1 Department of Paediatric Anaesthesia and 2 Department of Anaesthesia, University Hospital La Paz, Madrid, Spain. 3 Veterinary School, Complutense University of Madrid, Spain. 4 Department of Anaesthesia, University Hospital La Princesa, Madrid, Spain. 5 Medical School, Autonoma University of Madrid, Spain
* Corresponding author. Hospital La Paz, P° de la Castellana 261, Servicio de Anestesiología Pediátrica, 28046-Madrid, Spain. E-mail: ventilacionpediatrica{at}hotmail.com
Accepted for publication May 15, 2005.
![]() |
Abstract |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Methods. Five adult beagle dogs completed the study. In a randomized and blinded manner each animal received placebo (saline 0.1 ml kg1) and three concentrations of pure sevoflurane administered intrathecally (0.05, 0.075 and 0.1 ml kg1) by means of a permanent spinal catheter. Sensory and motor block and state of consciousness were determined at baseline and at predetermined regular intervals until at least 2 h after total recovery.
Results. None of the dogs presented a decrease in consciousness with either 0.05 or 0.075 ml kg1 of sevoflurane. Administration of 0.1 ml kg1 produced light sedation (2 on a four-point sedation scale) in three of the five dogs. A comparison of the duration of the sensory and motor blocks among the three sevoflurane dosages shows a significant dose-dependent increase that is greater in all cases than that for the saline solution.
Conclusions. Spinal administration of pure sevoflurane resulted in a dose-related and totally reversible motor and sensory regional block without any signs of clinical neurotoxicity or significant decrease in consciousness. Therefore the model allows us to comment on the analgesic effects at the spinal level in addition to the direct immobilizing effects of sevoflurane.
Keywords: anaestheticanalgesic regimens ; anaesthetic techniques, subarachnoid ; anaesthetics volatile, sevoflurane ; model, dog
![]() |
Introduction |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Existing in vitro and in vivo studies of the spinal effects of halogenated ethers have not determined whether the immobilizing effect at the spinal cord is due to inhibition of nociceptive transmission acting on dorsal neurones or inhibition of the motor neurones or a combination of both effects.811
This study presents a novel in vivo experimental model in dogs that has not been used before. Sevoflurane is directly administered to the spine instead of through the more traditional route of systemic inhalation. The main objective of our study with this model was to characterize the effects of sevoflurane, administered directly to the spine in pure liquid form for the first time, and to study the clinical effects of increasing concentrations of the drug on consciousness and on superficial and deep sensitivity, as well as on the motor response to a painful stimulus and the possibility of motor block. The study was designed to evaluate sensory effects separately from motor effects. In addition, we also assessed the reversibility and duration of clinical spinal effects and whether administration produces signs of medullar lesions.
![]() |
Materials and methods |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Catheter placement
Catheters were placed under general anaesthesia (premedication with medetomidine, induction with propofol and maintenance with isoflurane and fentanyl) with standard monitoring and mechanical ventilation. A 3 cm vertical incision was made in the medial lumbar region between L5 and L6 creating a tobacco sac to house the catheter connected to an injection site cap with a latex membrane through which the anaesthetic could be administered with a transdermic needle.
The subarachnoid space was located with a puncture between the L4 and L5 vertebrae. An epidural Tuohy calibre 20 epidural needle (Perisafe®, Becton Dickison, Bidford-on-Avon, UK) was inserted using the loss of resistance technique and, once the epidural space was located, advanced until a free flow of cerbrospinal fluid (CSF) was obtained. At this point the catheter was introduced into the subarachnoid space. Catheter placement was checked using a myelographic scope, introducing 0.5 ml of a low concentration of iodinated contrast (240 mg ml1) (Omnipaque®, Amersham Health, Cork, Ireland). The distance between the insertion point and placement in the spinal space was calculated, and another 5 cm was added to place the catheter at L2.
The catheters were left to stabilize for 1 week. Proper catheter placement and function were confirmed 72 h before beginning the studies by administering a test dose of lidocaine 0.1 mg kg1. If there were doubts as to proper catheter function, myelography was repeated.
