Direct Measurement of Nitrous Oxide MAC and Neurologic Monitoring in Rats During Anesthesia Under Hyperbaric Conditions Garfield B. Russell,
MD, FRCPC,
and John M. Graybeal,
CRTT
Department of Anesthesia, Division of Respiratory and Intensive Care, The Milton S. Hershey Medical Center, The Pennsylvania State University College of Medicine, Hershey, Pennsylvania
The minimum alveolar concentration (MAC) of nitrous oxide necessary to prevent purposeful movement in rats has not been directly measured; rather, it has been extrapolated because the required partial pressure exceeds 760 mm Hg, or 1 atm absolute pressure (ATA). Values reported have ranged from 1.36 to 2.20 ATA (136-220 vol%, or 1034-1672 mm Hg). By maintaining general anesthesia at 2.25 ATA (1710 mm Hg), we directly measured the nitrous oxide MAC in 17 Long-Evans rats during mechanical ventilation and monitoring of two-channel electroencephalogram, compressed spectral array and cortical evoked potentials, electrocardiograph, and respiratory and anesthetic gases by mass spectrometry. After a minimal stabilization period of 30 min during ventilation by 1.8 ATA nitrous oxide and 0.45 ATA oxygen, MAC measurements were begun. Each rat was given up to three noxious electrical stimulations of 50 V by 10-ms-durationpulses at 501s for 45 s. The
T
he minimum alveolar concentration (MAC) of nitrous oxide (N20)necessary to prevent repetitive, purposeful movements in rats has not been directly measured. The N,O MAC is a partial pressure value >760 mm Hg, or 1 atm absolute pressure (ATA); hyperbaric exposure is required for MAC measurement and to enable investigators to ventilate animals with high enough N,O partial pressures to exceed MAC as well as prevent hypoxemia. Although the MAC of N 2 0 has been directly measured and reported to be l .04 ATA in humans (1),the reported values for N 2 0 MAC in rats have been primarily extrapolated from the presumed linear adThiswork was supported by an Eric A. Walker Young Investigator Fellowship and by the Department of Anesthesia, The Pennsylvania State University College of Medicine, Hershey, Pennsylvania. Accepted for publication June 10, 1992. Address correspondence to Dr. Russell, Department of Anesthesia, P.O. Box 850, The Milton S. Hershey Medical Center, Hershey, PA 17033. 01992 by the International Anesthesia Research Society 0003-2999/92/$5.00
partial pressure of nitrous oxide was decreased by approximately 10% after each negative response. The MAC was taken as the nitrous oxide concentration midway between that at which there was no response and that at which the rat moved purposefully. The nitrous oxide MAC in Long-Evans rats was determined to be 1.55 0.16 ATA (mean SD). Hyperbaric nitrous oxide decreased electroencephalogram wave frequency to a predominantly theta rhythm of increased amplitude. Cortical evoked potentials had decreased wave amplitudes and increased latencies with increasing partial pressures >0.75 ATA. General anesthesia with nitrous oxide at hyperbaric pressures allows direct measurement of the nitrous oxide MAC in rats and demonstrates neurophysiologic depressant effects on the electroencephalogram and somatosensory evoked potentials.
*
*
(Anesth Analg 1992;75:995-9)
ditivity between N20 and volatile anesthetics and the correlation of anesthetic potency with lipid solubility (2,3). Values reported have ranged from 1.36 to 2.20 ATA. However, nonlinear additivity of N,O and other anesthetics has also been reported (3-5). Wardley-Smith and Halsey (6) found N,O potency to be 1.48 k 0.076 ATA (mean SD) in an unknown number of rats after 10-V electrical stimulation to the tail. Recent theories of anesthetic action have also postulated that there is anesthetic binding to specific membrane proteins (perhaps some degree of anesthetic-specific proteins) rather than disruption of cell membrane lipids (7). Although these theories may be formulative and not fully proved, the extrapolated values assigned to MAC based on lipid solubility or linear additivity because of the same mechanism of action in rats can be questioned. Because of the importance of knowing the true value for N,O MAC in rats, in particular for experiments in which anesthetic potency and relative dosages of anesthetics are an important part of the protocol, we determined to
*
Anesth Analg 1992;75:995-9
995
996
ANESTH ANALG
RUSSELL AND GRAYBEAL MEASURED N,O MAC IN RATS
directly measure the MAC of N,O in rats during general anesthesia in a hyperbaric chamber.
