Effects of guaiphenesin on the equine electroencephalogram during anaesthesia with halothane in oxygen

Veterinary Anaesthesia and Analgesia, 2000, 27, 6–12

Effects of guaiphenesin on the equine electroencephalogram during anaesthesia with halothane in oxygen CB Johnson BVSc, PhD, DVA, Dip ECVA Department of Clinical Veterinary Science, University of Bristol, Langford House, Langford, Bristol, UK

M Bloomfield MA, Vet MB, PhD, DVA, Dip ECVA and PM Taylor MA, Vet MB, PhD, DVA, Dip ECVA Department of Clinical Veterinary Medicine, University of Cambridge, Madingley Rd, Cambridge, UK

Correspondence: Dr CB Johnson, Department of Clinical Veterinary Science, University of Bristol, Langford House, Langford, Bristol BS18 7DU, UK.

Abstract Objective To identify and characterize the effects of guaiphenesin (GGE) on the electroencephalogram during halothane anaesthesia. Study design Prospective controlled study. Animals Eight healthy Welsh mountain pony geldings between 5 and 9 years old and weighing between 270 and 330 kg (mean 301 kg). Methods Anaesthesia was induced with thiopentone and maintained using halothane in oxygen. End tidal halothane was maintained above 0.75 and below 0.85%. The EEG was recorded continuously and a binaural broad band click stimulus was provided throughout the experiment at 6.1224 Hz. An infusion of 1500 mg GGE was given over 5 minutes. Samples were taken for blood gas analysis and plasma GGE assay (HPLC) 5 minutes prior to the start of the infusion and at 3, 5, 7, 10, 15, 20, 30, 45 and 60 minutes thereafter. The median and 95th percentile of the EEG were calculated using standard statistical techniques and the mid-latency of the auditory evoked response was generated. The values of EEG variables at each time point were compared to the average value for the 15 minute period before the infusion was started. Arterial blood gas values and plasma GGE concentration were compared to

Work was carried out at the University of Cambridge. 6

the baseline sample taken prior to the start of the infusion. Comparisons were made using analysis of variance for repeated measures followed by Dunnett’s test if a significant difference was detected. Results The peak serum plasma concentration was 49.6 2 7.8 mg mL−1 (mean 2 SD) occurring five minutes after the start of the infusion. The 95% spectral edge frequency (F95) of the EEG decreased by a maximum of 5.2 2 14.3% 5 minutes after the start of the GGE infusion. This change did not reach statistical significance (p = 0.07). When three nonresponders were excluded, the depression in F95 at 5 minutes in the remaining five animals became 13.0 2 12.0% and was statistically significant (p = 0.02). No changes were seen in median frequency of the EEG or the second differential of the middle latency auditory evoked potential. Conclusions These results did not demonstrate any statistically significant GGE-induced changes in the EEG. However, there was some visible depression of F95 in five of the animals studied even though the dose of GGE used was considerably less than that used in most clinical circumstances. Clinical relevance The EEG effects seen in this study concur with the commonly held view that while GGE has some sedative effects, it is not a reliable anaesthetic agent. Keywords horses.

electroencephalogram,

guaiphenesin,

EEG effects of guaiphenesin in horses CB Johnson et al.

