THE INFLUENCE OF CHANGES IN PaCO2 ON BISPECTRAL INDEX, SPECTRAL EDGE FREQUENCY AND MEDIAN FREQUENCY DURING PROPOFOL/FENTANYL ANESTHESIA

Abstract

S548 The changes in PaCO2 is known to influence EEG and the cerebral blood flow(CBF). The latter is associated with the amount of anesthetics entering the brain. In this study, we sought to determine the effects of changes in PaCO2 on the EEG processed parameters, bispectral index(BIS), spectral edge frequency(SEF) and median frequency (MF). METHODS: With approval of our Human Ethical committee and written informed consent, 14 ASA I-II patients (age 20-55 yr) scheduled for elective nonneurologic surgery were studied. The processed EEG parameters, BIS, SEF and MF were continuously recorded from a bifrontial montage (Fp1-Cz and Fp2-Cz). Anesthesia was induced and maintained with propofol / fentanyl / vecuronium. Fentanyl was administered at dose of 2 [micro sign] g[center dot]kg-1 [center dot]hr-1 and propofol at rate 5-10 mg [center dot] kg-1 [center dot] hr-1 to maintain adequate anesthesia, which was evaluated by vigilant observation of hemodynamic, autonomic and somatic responses as well as keeping BIS below 50. All patients were ventilated mechanically with air/O2 (FiO2 =0.3-0.35). End-tidal CO2 was monitored continuously. After at least 10 min adequate and steady anesthetic state was kept during operation period when the stimulation of surgery was weak and stable, in random sequence, PaCO (2) was modulated to following levels: 1) over 45 mmHg: 2) 35 +/- 2.5 mmHg: 3) lower than 25 mmHg, and maintained at each level for 10 min. At each case, the difference between each two PaCO2 levels was over 10 mmHg. A minimum of three arterial blood samples were withdrawn for determination of blood gas during each PaCO2 level. The rate of propofol and dosage of fentanyl were kept unchanged throughout the experimental period. Mean BIS, SEF, MF, BP and HR at that PaCO2 level for each case were obtained by recording and computing the values of each minute. Differences between treatment were compared using analyses of variance. Data are expressed as mean +/- SD. RESULTS: MBP and HR kept stable and showed no difference between the levels of PaCO2. The changes of PaCO2 had no significant effect on BIS and SEF. But, MF increased significantly when PaCO2 reached over 45 mmHg from 35 mmHg and 25 mmHg(P<0.01). (Table 1)Table 1CONCLUSIONS: The results suggest that the changes of PaCO2 do not alter the concentration of propofol in the brain, although the changes of PaCO2 within the range in our study can alter CBF markedly (2-3%/mmHg) even during anesthesia with propofol [1]. It is possible that the amount of propofol drawn from brain is altered similarly as that entering the brain after the equilibration of anesthetics between the blood and brain has been received. The change of MF possibly results from the effect of different levels of PaCO2 on the EEG, since it is said to reflect anesthetic depth more poorly than BIS and SEF. We conclude that the changes of PaCO (2) within 24-46 mmHg do not interfere with monitoring anesthetic depth with BIS or SEF, and produce no effect on the depressant level of brain during equilibratory state of anesthesia.