Sevoflurane administration
Each animal received three sevoflurane doses (Sevorane®, Abbot Laboratories, Queenborough, Kent, UK): 0.05 ml kg1 (0.076 mg kg1), 0.075 ml kg1 (0.114 mg kg1) and 0.1 ml kg1 (0.152 mg kg1), and saline solution 0.1 ml kg1 (0.9%) as control. The specific gravity of sevoflurane is 1.52 g l1 and its molecular weight is 200.05.12 The clinically relevant dose of sevoflurane is 0.4 mM (
1 MAC).13 We used a priori calculation to design the study based on the theoretical distribution of CSF volume in the dog (2.53 ml kg1).14 According to this, the maximum dose of 0.1 ml kg1 used in our study corresponds to 0.24 mM, which is <1 MAC, the medium sevoflurane dose of 0.075 ml kg1 corresponds to 0.18 mM, which is
0.5 MAC and the lowest dose of 0.05 ml kg1 corresponds to 0.12 mM (<0.33 MAC). These doses were assigned randomly and blindly. To ensure blinding, two people randomly chose the doses and administrated each dose in one room, and a third person (the same observer for the entire study) made the evaluations in another room. The animals were dosed at intervals of at least 72 h and after each administration the catheter was flushed with 0.3 ml saline solution. Immediately after administration, the animal was allowed to walk freely.
Data collection
We used four clinical tests following a modification of the method described by Feldman and colleagues15 (Table 1). The first test evaluated the level of consciousness on a four-point sedation scale, the second test evaluated motor function on a three-point motor block scale, and the third and fourth tests evaluated sensation. The painful stimulus test evaluated the response to a deep nociceptive sensory stimulus (ungueal base pressure with a Halstead clamp) on the dog's four legs on a three-point scale. The other sensory block test was the prick test or pannicular reflex exploration, which evaluated the response to a superficial sensory stimulus (skin pricking by piercing the skin with a needle) on a two-point scale (Table 1). All four tests were performed on all animals at predetermined regular intervals (0, 5, 15, 30, 45, 60, 75, 90, 105 and 120 min, and then every 30 min for as long as necessary until 2 h after the recovery was complete, or for a minimum of 2 h). The maximum degree of blockade of each dose was graded on a three-point scale (1=no effect; 2=partial block; 3=total block) in each animal for the purpose of comparing the maximum degree of motor and sensory blockade between doses.
|
Deep sensory response was evaluated by observing the response to pinching the space between the toes on both the front and hind paws using a Halstead clamp protected with plastic sheaths (painful stimulus test). Absences of vocalization or head movement toward the area being pinched were taken as indicating deep analgesia. The prick test was performed bilaterally using vertebral dermatome distribution to determine whether the reflex was present, but was grouped into lumbar block (lumbar and sacral) or purely sacral block, since either region was considered blocked when at least two of its dermatomes were blocked. This sensitivity was measured by skin pricking, i.e. superficially piercing the skin with a 25 gauge needle (skin prick test) in a caudocranial direction. We also checked that the reaction of the animal was a response to a nociceptive stimulus and not a habituation phenomenon. For this purpose, we checked that the animal did not respond similarly to an painless stimulus, such as a pat on the hind leg. Motor blockade was evaluated by assessing gait and the ability to stand on four legs.
Exclusion criteria
Exclusion criteria were as follows: dogs presenting with any alteration in neurological status before initiating drug dosing; dogs showing no symmetrical motor blockade of the hind legs after the test administration with lidocaine; dogs with any sign of alterations due to catheter malpositioning that might affect the results; dogs in which it was not possible to perform all four experiments with the same catheter were excluded from the study.
Statistical analysis
The difference between the value found during sevoflurane administration and the immediately preceding control was analysed for each variable considered. Differences obtained for a range of sevoflurane doses were analysed by one-way analysis of variance (ANOVA with repeated measures) with Dunnett post hoc tests on raw data using an iterative procedure with commercial software. Two-way ANOVA was used to compare pairs of curves (Graph-Pad Prism 3.0, Graph-Pad Software, San Diego, CA, USA). The Friedman test was used to compare the degrees of intensity of maximum blockade at different doses.
![]() |
Results |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Level of consciousness
None of the dogs displayed a decrease in consciousness with the saline solution, sevoflurane 0.05 ml kg1 or sevoflurane 0.075 ml kg1. Administration of sevoflurane 0.1 ml kg1 produced light sedation (2 on a four-point sedation scale) in three of the five dogs. None of the sevoflurane doses produced a state of moderate sedation or of general anaesthesia in any of the five dogs evaluated.