Methods After approval by the Pennsylvania State University College of Medicine Animal Utilization Committee, we studied 17 Long-Evans rats of similar ages (-10 wk) and weights (370 & 69 g) to directly measure the MAC of N,O. All rats were fed a similar unrestricted diet of Purina Rat Chow. All studies were performed between 8 A M and 4 PM. Anesthesia was induced in a 2-L plexiglass box with a total gas flow of 2 Limin of oxygen with 4.0% isoflurane. Each rat was orotracheally intubated and mechanically ventilated with a small-animal volume ventilator (Harvard Apparatus, Cambridge, Mass.) to mild hypocapnea. Each animal was monitored by compressed spectral array with two-channel electroencephalogram (EEG) and somatosensory cortical evoked potentials (Neurotrac, Interspec Medical, Conshohocken, Pa.), electrocardiograph, and respiratory gas measurements, including inspired oxygen, end-tidal carbon dioxide, N20, and isoflurane by a calibrated, accurate mass spectrometer (MGA 1100, Marquette Gas Monitoring Corp, St. Louis, Mo.) (8). Each animal was placed on a heating blanket for maintenance of body temperature stability. Temperature did not vary by >1"C during the experiment. All equipment was positioned inside an animal hyperbaric chamber (Reimers Engineering, Alexandria, Va.). Isoflurane was discontinued from the ventilating gas and flushed from the respiratory circuit. The chamber was sealed and chamber pressure increased to 2.25 ATA (1710 mm Hg). After reaching stable pressure, ventilation was continued with 1.8 ATA N 2 0 and 0.45 ATA 0,. The cortically recorded somatosensory evoked potentials (CEPs), two-channel analogue EEG, and compressed spectral array were recorded constantly throughout the experiment. Needle electrode impedance was maintained at 13 Hz) and alpha (8-13 Hz) brain wave activity both decreased with increasing N,O partial pressures. There was a preponderance of higher amplitude, slow brain wave activity with N,O anesthesia. However, increased delta (0-3 Hz) wave activity was not documented, but theta ( 4 7 Hz) wave activity commonly occurred at >1.0 ATA N20. The EEG activity in the theta range was predominant at 21.5 ATA N20. Both somatosensory and auditory CEP amplitude and latency have been shown to be decreased and increased, respectively, in humans during inhalation of 33% or 40% N,O (23,24). The effects of N,O on CEP may be more graded in humans than those demonstrated in rats (25). The MAC of N,O in rats is 1.55 ATA when directly
*
ANESTH ANALG 1992;75:995-9
measured with electrical stimulation as a noxious stimulus. During anesthesia with N,O alone in rats, cortical neuroelectrical activity was progressively depressed as the partial pressure increased, although the EEG fast-wave suppression and amplitude increase may not occur at as low a partial pressure as previously demonstrated in humans.
