|Year : 2007 | Volume
| Issue : 5 | Page : 401
Comparison of Bispectral Index Values Produced by Isoflurane and Halothane at Equal End-tidal MAC Concentrations
Neerja Bharti1, Jagan Devrajan2
1 MD, Assistant Professor, PGIMER, Chandigarh, India
2 MD, Senior Resident, AIIMS, New Delhi, India
|Date of Acceptance||04-Jul-2007|
|Date of Web Publication||20-Mar-2010|
Assistant Professor, Department of Anaesthesia & Intensive Care, PGIMER, Chandigarh-160012
The bispectral index (BIS) has been validated as a measure of hypnotic effect for various anaesthetic agents. There are limited data correlating the BIS with minimum alveolar concentration (MAC) of different volatile anaesthetics. We compared the BIS values using halothane and isoflurane at equal end-tidal MAC multiples. Forty adult patients of ASAI-II undergoing elective abdominal surgeries were randomly assigned into two groups. After a standardized induction, anaesthesia was maintained with either halothane (Group H) or isoflurane (Group I) in combination with nitrous oxide 66% in oxygen and fentanyl.BIS values were measured before induction, during anaesthesia at 0.5 MAC, 1 MAC, 1.5 MAC concentration of the volatile anaesthetics and then at awakening of the patient. BIS measurements were made after meeting steady state concentrations of volatile anaesthetic agents.
BIS values at 0.5 MAC, 1.0 MAC and 1.5 MAC for patients receiving halothane exceeded those for patients receiving isoflurane. BIS value at awakening did not differ between the halothane and isoflurane groups.
We concluded that at equal MAC concentrations of halothane and isoflurane BIS values were significantly lower with isoflurane.
Keywords: Bispectral index; Halothane; Isoflurane, Minimum alveolar concentration
|How to cite this article:|
Bharti N, Devrajan J. Comparison of Bispectral Index Values Produced by Isoflurane and Halothane at Equal End-tidal MAC Concentrations. Indian J Anaesth 2007;51:401
|How to cite this URL:|
Bharti N, Devrajan J. Comparison of Bispectral Index Values Produced by Isoflurane and Halothane at Equal End-tidal MAC Concentrations. Indian J Anaesth [serial online] 2007 [cited 2014 Nov 21];51:401. Available from: http://www.ijaweb.org/text.asp?2007/51/5/401/61170
| Introduction|| |
Minimum alveolar concentration (MAC) is the most common method of titration of volatile anaesthetics in clinical practice. However, this value describes mainly the immobilizing potency (prevents movement to skin incision in 50% of patients) of the anaesthetic agent and most likely reflects more the effects on the spinal cord rather than the brain  . To obtain more complete concentration-response curves for the hypnotic effects of various volatile anaesthetics a novel EEG-derived parameter bispectral index (BIS) is more specific. Previous studies have shown that BIS correlates well with the brain metabolism and with hypnotic and sedative effects of various anaesthetic agents including isoflurane, sevoflurane, midazolam and propofol , .
Anaesthetic agents vary in their relative hypnotic and immobilizing potentials. Therefore, equal MAC concentrations of various volatile agents may produce different BIS values. Limited data allow a comparison of BIS values with equal end-tidal MAC concentrations of volatile anaesthetics , . Olofsen and Dhaman  demonstrated an inhibitory sigmoid E max model for the dynamic relationship between isoflurane and sevoflurane end-tidal concentrations and the BIS values, and found that isoflurane is more potent hypnotic than sevoflurane. In a comparative study Schwad et al  found that sevoflurane decreases BIS values more than does halothane at equal MAC multiples. Halothane has different EEG effects from sevoflurane and isoflurane , . The present study was aimed to compare the BIS values at various end-tidal MAC concentrations of halothane and isoflurane.
| Methods|| |
After obtaining institutional ethical committee approval and informed written consent, forty patients with ASA physical status I-II, scheduled for elective abdominal surgeries lasting >60min were randomly allocated into two groups receiving either halothane (Group H) or isoflurane (Group I). Patients with morbid obesity (weight >130% of IBW), uncontrolled hypertension or diabetes mellitus, impaired renal or hepatic function, or having history of drug or alcohol abuse were excluded.
All patients were fasted overnight and received diazepam 5 mg orally at night and 2 hr before surgery. On arrival in the operation theatre, intravenous cannulation was secured and routine monitoring of ECG, oxygen saturation and blood pressure was started. The BIS was recorded continuously with soothing rate of 15sec by using an Aspect EEG monitor. After obtaining base line values of heart rate, blood pressure and BIS, anaesthesia was induced with fentanyl 2 µg.kg -1 followed by propofol 2-3 mg.kg -1 till loss of verbal response. Tracheal intubation was facilitated with vecuronium 0.1 mg.kg -1 . Anaesthesia was maintained with 66% N 2 O in oxygen along with either halothane or isoflurane. The patients' lungs were mechanically ventilated to maintain an end-tidal carbon-di-oxide concentration of 32-35 mmHg. Total flow through a semicircle system was kept constant to 3L.min -1 throughout the study period. Fentanyl infusion was started after induction of anaesthesia at the rate of 0.5 µg.kg -1 .hr -1 . Intermittent bolus doses of vecuronium 0.02 mg.kg -1 were administered at one twitch response using the train-of-four (TOF) monitor for assessing the degree of neuromuscular blockade. Supplemental doses of fentanyl (25-50µg) were given if there was a persistent increase in heart rate (> 100 beats/min) or blood pressure (> 20% of baseline). Body temperature was maintained above 35.5 0 C in all cases. The inspired and end-tidal concentrations of inhalational agents, oxygen and carbon-di-oxide were measured by using a Datex Capnomac monitor.