Motor blockade
The saline solution did not produce any degree of motor block. Blockade onset, total duration of blockade and recovery times for each sevoflurane dose are shown in Table 2. None of the dogs showed maximum motor blockade of the hind legs with 0.05 ml kg1 and none presented any blockade whatsoever of the front legs. With 0.075 ml kg1 a complete motor blockade was obtained in the hind legs of one dog, which also had partial front leg blockade, but there was only partial blockade in the other four dogs. At the highest dose (0.1 ml kg1) all dogs had a complete motor blockade of their hind legs, three had a partial blockade of the front legs and the other two had complete blockade of all four legs. Comparison of the degree of intensity of maximum motor blockade achieved with the different doses showed significant differences between the maximum, medium and minimum doses (Friedman test, P<0.05), without a statistically significant difference for maximum blockade intensity between the medium and minimum dosages (post-hoc difference of Friedman test, P>0.05). Comparison of the total duration of the motor blockade among the three sevoflurane doses shows a significant dose-dependent increase between doses (post-hoc difference of Friedman test, P=0.003) that is greater than the effect of the saline solution at any of the three sevoflurane doses (post-hoc difference of Friedman test, P=0.004).
|
Prick test in the sacral region
The saline solution did not produce any grade of sensory block. Blockade onset, total duration of blockade and recovery times for each sevoflurane dose are shown in Table 2. A comparison of the duration of the sensory block in the sacral region among the three sevoflurane doses shows a significant dose-dependent increase (one-way ANOVA with repeated measures, P=0.03) that is always greater than that for the saline solution (one-way ANOVA with repeated measures, P=0.001).
Prick test in the lumbar region
The results for the sensory block of the lumbar region are shown in Table 2. Comparison of the duration of the lumbar region sensory block among the three sevoflurane doses shows a significant dose-dependent relationship between dose and duration of block (one-way ANOVA with repeated measures, P=0.01) that is always greater than that for the saline solution (one-way ANOVA with repeated measures, P=0.002) (Table 2).
Ratio of motor block to sensory block (painful stimulus test)
The duration of the motor block was compared with that of the sensory block at the different doses by determining the ratio between the motor block and the sensory block as measured by the prick test in which the response is independent of the degree of motor block. The ratio was 1.3(SD 0.5) at 0.05 ml kg1, 1(0.2) at 0.075 ml kg1 and 1.2(0.2) at 0.1 ml kg1. The average was 1.17(0.12).
![]() |
Discussion |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Subarachnoid sevoflurane did not alter the level of consciousness at doses of 0.05 or 0.075 ml kg1, and had only a slight sedative effect in three of the five dogs at the highest dose of 0.1 ml kg1. This made it possible to evaluate the animal's response to a painful stimulus clinically. The response took the form of a head movement towards the stimulated area, although the animal could not move its legs because of the motor blockade produced by the sevoflurane. Thus it was possible to discriminate whether the drug-induced blockade of the motor response to a painful stimulus was the result of motor blockade, sensory blockade or, as shown in this study, both blockades simultaneously.
Sevoflurane administration via the spinal route always produced an effect that followed the homogeneous distribution in a caudocranial distribution, and the duration and intensity of the blockade increased with the increase in drug dose. Thus the ad integrum recovery from the clinical effects in all dogs without neurological sequelae supports the validity of the method employed here as well as the results obtained for the spinal effects of sevoflurane.
Sensory block was dose dependent in duration, intensity and extension, allowing confirmation of the analgesic effects of sevoflurane administration to the spinal medulla. The hypothesis of an analgesic effect by general anesthetics9 1618 has been widely documented in many experiments. Our model, in which the level of consciousness is unaffected, allows analgesia and not interruption of nociceptive transmission to be noted. This was not possible in earlier studies in which the subjects were unconscious.11
The duration, intensity and extension of the motor block were dose dependent, in agreement with the theory that the immobilizing clinical effect of sevoflurane is produced by the spinal action of the drug. Several studies have demonstrated that inhalational anaesthetics have a direct effect on motoneurone excitability and hyperpolarize motoneurones.9 19 20 Other studies have observed an inhibition of the F wave (directly related to motoneurone excitability) with the suppression of movement in response to surgical stimulus.10 2125 Our study has shown that the clinical effect of the motor block produced by the intradural administration of sevoflurane does not alter the conscious awareness of the dog.
The average intensity and duration of the motor and prick test sensitivity/pannicular reflex blocks was similar for each of the sevoflurane doses, and so we believe that the clinical effects of sevoflurane on sensory transmission and the effect on the motoneurone are equally strong.
In summary, subarachnoid sevoflurane administration in dogs produces a reversible dose-dependent sensory and motor block that does not affect the level of consciousness.