Introduction The electroencephalogram (EEG) is the electrical activity generated by the surface of the cerebral cortex. It has been found to alter in various ways under the effects of different anaesthetic agents (Gibbs et al. 1937). These changes have been quantified using the technique of Fast Fourier Transformation (FFT) which allows frequency spectra to be generated from raw EEG data (Cooley & Tukey 1965). These power spectra show consistent changes with increasing doses of particular anaesthetics and have been used in a limited way to predict adequacy of anaesthesia (Thomsen et al. 1989). The middle latency auditory evoked potential (MLAEP) has been shown to be a discriminator between patients who moved in response to placement of a laryngeal mask airway and those who did not, suggesting that MLAEP has a greater ability to predict responses to noxious stimuli compared with EEG derivatives (Doi et al. 1999). Guaiphenesin (GGE) is a centrally acting muscle relaxant (Funk 1970) which has been used to cast horses (Schatzman 1974) for induction of anaesthesia in combination with a short acting anaesthetic agent (usually a barbiturate) (Dodman 1980) and as a component of infusions for total intravenous anaesthesia (Greene et al. 1986; McCarty et al. 1990; Young et al. 1991; Taylor et al. 1992). Guaiphenesin is thought to act at neuronal synapses in the spinal cord (Goebel & Kohlhas 1951) to produce ataxia and muscle relaxation; it may have some sedative action via the ascending reticular formation (Gycha 1952; Chemnitius et al. 1957). Despite this, clinical doses have no reliable anaesthetic effects (Funk 1973). There are no published reports of the EEG effects of GGE. To examine the potential of the EEG as a monitoring technique in equine anaesthesia it is first necessary to characterize the electroencephalographic effects of both constant, and changing plasma concentrations of anaesthetics. The aim of this study was to investigate the effects of GGE on the EEG and auditory evoked potential in the horse.

Materials and methods This study was conducted in accordance with the Animals (Scientific Procedures) Act 1986 (Home Office Licence 80/666). Eight healthy Welsh mountain pony geldings were used. They weighed between 270 and 330 kg (mean 301 kg) and were between 5 Veterinary Anaesthesia and Analgesia, 2000, 27, 6–12

and 9 years old. Each pony had previously undergone surgery in which the right carotid artery had been raised to a subcutaneous position using a surgical technique adapted from that of Tavernor (1969). The interval between carotid elevation and this experiment was at least one year. After placement of a 14 SWG cannula in the left jugular vein, anaesthesia was induced using 3 g thiopentone. Additional thiopentone was given in 0.5 or 1 g increments to allow endotracheal intubation, resulting in a total dose of 11.8 2 1.87 mg kg−1, (mean 2 SD). Anaesthesia was maintained using halothane in oxygen delivered through a circle breathing system (JD Medical, Phoenix, AZ, USA). Airway gases were sampled continuously from the end of the endotracheal tube adjacent to the circle system. Halothane concentration was measured using a piezo-electric agent monitor (Kontron 7860, Kontron, Watford, Herts, UK) and carbon dioxide using an infrared monitor (Datex CD 200–02, Helsinki, Finland). Intermittent positive pressure ventilation was applied and adjusted to maintain end tidal CO2 between 5.31 and 5.98 kPa (40 and 45 mm Hg). The vaporizer was adjusted to maintain end-tidal halothane concentration above 0.75 and below 0.85%. A 20 SWG cannula was placed in the raised carotid artery. This cannula was used for all subsequent blood samples and for monitoring arterial blood pressure. The electrocardiogram and arterial blood pressure were monitored using an electronic system (Minimon 7132 A, Kontron, Watford, Herts, UK) and recorded on a paper trace recorder (Lectromed, Letchworth, Herts, UK). The blood pressure transducer (Ohmeda, Swindon, Wilts, UK) was calibrated against a mercury column on each occasion. The EEG was recorded continuously using a digital system developed by Medelec (Alert system, Medelec, Woking, Surrey, UK) and stored on a personal computer (450/L, Dell Computer Corporation, Bracknell, Berkshire, UK). The EEG was recorded with a passband of 0.5–400 Hz using three stainless steel needle electrodes placed transdermally. The electrodes were positioned as described by Mayhew & Washbourne (1990) with the active electrode over the right zygomatic process, the reference electrode in the midline over the parietal suture rostral to the divergence of the temporalis muscles and the ground electrode caudal to the poll. Ear pieces (Ross Audio FEN20, Maplin, Rayleigh, Essex, UK) were placed in the external ear canal of each pony and a binaural broad band click stimulus was provided throughout the experiment at 6.1224 Hz. 7

EEG effects of guaiphenesin in horses CB Johnson et al.

