E. Hammaren et al., CARDIOPULMONARY BYPASS-INDUCED CHANGES IN PLASMA-CONCENTRATIONS OF PROPOFOL AND IN AUDITORY-EVOKED POTENTIALS, British Journal of Anaesthesia, 77(3), 1996, pp. 360-364
Unbound, rather than total, plasma concentrations may be related to th
e anaesthetic action of propofol. Therefore, we measured plasma concen
trations of propofol and recorded Nb wave latencies of auditory evoked
potentials (AEP) during continuous infusion of propofol in 15 patient
s undergoing coronary artery bypass grafting (CABG) surgery. After ind
uction 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 red
uced to 3 mg kg(-1) h(-1). Administration of heparin before cardiopulm
onary bypass (CPB) did not affect total or unbound propofol concentrat
ion. Initiation of CPB decreased mean total propofol concentration fro
m 2.6 to 1.7 mu g ml(-1) (P < 0.01). Simultaneously, mean unbound prop
ofol concentration remained at 0.06 mu g ml(-1) because of a slight in
crease 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 mu g ml(-1)
; P > 0.05 vs value before CPB) and the mean unbound propofol concentr
ation was at its highest (0.07 mu g ml(-1); P < 0.05 vs value before h
eparin). After CPB and administration of protamine, the mean total pro
pofol concentration remained lowered (1.7 mu g ml(-1); P < 0.05 vs val
ue before heparin) and th mean unbound propofol concentration returned
to the level measured before heparin (P < 0.001 vs value during hypot
hermia). The latency of the Nb wave from recordings of AEP increased a
fter induction of anaesthesia, reached its maximum during hypothermia
and was prolonged during the subsequent phases of the study. The laten
cy of the Nb wave did not correlate with total or unbound propofol con
centration. We conclude that the changes in total and unbound concentr
ations of plasma propofol were not parallel in patients undergoing CAB
G. During CPB or at any other time during the CABG procedure, the unbo
und propofol concentration did not decrease and Nb wave latency was pr
olonged compared with baseline values measured after induction of anae
sthesia before the start of CPB.