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From the Department of Anesthesia, St Savas Hospital, Athens, Greece.
Address correspondence to: Dr. A. Fassoulaki, 57-59 Raftopoulou Street, 11744 Athens, Greece. Phone: 301- 9024530; Fax: 301-9024530; E-mail: afassou1{at}otenet.gr
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Abstract |
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Methods: Forty female patients undergoing breast surgery received propofol 3 mg•kg–1 followed by propofol infusion adjusted to maintain a 40% BIS value, supplemented by 50% nitrous oxide. Proprietary or generic propofol was administered in a randomized double-blind manner.
Results: The propofol mg/BIS% ratio obtained after the bolus dose, the cumulative infused propofol mg/BIS% ratio at the end of each five-minute interval and the total dose of propofol administered as bolus + infusion were similar between the two groups. The two groups did not differ with regard to systolic and diastolic blood pressure, heart rate, end-tidal carbon dioxide and arterial oxygen saturation.
Conclusion: The two propofol formulations are equipotent when hypnotic effect is assessed by BIS monitoring.
DIPRIVAN® (AstraZeneca, Sweden), is an oil-in-water emulsion, which contains soybean oil and purified egg phosphatide. Propofol is dissolved in the oil phase of the emulsion, which is dispersed to submicron size in the aqueous phase giving a stable product. Propofol prepared by Abbott Laboratories, using the same solvent, has become available recently. The present study compares the hypnotic potency using the Bispectral IndexTM (BIS) and the hemodynamics of the 1% propofol manufactured by AstraZeneca (proprietary) and that manufactured by Abbott Laboratories (generic).
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Methods |
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In the operating room, a 17-gauge catheter was inserted in a peripheral vein for the bolus and subsequent infusion of propofol. The electroencephalogram (EEG) signal was obtained from ZipprepTM electrodes (Aspect Medical System, MA, USA) applied to the forehead and temple using a frontal- temporal montage. The real-time EEG and BIS were continuously displayed on an EEG monitor (Bispectral IndexTM, software version 3.66.24, Spacelabs Medical, Redmond, WA, USA).
After preoxygenation, patients received a bolus dose of iv propofol 3 mg•kg–1 and rocuronium 0.6 mg•kg–1. Anesthesia was maintained with 50% nitrous oxide in oxygen and a continuous propofol infusion titrated to maintain a BIS of 40%. Blood pressure, heart rate, peripheral oxygen saturation, end-tidal carbon dioxide and BIS values were recorded at five-minute intervals. Propofol volume delivered, BIS% and hemodynamic parameters were recorded for 30 min.
The cumulative propofol infusion per kg body weight delivered every five minutes was normalized per BIS% unit, by dividing the dose of propofol infused at each predetermined time interval by the BIS value corresponding to the end of the same interval. The same calculations apply to the bolus doses of propofol administered to induce anesthesia. To obtain numbers greater than one, all the propofol (mg)/BIS% ratios were multiplied by 100.
Statistical analysis was performed with the SPSS 8.0 package. From an initial estimate of the dose of propofol/kg body weight infused in 30 min (4 mg•kg–1) and the standard deviation (0.8), we concluded that our study would have sufficient power (>80%) to detect a 20% difference between the two groups with more than 15 patients in each group. Age and body mass index were compared with Student's t test. Mean arterial pressure (MAP), heart rate, peripheral oxygen saturation, end-tidal carbon dioxide, BIS values and propofol normalized for the obtained BIS values were compared by the repeated measures general linear model procedures.
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Results |
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No difference was found in the BIS values between the two groups at any time interval (Figure 1
). The BIS values recorded after the bolus induction dose were lower compared to the ten, 15, 20, 25 and 30 min recordings (P <0.001) in both groups. The dose of propofol infused after each time interval did not differ between the two groups, nor did the dose of propofol infused per kg body weight and normalized for the corresponding BIS value (Figure 2).
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Discussion |
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In the present study we used BIS, as the endpoint of hypnosis to control the administration of propofol and to assess the potency of the two different propofol formulations. BIS was developed from a database of EEG segments, which correlated well with the hypnotic and sedation levels in volunteers given increasing and decreasing doses of several anesthetics.1,2 BIS has been used to monitor propofol-induced sedation and anesthesia. Endpoints for induction of anesthesia during propofol infusion with or without fentanyl were compared with BIS values.3 The impact of alfentanil on propofol requirements for loss of consciousness and lack of recall was also assessed using the BIS monitoring.4
The propofol (3 mg•kg–1)/BIS ratio and, thus, the hypnotic effect obtained after the predetermined bolus dose did not differ between groups. The cumulative infusion doses required to maintain a BIS 40% normalized by the corresponding BIS values did not differ between the two groups as well. As BIS is not altered by nitrous oxide,5 this agent was chosen to provide intraoperative analgesia for a procedure which is not associated with severe pain.
We chose a 40% BIS value as the anesthetic endpoint because no awareness has been described in the literature with a BIS less than 50%.6 BIS values correlate well with the hypnotic level produced by general anesthetics but not with the hemodynamic changes associated with endotracheal intubation and sternotomy.7 Since BIS does not predict equally effectively the somatic and autonomic responses, our results concerning the equipotency of the two propofol formulations studied apply to sedation alone.
In summary, the two propofol formulations (proprietary and generic) compared by BIS monitoring were similar with regard to their hypnotic and hemodynamic effects.
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Footnotes |
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Accepted for publication January 22, 2001.
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References |
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2 Rampil IJ. A primer for EEG signal processing in anesthesia. Anesthesiology 1998; 89: 980–1002.[Medline]
3 Mi WD, Sakai T, Singh H, Kudo T, Matsuki A. Hypnotic endpoints vs. the bispectral index, 95% spectral edge frequency and median frequency during propofol infusion with or without fentanyl. Eur J Anaesthesiol 1999; 16: 47–52.[Medline]
4
Iseline-Chaves IA, Flaishon R, Sebel PS, et al. The effect of the interaction of propofol and alfentanil on recall, loss of consciousness, and the bispectral index. Anesth Analg 1998; 87: 949–55.
5
Barr G, Jakobsson JG, Öwall A, Anderson RE. Nitrous oxide does not alter bispectral index: study with nitrous oxide as a sole agent and as an adjunct to i.v. anaesthesia. Br J Anaesth 1999; 82: 827–30.
6 Chan MTV, Gin T. What does the bispectral EEG index monitor? (Editorial). Eur J Anaesthesiol 2000; 17: 146–8.[Medline]
7 Driessen JJ, Harbers JBM, van Egmond J, Booij LHDJ. Evaluation of the electroencephalographic bispectral index during fentanyl-midazolam anaesthesia for cardiac surgery. Does it predict haemodynamic responses during endotracheal intubation and sternotomy? Eur J Anaesthesiol 1999; 16: 622–7.[Medline]
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