Data collection was started after 60 minutes of anaesthesia and was continuous over the course of the experiment. Fifteen minutes after the start of data collection an infusion of 1500 mg GGE (Gujatal, Aesculap, Tuttlingen, Germany) was given over 5 minutes resulting in an infusion rate of 300 mg min−1 and a mean total dose of 5 mg kg−1. The infusion was given into the jugular cannula in a stream of rapidly flowing 0.9% saline. Blood samples were taken for blood gas analysis and plasma GGE assay 5 minutes prior to the start of the infusion and 3, 5, 7, 10, 15, 20, 30, 45 and 60 minutes after the start of the infusion. Electroencephalographic data were collected until 60 minutes after the start of the infusion to allow time for the EEG variables to return to baseline. The blood samples collected for blood gas analysis were stored on ice and analysed within two hours of collection using an acid base laboratory (ABL, Radiometer, Copenhagen, Denmark). Samples taken for GGE assay were centrifuged for 10 minutes at 4000 r.p.m. × 3000 g using a refrigerated centrifuge cooled to 4 °C (Koolspin, Burkard, Rickmansworth, Herts, UK). Immediately after centrifugation, the plasma was separated by pipette and stored at −20 °C until analysis. All samples were frozen within 1 hour of collection. Guaiphenesin was assayed in the stored plasma using high performance liquid chromatography (HPLC). The plasma was thawed at room temperature and centrifuged at 4 °C for 10 minutes at 4000 r.p.m. × 3000 g. The supernatant was removed and 100 mL was added to 200 mL of acetonitrile in a 1.5-mL microcapped centrifuge tube (Elkay, Basingstoke, Hampshire, UK). After vortexing for 30 seconds, the mixture was centrifuged at 3000g for 10 minutes and the supernatant removed for

Table 1 Blood gas values at time points after the start of the GGE infusion. All values are mean 2 SD. No values are significantly different from the control (time = 0)

8

analysis. The HPLC assay was performed using a mobile phase composed of 25% acetonitrile and 75% 0.1 M phosphate buffer with a pH of 7.6 at a flow rate of 1.2 mL min−1. The stationary phase comprised a 250 × 4.6 mm sperisorb 50DS2 (HPLC technology, Macclesfield, Cheshire, UK). An ultraviolet detector was used with the wavelength set at 276 nm. The linearity of the assay was confirmed between 0 and 200 mg mL−1. All samples assayed lay within these limits. The intra-assay coefficient of variation was 2.5%. All samples were assayed in the same batch. Electroencephalographic analysis was performed off line after the completion of each experiment. The raw EEG data were inspected manually in 2-second epochs and any epochs contaminated by overscale, underscale or artefact due to nystagmus or other muscular activity were excluded from analysis (Walter 1987). The median frequency (F50) and 95th percentile or spectral edge frequency (F95) were calculated for each epoch. A further low pass filter at 30 Hz was applied and the data multiplied by a 10% raised cosine window prior to spectral analysis. A fast Fourier transform (FFT) was carried out on each epoch generating a frequency histogram with 0.5 Hz frequency bins. The FFT algorithm has been described by Welch (1967). For each frequency histogram, the median and 95th percentile were calculated using standard statistical techniques. These results were averaged over five epochs to give 10second data windows with no overlap. The data were then smoothed using a 10 point moving average. The mid-latency of the auditory evoked potential (MLAEP) was generated by averaging the EEG for 125 milliseconds following each auditory stimulus. Each average was composed of 512 stimuli with no overlap. The MLAEP was filtered with a pass band of 15–100 Hz. The second differential of the MLAEP

Time (minutes)

pH

PO2 (kPa)

PCO2 (kPa)

0 3 5 7 10 15 20 30 45 60

7.375 2 0.056 7.389 2 0.054 7.387 2 0.048 7.377 2 0.040 7.390 2 0.051 7.383 2 0.047 7.377 2 0.057 7.370 2 0.060 7.369 2 0.058 7.353 2 0.057

35.8 2 14.4 27.1 2 12.6 25.5 2 12.4 33.0 2 14.9 33.2 2 17.0 30.1 2 17.1 34.2 2 16.9 38.2 2 14.3 34.6 2 14.8 39.3 2 17.3

6.5 2 0.4 6.1 2 0.5 6.1 2 0.4 6.3 2 0.6 6.1 2 0.7 6.0 2 0.5 6.1 2 0.6 6.4 2 0.6 6.3 2 0.6 6.7 2 0.8

Veterinary Anaesthesia and Analgesia, 2000, 27, 6–12

EEG effects of guaiphenesin in horses CB Johnson et al.