References 1. Hornbein TF, Eger EI 11, Winter PM, Smith G, Wetstone D, Smith KH. The minimum alveolar concentration of nitrous oxide in man. Anesth Analg 1982;61:553-6. 2. DiFazio CA, Brown RE, Ball CG, Heckel, Kennedy SS. Additive effects of anesthetics and theories of anesthesia. Anesthesiology 1972;36:57-63. 3. Cole DJ, Kalichman MW, Shapiro Hh4. Nonlinear additive anesthetic effects of nitrous oxide-enflurane combinations. Anesth Analg 1989;68:556-62. 4. Cole DJ, Kalichman MW, Shapiro HM, Drummond JC. The nonlinear potency of sub-MAC concentrations of nitrous oxide in decreasing the anesthetic requirement of enflurane, halothane, and isoflurane in rats. Anesthesiology 1990;73:93-9. 5. Eisele PH, Taklen L, Eisele JH Jr. Potency of isoflurane and nitrous oxide in conventional swine. Lab Anim Sci 1985;3576-8. 6. Wardley-Smith B, Halsey MJ. Mixtures of inhalation and i.v. anaesthetics at high pressure. Br J Anaesth 1985;571248-56. 7. Franks NP, Lieb WR. Stereospecific effects of inhalational general anesthetic optical isomers on nerve ion channels. Science 1991;254:42730. 8. Richard RB, Loomis JL, Russell GB, Snider MT. Breath-bybreath monitoring of respiratory gas concentrations during compression and decompression. Undersea Biomed Res 1991; 18:117-26. 9. Eger EI 11, Saidman LJ, Brandstater B. Minimum alveolar anesthetic concentration: a standard of anesthetic potency. Anesthesiology 1965;26:756-63. 10. Quasha AI, Eger EI, Tinker JH. Determination and applications of MAC. Anesthesiology 1980;53:315-34. 11. Evers AS, Elliott WJ, Lefkowith JB, Needleman P. Manipulation of rat brain fatty acid composition alters volatile anesthetic potency. J Clin Invest 1985;77:102&%33. 12. Saidman LJ, Eger EI 11. Effect of nitrous oxide and of narcotic
RUSSELL AND GRAYBEAL MEASURED N,O MAC IN RATS
999
premedications on the alveolar concentration of halothane required for anesthesia. Anesthesiology 1964;25:302-6. 13. Cohen PJ. Hyperbaric pressure does not affect the analgesia produced by nitrous oxide in the mouse. Can J Anaesth 1989;36:403. 14. Eger EI 11, Lundgren C, Miller SL, Stevens WC. Anesthetic potencies of sulfur hexafluoride, carbon tetrafluoride, chloroform and Ethrane in dogs: correlation with the hydrate and lipid theories of anesthetic actions. Anesthesiology 1969;30: 129-35. 15. Eger EI 11, Brandstater 8, Saidman LJ, Regan MJ, Severinghaus JW, Munson ES. Equipotent alveolar concentrations of methoxyflurane, halothane, diethyl ether, fluroxene, cyclopropane, xenon and nitrous oxide in the dog. Anesthesiology 1965;26 771-7. 16. Saidman LJ, Eger EI 11. Effect of nitrous oxide and of narcotic premedication on the alveolar concentration of halothane required for anesthesia. Anesthesiology 1964;25:302-6. 17. White PF, Johnston RR, Eger EI 11. Determination of anesthetic requirements in rats. Anesthesiology 1974;40:52-7. 18. Smith RA, Winter PM, Smith M, Eger El 11. Rapidly developing tolerance to acute exposures to anesthetic agents. Anesthesiology 1979;50:496-500. 19. Avramov MN, Shingu K, Mori K. Progressive changes in electroencephalographic responses to nitrous oxide in humans: a possible acute drug tolerance. Anesth Analg 1990;70: 369-74. 20. Faulconer A, Pender JW. The influence of partial pressure of nitrous oxide on the depth of anesthesia and the electroencephalograph. Anesthesiology 1949;10:601-9. 21. Russell GB, Snider MT, Richard RB, Loomis JL. Hyperbaric nitrous oxide as a sole anesthetic agent in humans. Anesth Analg 1990;70:289-95. 22. Smith WDA, Mapleson WW, Siebold K, Hargreaves MD, Clarke GM. Nitrous oxide anesthesia induced at atmospheric and hyperbaric pressures. Part 1. Measured pharmacokinetic and EEG data. Br J Anaesth 1974;46:>12. 23. Chapman CR, Benedetti C. Nitrous oxide effects on cerebral evoked potential to pain: partial reversal with a narcotic antagonist. Anesthesiology 1979;51:1354. 24. Houston HG, McClelland RJ, Fenwick PBC. Effects of nitrous oxide on auditory cortical evoked potentials and subjective thresholds. Br J Anaesth 1988;61:606-10. 25. Sebel PS, Flynn PJ, Ingram DA. Effect of nitrous oxide on visual, auditory and somatosensory evoked potentials. Br J Anaesth 1984;56:1403-7.
