue 3 360-364, Copyright © 1996 by The Board of Management and Trustees of the British Journal of Anaesthesia
E. Hammaren, A. Yli-Hankala, P. H. Rosenberg and M. Hynynen
Unbound, rather than total, plasma concentrations may be related to the
anaesthetic action of propofol. Therefore, we measured plasma
concentrations of propofol and recorded Nb wave latencies of auditory
evoked potentials (AEP) during continuous infusion of propofol in 15
patients undergoing coronary artery bypass grafting (CABG) surgery. After
induction of anaesthesia with fentanyl, propofol was infused continuously
at a rate of 10 mg kg-1 h-1 for 20 min, and then the rate was reduced to 3
mg kg-1 h-1. Administration of heparin before cardiopulmonary bypass (CPB)
did not affect total or unbound propofol concentration. Initiation of CPB
decreased mean total propofol concentration from 2.6 to 1.7 micrograms ml-1
(P < 0.01). Simultaneously, mean unbound propofol concentration remained
at 0.06 micrograms ml-1 because of a slight increase in the mean free
fraction of plasma propofol (from 2.3 to 3.5%; P > 0.05). During
hypothermic CPB, mean total propofol concentration increased to
concentrations measured before bypass (to 2.1 micrograms ml-1; P > 0.05
vs value before CPB) and the mean unbound propofol concentration was at its
highest (0.07 microgram ml-1; P < 0.05 vs value before heparin). After
CPB and administration of protamine, the mean total propofol concentration
remained lowered (1.7 micrograms ml-1; P < 0.05 vs value before heparin)
and the mean unbound propofol concentration returned to the level measured
before heparin (P < 0.001 vs value during hypothermia). The latency of
the Nb wave from recordings of AEP increased after induction of
anaesthesia, reached its maximum during hypothermia and was prolonged
during the subsequent phases of the study. The latency of the Nb wave did
not correlate with total or unbound propofol concentration. We conclude
that the changes in total and unbound concentrations of plasma propofol
were not parallel in patients undergoing CABG. During CPB or at any other
time during the CABG procedure, the unbound propofol concentration did not
decrease and Nb wave latency was prolonged compared with baseline values
measured after induction of anaesthesia before the start of CPB.
CLINICAL INVESTIGATIONS
Cardiopulmonary bypass-induced changes in plasma concentrations of propofol and in auditory evoked potentials
Department of Anaesthesia, Helsinki University Hospital, Haartmaninkatu 4, FIN-00290 Helsinki, Finland
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