Table 2 Change in plasma GGE concentration and EEG variables at time points after the start of the GGE infusion

Time (minutes)

Plasma GGE concentration (mg ml−1)

F50 (%)

F95 (%)

DD (%)

0 3 5 7 10 15 20 30 45 60

020 32.9 2 2.9* 49.6 2 7.8* 17.7 2 3.6* 11 2 3.5* 6.1 2 2 3.6 2 1.7 1 2 1.3 0.1 2 0.3 020

99.8 2 7.1 99.5 2 17.2 106.1 2 17.6 104.6 2 22.7 92.1 2 16.9 101.4 2 35.5 103.8 2 26.6 110.9 2 19.7 90.4 2 12.9 105.6 2 28.3

101.1 2 2.9 99.4 2 14.7 94.8 2 14.3 98 2 10.4 95.6 2 10.3 105.9 2 13.2 101.9 2 12.4 107.5 2 3.5 104.7 2 13.1 111.4 2 16.2

100.9 2 28.6 86.6 2 39.2 91 2 47.8 89.3 2 54 85.6 2 33.6 99.7 2 28.8 93.9 2 35 125.4 2 49.4 105.4 2 15.5 89.2 2 23.3

All data are shown as mean 2 SD. * denotes a statistically significant difference from the value at time 0.

(DD) between 30 and 90 milliseconds (Newton et al. 1993) was determined by calculating the second differential of each recorded MLAEP using the following formula (Jordan 1994):

y(n) = x(n + d) + x(n − d) − 2*x(n) Where: x(n) is the nth sample value of the recorded MLAEP. d is the spread either side of the nth sample over which the estimate of the second rate of change is performed. y(n) is the second differential estimate for sample n. The value of y(n) was summated for each sample point to give a result for the function over the period of the MLAEP considered. The result of this function was scaled to give a value between 0 and 100. This value was taken as the DD. The values of EEG variables at each time point were compared to the average value for the 15minute period before the infusion was started. Arterial blood gas values and the plasma GGE concentration were compared to the baseline sample taken prior to the start of the infusion. Comparisons were made using analysis of variance for repeated measures followed by Dunnett’s test (Dunnett 1964) if a significant difference was detected. All data are shown as mean 2 SD. A value of p ¾ 0.05 was taken as statistically significant. Veterinary Anaesthesia and Analgesia, 2000, 27, 6–12

Results The values of pH, PO2 and PCO2 are shown in Table 1. There were no statistically significant changes in any of these variables. The changes in EEG variables and plasma GGE concentrations are shown in Table 2. The changes in F50, F95 and DD are illustrated in Figs 1, 2 and 3, respectively. The plasma GGE concentrations are illustrated in Fig. 4. The plasma GGE concentrations were significantly increased above the baseline from 3 to 10 minutes after the start of the infusion with a

Figure 1 Percentage change in F50 after infusion of GGE. Each line represents an individual animal. For statistical information see Table 2. 9

EEG effects of guaiphenesin in horses CB Johnson et al.

Figure 2 Percentage change in F95 after infusion of GGE. Each line represents an individual animal. For statistical information see Table 2.

Figure 3 Percentage change in DD after infusion of GGE. Each line represents an individual animal. For statistical information see Table 2.

Figure 4 Plasma GGE concentration. Values are shown as mean 2 SD. Ž denotes a value significantly different from baseline (t = 0).

Figure 5 Percentage change in F95 after infusion of GGE following removal of three ponies without obvious depression over the period of infusion. Each line represents an individual animal.