MD, FRCPC,
and John M. Graybeal,
CRTT
Department of Anesthesia, Division of Respiratory and Intensive Care, The Milton S. Hershey Medical Center, The Pennsylvania State University College of Medicine, Hershey, Pennsylvania
The minimum alveolar concentration (MAC) of nitrous oxide necessary to prevent purposeful movement in rats has not been directly measured; rather, it has been extrapolated because the required partial pressure exceeds 760 mm Hg, or 1 atm absolute pressure (ATA). Values reported have ranged from 1.36 to 2.20 ATA (136-220 vol%, or 1034-1672 mm Hg). By maintaining general anesthesia at 2.25 ATA (1710 mm Hg), we directly measured the nitrous oxide MAC in 17 Long-Evans rats during mechanical ventilation and monitoring of two-channel electroencephalogram, compressed spectral array and cortical evoked potentials, electrocardiograph, and respiratory and anesthetic gases by mass spectrometry. After a minimal stabilization period of 30 min during ventilation by 1.8 ATA nitrous oxide and 0.45 ATA oxygen, MAC measurements were begun. Each rat was given up to three noxious electrical stimulations of 50 V by 10-ms-durationpulses at 501s for 45 s. The
T
he minimum alveolar concentration (MAC) of nitrous oxide (N20)necessary to prevent repetitive, purposeful movements in rats has not been directly measured. The N,O MAC is a partial pressure value >760 mm Hg, or 1 atm absolute pressure (ATA); hyperbaric exposure is required for MAC measurement and to enable investigators to ventilate animals with high enough N,O partial pressures to exceed MAC as well as prevent hypoxemia. Although the MAC of N 2 0 has been directly measured and reported to be l .04 ATA in humans (1),the reported values for N 2 0 MAC in rats have been primarily extrapolated from the presumed linear adThiswork was supported by an Eric A. Walker Young Investigator Fellowship and by the Department of Anesthesia, The Pennsylvania State University College of Medicine, Hershey, Pennsylvania. Accepted for publication June 10, 1992. Address correspondence to Dr. Russell, Department of Anesthesia, P.O. Box 850, The Milton S. Hershey Medical Center, Hershey, PA 17033. 01992 by the International Anesthesia Research Society 0003-2999/92/$5.00
partial pressure of nitrous oxide was decreased by approximately 10% after each negative response. The MAC was taken as the nitrous oxide concentration midway between that at which there was no response and that at which the rat moved purposefully. The nitrous oxide MAC in Long-Evans rats was determined to be 1.55 0.16 ATA (mean SD). Hyperbaric nitrous oxide decreased electroencephalogram wave frequency to a predominantly theta rhythm of increased amplitude. Cortical evoked potentials had decreased wave amplitudes and increased latencies with increasing partial pressures >0.75 ATA. General anesthesia with nitrous oxide at hyperbaric pressures allows direct measurement of the nitrous oxide MAC in rats and demonstrates neurophysiologic depressant effects on the electroencephalogram and somatosensory evoked potentials.
*
*
(Anesth Analg 1992;75:995-9)
ditivity between N20 and volatile anesthetics and the correlation of anesthetic potency with lipid solubility (2,3). Values reported have ranged from 1.36 to 2.20 ATA. However, nonlinear additivity of N,O and other anesthetics has also been reported (3-5). Wardley-Smith and Halsey (6) found N,O potency to be 1.48 k 0.076 ATA (mean SD) in an unknown number of rats after 10-V electrical stimulation to the tail. Recent theories of anesthetic action have also postulated that there is anesthetic binding to specific membrane proteins (perhaps some degree of anesthetic-specific proteins) rather than disruption of cell membrane lipids (7). Although these theories may be formulative and not fully proved, the extrapolated values assigned to MAC based on lipid solubility or linear additivity because of the same mechanism of action in rats can be questioned. Because of the importance of knowing the true value for N,O MAC in rats, in particular for experiments in which anesthetic potency and relative dosages of anesthetics are an important part of the protocol, we determined to
*
Anesth Analg 1992;75:995-9
995
996
ANESTH ANALG
RUSSELL AND GRAYBEAL MEASURED N,O MAC IN RATS
directly measure the MAC of N,O in rats during general anesthesia in a hyperbaric chamber.