During the maintenance phase, 10 min after incision, the end-tidal concentration of inhalational anaesthetic was adjusted at 0.5 MAC for 10 min. When BIS values were stabilized, the inspired concentrations of halothane or isoflurane were increased from 0.5 to 1.0 MAC and then 1.5 MAC, and then decreased to 0.5 MAC in same graded manner. The end tidal MAC was
calculated according to the use of nitrous oxide. After each increment or decrement, the concentration was kept constant for 10 min, to reduce the difference between inspired and end-tidal concentration of inhalational anaesthetic to <10%. The BIS values were recorded at the end of 10 min at each end-tidal concentration.
Fentanyl infusion was stopped 15 min before the end of procedure. The inhalational anaesthetic was discontinued just before skin closure. The residual neuromuscular blockade was reversed with neostigmine 0.05 mg.kg -1 and atropine 0.02 mg.kg -1 at the end of surgery. While the end-tidal concentration of inhalational anaesthetic was decreasing the patients were asked to open their eyes. The BIS value (BIS-awake) and end-tidal concentration of inhalational agent at eye opening was recorded. Patients were extubated and shifted to postoperative care unit for monitoring of vitals and recovery.
| Statistical analysis|| |
The data were collected and presented as mean ± SD. Two sample unpaired student's t-test was used to compare demographic data and duration of surgery. Friedman two-way analysis of variance was applied to see the trends of BIS value in each group. MannWhitney U test was used to compare the BIS values between the two groups. P value <0.05 was considered significant.
| Results|| |
The demographic data and duration of surgery were comparable among groups [Table 1].
In both groups the pre-induction BIS values were between 96 and 98, and then decreased precipitously after induction. The BIS values for each milestone are presented in [Figure 1].
[Figure 1] shows a significant correlation of BIS with the end-tidal anaesthetic concentrations in each group. The BIS values decreased significantly with increasing MAC concentrations during wash-in phase and increased significantly with decreasing MAC concentrations during wash-out phase in both the groups. However, for similar anaesthetic agent the BIS values were comparable at equi-MAC concentrations during wash-in and wash-out phases. The BIS values were 66.5 ± 3.1 and 55.3 ± 5.5 at 0.5 MAC, and 54.2 ± 3.7 and 42.4 ± 5.8 at1 MAC in halothane and isoflurane groups respectively. At 1.5 MAC, BIS values were 40.1 ± 5.2 and 34.7 ± 4.5 respectively. BIS values were significantly (P<0.05) lower in isoflurane group as compared to halothane group at each target MAC value during anaesthesia. However, BIS-awake did not differ between the halothane and isoflurane groups (89.6 ± 4.2 vs 88.2 ± 3.3). The time taken for awakening was 9.2 ± 3.4 min in halothane group and 6.3 ± 2.8 min in isoflurane group (P<0.05).
There was no significant difference in heart rate, mean arterial pressure, end-tidal carbon-di-oxide concentration and oxygen saturation in both groups at various MAC concentrations. At 1.5 MAC 3 patients in halothane group developed mild hypotension which was corrected by fluid administration. No other adverse effect was recorded in any group.
| Discussion|| |
The results of our study showed that the BIS values were less during isoflurane anaesthesia than during halothane anaesthesia at equivalent MAC levels. This observation is probably consistent with the greater metabolic suppression caused by isoflurane and also its ability to produce a higher degree of brain electrical activity suppression than halothane  . BIS incorporates different information from the raw EEG : power and frequency, (3 activation, and burst suppression are integrated in a single number. Halothane and isoflurane differently affect spectral power and median power of EEG . Halothane produces relatively fast EEG rhythms whereas isoflurane produces mainly slow waves. In addition isoflurane produces burst suppression within the clinical dosage range whereas halothane does not , . These known differences in the effects of halothane and isoflurane on EEG are expected to influence the BIS value differently at a similar depth of anaesthesia.
The difference in BIS values between the two groups may be due to the different mechanism of anaesthetic action of the two agents. Halothane is known to have a greater analgesic and immobilizing effect (through its spinal action) as compared to isoflurane  . The BIS does indeed selectively measure the hypnotic or obtunding aspects of anaesthesia rather than the immobilizing action of anaesthesia. At low concentrations of anaesthetic agent the predominant EEG determinant is arousal and the BIS is less agent specific. This is consistent with the findings that BIS-awake values were similar between the agents. As the concentration increases, the effects of arousal are less, and the effects of anaesthetics are greater and the BIS may be more agent-specific  . In a previous study in children, Davidson et al, also reported significant low BIS values with isoflurane than halothane at 1MAC but not at awakening10.
BIS values in a range of 40-60 have been proposed for producing adequate degree of hypnosis during anaesthesia. In present study we found that the BIS value at 1 MAC of halothane (54.2 ± 3.7) was adequate for hypnotic effect .In contrast the BIS value at 1 MAC of isoflurane was 42.4 ± 5.8. which shows the possibility that 1 MAC of isoflurane is more than enough for adequate hypnotic effect. This finding is in consistent with the fact that the use of BIS monitoring for titration of anaesthetic agent reduces intraoperative isoflurane consumption  . However, the use of BIS for titration of anaesthesia in unparalyzed patients cannot predict the likelihood of movement  .
In conclusion, at equi-MAC end tidal concentrations isoflurane produces less BIS values than halothane. The BIS values for volatile anaesthetics may be drug specific.
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