Discussion maximum plasma GGE concentration of 49.6 2 7.8 mg kg−1 at 5 minutes. There was an apparent decrease in F95 in five animals (Fig. 5), but no statistically significant change when all animals were considered together. When the three nonresponders were excluded, F95 at five minutes in the remaining five animals decreased to 87.0 2 12.0% and was statistically significant (p = 0.02). No consistent changes were seen in F50 or DD. 10

In studies of the pharmacodynamics of centrallyacting agents it is usual to restrict administration of centrally-acting drugs as far as possible to that under study (Scott et al. 1985; Smith et al. 1985; Wauquier et al. 1988). This is difficult in the horse because of its size and occasionally uncooperative nature. For this reason a slightly more complex protocol was used to study the effect of changing plasma drug concentrations against a stable background (Johnson & Taylor 1997). The aim was to produce a state Veterinary Anaesthesia and Analgesia, 2000, 27, 6–12

EEG effects of guaiphenesin in horses CB Johnson et al.

of CNS function which was reproducible, resulted in minimal depression consistent with physical immobility, and against which the effects of a changing plasma concentration of the trial drug could be measured. Although induction of anaesthesia with halothane delivered by mask is possible in the horse (Pascoe et al. 1993), intravenous induction is usually smoother and safer for both the animals and handlers when specialized physical restraining systems are unavailable. Thiopentone was chosen for induction of anaesthesia, as it is reasonably predictable in the horse when used alone without pre-anaesthetic medication. Thiopentone serum concentrations had fallen to below the threshold concentration for discernable EEG alterations in humans by the time of the start of the experimental period (Hudson et al. 1983). The chosen EEG derivatives were all stable within 60 minutes of induction of anaesthesia. The total infusion dose was chosen to produce peak plasma concentrations similar to those of a study investigating the EEG effects of total intravenous anaesthesia using a combination of detomidine, ketamine and GGE (Johnson 1996). The dose of GGE used was approximately 10% of that routinely used with thiopentone in the induction of general anaesthesia (Hubbell 1996). This study was unable to demonstrate any statistically significant GGE-induced changes in the EEG. However, there was some visible depression of F95 in five of the animals studied (Fig. 5) indicating some CNS depression. It is possible that higher doses of GGE would have a more consistent EEG effect than the one observed in the present study.

Acknowledgements The equipment used to measure the EEG was provided by The Elise Pilkington Trust. The authors would like to thank R. Eastwood for his help in conducting the experimental work.

References Chemnitius KH, Boltze KH, Hofmann H (1957) Die wirkung von gaujacol-glycerinaether in kombination mit pyrazolon im tierversuch. Pharmazie 12, 391. Cooley JW, Tukey JW (1965) An algorithm for the machine calculation of complex Fourier series. Math Comput 19, 297–301. Doi N, Garaj RJ, Mantzaridis et al. (1999) Prediction of movement at laryngeal mask airway insertion: comVeterinary Anaesthesia and Analgesia, 2000, 27, 6–12