Methods After approval by the Pennsylvania State University College of Medicine Animal Utilization Committee, we studied 17 Long-Evans rats of similar ages (-10 wk) and weights (370 & 69 g) to directly measure the MAC of N,O. All rats were fed a similar unrestricted diet of Purina Rat Chow. All studies were performed between 8 A M and 4 PM. Anesthesia was induced in a 2-L plexiglass box with a total gas flow of 2 Limin of oxygen with 4.0% isoflurane. Each rat was orotracheally intubated and mechanically ventilated with a small-animal volume ventilator (Harvard Apparatus, Cambridge, Mass.) to mild hypocapnea. Each animal was monitored by compressed spectral array with two-channel electroencephalogram (EEG) and somatosensory cortical evoked potentials (Neurotrac, Interspec Medical, Conshohocken, Pa.), electrocardiograph, and respiratory gas measurements, including inspired oxygen, end-tidal carbon dioxide, N20, and isoflurane by a calibrated, accurate mass spectrometer (MGA 1100, Marquette Gas Monitoring Corp, St. Louis, Mo.) (8). Each animal was placed on a heating blanket for maintenance of body temperature stability. Temperature did not vary by >1"C during the experiment. All equipment was positioned inside an animal hyperbaric chamber (Reimers Engineering, Alexandria, Va.). Isoflurane was discontinued from the ventilating gas and flushed from the respiratory circuit. The chamber was sealed and chamber pressure increased to 2.25 ATA (1710 mm Hg). After reaching stable pressure, ventilation was continued with 1.8 ATA N 2 0 and 0.45 ATA 0,. The cortically recorded somatosensory evoked potentials (CEPs), two-channel analogue EEG, and compressed spectral array were recorded constantly throughout the experiment. Needle electrode impedance was maintained at 13 Hz) and alpha (8-13 Hz) brain wave activity both decreased with increasing N,O partial pressures. There was a preponderance of higher amplitude, slow brain wave activity with N,O anesthesia. However, increased delta (0-3 Hz) wave activity was not documented, but theta ( 4 7 Hz) wave activity commonly occurred at >1.0 ATA N20. The EEG activity in the theta range was predominant at 21.5 ATA N20. Both somatosensory and auditory CEP amplitude and latency have been shown to be decreased and increased, respectively, in humans during inhalation of 33% or 40% N,O (23,24). The effects of N,O on CEP may be more graded in humans than those demonstrated in rats (25). The MAC of N,O in rats is 1.55 ATA when directly
*
ANESTH ANALG 1992;75:995-9
measured with electrical stimulation as a noxious stimulus. During anesthesia with N,O alone in rats, cortical neuroelectrical activity was progressively depressed as the partial pressure increased, although the EEG fast-wave suppression and amplitude increase may not occur at as low a partial pressure as previously demonstrated in humans.