![]() |
References |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
2 Rampil IJ, Mason P, Singh H. Anesthetic potency (MAC) is independent of forebrain structures in the rat. Anesthesiology 1993; 78: 70712[ISI][Medline]
3 Collins JG, Kendig JJ, Mason P. Anesthetic action within the spinal cord: contributions to the state of general anesthesia. Trends Neurosci 1995; 18: 54953[CrossRef][ISI][Medline]
4 Savola MK, Woodley SJ, Maze M, Kendig JJ. Isoflurane and an alpha 2-adrenoceptor agonist suppress nociceptive neurotransmission in neonatal rat spinal cord. Anesthesiology 1991; 75: 48998[ISI][Medline]
5 Matute E, Lopez-Garcia JA. Characterisation of sevoflurane effects on spinal somato-motor nociceptive and non-nociceptive transmission in neonatal rat spinal cord: an electrophysiological study in vitro. Neuropharmacology 2003; 44: 81116
6 Zhou HH, Jin TT, Qin B, Turndorf H. Suppression of spinal cord motoneuron excitability correlates with surgical immobility during isoflurane anesthesia. Anesthesiology 1998; 88: 95561[CrossRef][ISI][Medline]
7 Antognini JF, Atherley R, Carstens E. Isoflurane action in spinal cord indirectly depresses cortical activity associated with electrical stimulation of the reticular formation. Anesth Analg 2003; 96: 9991003.
8 Antognini JF, Carstens E. In vivo characterizacion of clinical anaesthesia and its components. Br J Anaesth 2002; 89: 15666
9 Matute E, Rivera-Arconada I, Lopez-Garcia JA. Effects of propofol and sevoflurane on the excitability of rat spinal motoneurones and nociceptive reflexes in vitro. Br J Anaesth 2004; 93: 4227
10 Matute E, Rojo P, Lopez-Garcia JA. Effects of sevoflurane and propofol at different concentrations on F-wave and H-reflex in humans. Anesthesiology 2004; 101: A308
11 Urban BW. Current assessment of targets and theories of anaesthesia. Br J Anaesth 2002; 89: 16783
12 Laster MJ, Fang Z, Eger EI 2nd. Specific gravities of desflurane, enflurane, halothane, isoflurane, and sevoflurane. Anesth Analg 1994; 78: 11523[Abstract]
13 Nishikawa K, MacIver MB. Agent-selective effects of volatile anesthetics on GABA receptor-mediated synaptic inhibition in hippocampal interneurons Anesthesiology 2001; 94: 3407[ISI][Medline]
14 Takanashi Y, Ishida T, Meguro T, Kiwada H, Zhang JH, Yamamoto I. Efficacy of intrathecal liposomal fasudil for experimental cerebral vasospasm after subarachnoid hemorrhage. Neurosurgery 2001; 48: 894900[CrossRef][ISI][Medline]
15 Feldman SH, Covino BG. A chronic model for investigation of experimental spinal anesthesia in the dog. Anesthesiology 1981; 54: 14852[ISI][Medline]
16 Namiki A, Collins JG, Kitahata LM, Kikuchi H, Homma E, Thalhammer JG. Effects of halothane on spinal neuronal responses to graded noxious heat stimulation in the cat. Anesthesiology 1980; 53: 47580[ISI][Medline]
17 Jinks SL, Antognini JF, Carstens E. Isoflurane depresses diffuse noxious inhibitory controls in rats between 0.8 and 1.2 minimum alveolar anesthetic concentration. Anesth Analg 2003; 97: 11116
18 Hao S, Takahata O, Mamiya K, Iwasaki H. Sevoflurane suppresses noxious stimulus-evoked expression of Fos-like immunoreactivity in the rat spinal cord via activation of endogenous opioid systems. Life Sci 2002; 71: 57180[CrossRef][ISI][Medline]
19 Nicoll RA, Madison DV. General anesthetics hyperpolarize neurons in the vertebrate central nervous system. Science 1982; 217: 10557[ISI][Medline]
20 Takenoshita M, Takahashi T. Mechanisms of halothane action on synaptic transmission in motoneurons of the newborn rat spinal cord in vitro. Brain Res 1987; 402: 30310
21 Rampil IJ, King BS. Volatile anesthetics depress spinal motor neurons. Anesthesiology 1996; 85: 12934[CrossRef][ISI][Medline]
22 Kammer T, Rehberg B, Menne D, Wartenberg HC, Wenningmann I, Urban BW. Propofol and sevoflurane in subanesthetic concentrations act preferentially on the spinal cord: evidence from multimodal electrophysiological assessment. Anesthesiology 2002; 97: 141625[CrossRef][ISI][Medline]
23 Antognini JF, Carstens E, Buzin V. Isoflurane depresses motoneuron excitability by a direct spinal action: an F-wave study. Anesth Analg 1999; 88: 6815
24 Pereon Y, Bernard JM, Nguyen The Tich S, Genet R, Petitfaux F, Guiheneuc P. The effects of desflurane on the nervous system: from spinal cord to muscles. Anesth Analg 1999; 89: 4905
25 Yasuda N, Targ AG, Eger EI 2nd. Solubility of I-653, sevoflurane, isoflurane and halothane in human tissues. Anesth Analg 1989; 69: 3703[Abstract]