parison of auditory evoked potential, bispectral index, spectral edge frequency and median frequency. Br J Anaesth 82, 203–207. Dodman NH (1980) Chemical restraint in the horse. Equine Vet J 12, 166–170. Dunnett CW (1964) New tables for multiple comparisons with a control. Biometrics 20, 482–491. Funk KA (1970) Glyceryl Guaiacolate: a centrally acting muscle relaxant. Equine Vet J 2, 173–178. Funk KA (1973) Glyceryl guaiacolate: some effects and indications in horses. Equine Vet J 5, 15–19. Gibbs FA, Gibbs EL, Lennox WG (1937) Effect on the electroencephalogram of certain drugs which influence nervous activity. Arch Intern Med 60, 154–166. Goebel F, Kohlhas W (1951) Die kombinierte eunarconcurarythan narkose bei hund und katze. Tieraerztl Umsch 6, 348. Greene SA, Thurmon JC, Tranquilli WJ, et al. (1986) Cardiopulmonary effects of continuous intravenous infusion of guaiafenesin, ketamine and xylazine in ponies. Am J Vet Res 47, 2364–2367. Gycha FP (1952) Ueber tierexperimentelle untersuchungen und selbstversuche mid dem muskelrelaxans MY 301. Medsche Mschr Stuttg 6, 226. Hubbell JAE (1996) Anesthesia and immobilisation of horses. In: Lumb and Jones’ Veterinary Anesthesia. (3rd edn). Thurmon JC, Tranquilli WJ, Benson GJ (eds). Williams & Wilkins, Baltimore, MD, pp. 599–610. Hudson RJ, Stanski DR, Saidman LJ, et al. (1983) A model for studying depth of anesthesia and acute tolerance to thiopental. Anesthesiology 59, 301–308. Johnson CB (1996) Some effects of anaesthesia on the electrical activity of the equine brain. PhD Thesis. University of Cambridge, Cambridge, UK. Johnson CB, Taylor PM (1997) Effects of alfentanil on the equine electroencephalogram during anaesthesia with halothane in oxygen. Res Vet Sci 62, 159–163. Jordan C (1994) Auditory Evoked Response Program AVG. Manual. C Jordan, London. Mayhew IG, Washbourne JR (1990) A method of assessing auditory and brainstem function in horses. Br Vet J 146, 509–518. McCarty JE, Trim CM, Ferguson D (1990) Prolongation of anesthesia with xylazine, ketamine and guaifenesin in horses: 64 cases (1986–89). J Am Vet Med Assoc 197, 1646–1650. Newton DEF, Thornton C, Jordan C (1993) The auditory evoked response as a monitor of anesthetic depth. In: Memory and Awareness in Anesthesia. Sebel PS, Bonke B, Winograd E (eds). Prentice Hall, Englewood Cliffs, NJ, pp. 274–280. Pascoe PJ, Steffey EP, Black WD, et al. (1993) Evaluation of the effect of alfentanil on the MAC of halothane in horses. Am J Vet Res 54, 1327–1332. Schatzman U (1974) The induction of general anaesthesia in the horse with glyceryl guaiacolate. Comparison when 11

EEG effects of guaiphenesin in horses CB Johnson et al.

used alone and with sodium thiamylal (Surital). Equine Vet J 6, 164–169. Scott JC, Ponganis KV, Stanski DR (1985) EEG quantitation of narcotic effect: the comparative pharmacodynamics of fentanyl and alfentanil. Anesthesiology 62, 234–241. Smith NT, Westover CJ Jr, Quinn M, et al. (1985) An electroencephalographic comparison of alfentanil with other narcotics and with thiopental. J Clin Monitor 1, 236–244. Tavernor WD (1969) Technique for the subcutaneous relocation of the common carotid artery in the horse. Am J Vet Res 30, 1899. Taylor PM, Luna SPL, Brearley JC, et al. (1992) Physiological effects of total intravenous surgical anaesthesia using detomidine-guaiaphenesin-ketamine in horses. J Vet Anaesth 19, 24–31. Thomsen CE, Christensen KN, Rosenfalck A (1989) Computerised monitoring of depth of anaesthesia with isoflurane. Br J Anaesth 63, 36–43. Walter DO (1987) Introduction to computer analysis in electroencephalography. In: Electroencephalography –

12

Basic Principles, Clinical Applications and Related Fields. Niedermeyer E, Lopes da Silva F (eds). Urban and Schwarzenberg, Baltimore, MD, pp. 863–870. Wauquier A, De Ryck M, Van den Broeck W, et al. (1988) Relationships between quantitative EEG measures and pharmacodynamics of alfentanil in dogs. Electroen Clin Neuro 69, 550–560. Welch PD (1967) The use of Fast Fourier Transform for the estimation of power spectra: a method based on time averaging over short, modified periodograms. IEEE Transactions Audio Electroacoustics 15, 70–73. Young LE, Bartram DH, Diamond M, et al. (1991) Preliminary findings following the use of a continuous infusion of xylazine, GGE and ketamine to maintain general anaesthesia in 40 horses undergoing a wide variety of surgical procedures. Proceedings of the 4th International Congress of Veterinary Anaesthesia, Utrecht, 1991, pp. 171–174. Received 21 September 1999; revised 20 October 1999; accepted 28 October 1999

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