References 1. Hornbein TF, Eger EI 11, Winter PM, Smith G, Wetstone D, Smith KH. The minimum alveolar concentration of nitrous oxide in man. Anesth Analg 1982;61:553-6. 2. DiFazio CA, Brown RE, Ball CG, Heckel, Kennedy SS. Additive effects of anesthetics and theories of anesthesia. Anesthesiology 1972;36:57-63. 3. Cole DJ, Kalichman MW, Shapiro Hh4. Nonlinear additive anesthetic effects of nitrous oxide-enflurane combinations. Anesth Analg 1989;68:556-62. 4. Cole DJ, Kalichman MW, Shapiro HM, Drummond JC. The nonlinear potency of sub-MAC concentrations of nitrous oxide in decreasing the anesthetic requirement of enflurane, halothane, and isoflurane in rats. Anesthesiology 1990;73:93-9. 5. Eisele PH, Taklen L, Eisele JH Jr. Potency of isoflurane and nitrous oxide in conventional swine. Lab Anim Sci 1985;3576-8. 6. Wardley-Smith B, Halsey MJ. Mixtures of inhalation and i.v. anaesthetics at high pressure. Br J Anaesth 1985;571248-56. 7. Franks NP, Lieb WR. Stereospecific effects of inhalational general anesthetic optical isomers on nerve ion channels. Science 1991;254:42730. 8. Richard RB, Loomis JL, Russell GB, Snider MT. Breath-bybreath monitoring of respiratory gas concentrations during compression and decompression. Undersea Biomed Res 1991; 18:117-26. 9. Eger EI 11, Saidman LJ, Brandstater B. Minimum alveolar anesthetic concentration: a standard of anesthetic potency. Anesthesiology 1965;26:756-63. 10. Quasha AI, Eger EI, Tinker JH. Determination and applications of MAC. Anesthesiology 1980;53:315-34. 11. Evers AS, Elliott WJ, Lefkowith JB, Needleman P. Manipulation of rat brain fatty acid composition alters volatile anesthetic potency. J Clin Invest 1985;77:102&%33. 12. Saidman LJ, Eger EI 11. Effect of nitrous oxide and of narcotic
RUSSELL AND GRAYBEAL MEASURED N,O MAC IN RATS
999
premedications on the alveolar concentration of halothane required for anesthesia. Anesthesiology 1964;25:302-6. 13. Cohen PJ. Hyperbaric pressure does not affect the analgesia produced by nitrous oxide in the mouse. Can J Anaesth 1989;36:403. 14. Eger EI 11, Lundgren C, Miller SL, Stevens WC. Anesthetic potencies of sulfur hexafluoride, carbon tetrafluoride, chloroform and Ethrane in dogs: correlation with the hydrate and lipid theories of anesthetic actions. Anesthesiology 1969;30: 129-35. 15. Eger EI 11, Brandstater 8, Saidman LJ, Regan MJ, Severinghaus JW, Munson ES. Equipotent alveolar concentrations of methoxyflurane, halothane, diethyl ether, fluroxene, cyclopropane, xenon and nitrous oxide in the dog. Anesthesiology 1965;26 771-7. 16. Saidman LJ, Eger EI 11. Effect of nitrous oxide and of narcotic premedication on the alveolar concentration of halothane required for anesthesia. Anesthesiology 1964;25:302-6. 17. White PF, Johnston RR, Eger EI 11. Determination of anesthetic requirements in rats. Anesthesiology 1974;40:52-7. 18. Smith RA, Winter PM, Smith M, Eger El 11. Rapidly developing tolerance to acute exposures to anesthetic agents. Anesthesiology 1979;50:496-500. 19. Avramov MN, Shingu K, Mori K. Progressive changes in electroencephalographic responses to nitrous oxide in humans: a possible acute drug tolerance. Anesth Analg 1990;70: 369-74. 20. Faulconer A, Pender JW. The influence of partial pressure of nitrous oxide on the depth of anesthesia and the electroencephalograph. Anesthesiology 1949;10:601-9. 21. Russell GB, Snider MT, Richard RB, Loomis JL. Hyperbaric nitrous oxide as a sole anesthetic agent in humans. Anesth Analg 1990;70:289-95. 22. Smith WDA, Mapleson WW, Siebold K, Hargreaves MD, Clarke GM. Nitrous oxide anesthesia induced at atmospheric and hyperbaric pressures. Part 1. Measured pharmacokinetic and EEG data. Br J Anaesth 1974;46:>12. 23. Chapman CR, Benedetti C. Nitrous oxide effects on cerebral evoked potential to pain: partial reversal with a narcotic antagonist. Anesthesiology 1979;51:1354. 24. Houston HG, McClelland RJ, Fenwick PBC. Effects of nitrous oxide on auditory cortical evoked potentials and subjective thresholds. Br J Anaesth 1988;61:606-10. 25. Sebel PS, Flynn PJ, Ingram DA. Effect of nitrous oxide on visual, auditory and somatosensory evoked potentials. Br J Anaesth 1984;56:1403-7.
