Nitrous oxide elevates local anesthetic seizure threshold

EXPERIMENTAL

Nitrous

E*‘EUROLOGY

Oxide

RUDOLPH

H.

Departments of Ultivevsity

DE

35, 558-564 (1972)

Elevates JONC,

Local E.

JAMES

Awsthesiology of Washbtgton

Anesthetic HEAVNER

and Pharmacology School of Medicine, Received

February

AND

Seizure LUIZ F.

Threshold DE

OLIVEIRA

1

and .Iuesthesia Research Center, Seattle, Washingtolr 98105 8, 1972

The median intravenous convulsant dose (CD,,) of lidocaine in awake unoperated cats breathing air was 7.65 mg/kg. In acute experiments on cats ventilated with nitrous oxide and having noninvasive extradural cortical recording electrodes, the lidocaine CD,, was 11.43 mg/kg-an elevation of the CD,, in air-breathing animals of nearly 50%. When cortical as well as depth (limbic) electrodes were placed in cats ventilated with nitrous oxide the lidocaine CD,, was 12.55 mg/kg. This too was some 50% greater than the 8.16 mg/kg CD,, in awake cats with chronically implanted cortical and limbic electrodes. Nitrous oxide or unidentified intraoperative factors (or both) alter CNS responses to local anesthetics. Cats with chronically implanted electrodes appear to respond normally. introduction

The neuroscientist often must decide whether to use an acute or a chronic preparation for his studies. The acute preparation has the advantage of providing experimental conditions largely under the investigator’s control. This advantage is offset somewhat by the requirement for anesthetic and other drugs, and by the stressesof surgery. These factors, singly or combined, may modify CNS function sufficiently to distort interpretation of results. The latter problem is neatly sidestepped by turning to a chronic preparation where the surgical procedures have been carried out at an earlier date. With the animal recovered prior to experiments, CNS responses ought to resemble those in the normal animal. Full control over experimental conditions, on the other hand, is difficult to attain in the chronic preparation, and the scope of prior surgical preparation is limited by humane as well as practical considerations. This problem was driven home to us in our work on convulsions induced by local anesthetics. One reason for investigating this class of seizures is that they have been likened to (2, 4), and proposed as a model of (7), temporal lobe seizures. We used acute as well as chronic experimen1 Supported by Career Development Award 5K3-GM-28,168 (RdeJ). Research Training Grant GM01160 (JEH), and Anesthesia Research Center Grant GM1.5991. 558 63 1972

by Academic

Press.

Inc.

SEIZURE

THRESHOLD

559

tal methods in our studies, but we soon noted that the lidocaine threshold was much higher in anesthetized than in awake animals. We sought to minimize the shortcomings of the acute preparation by using nitrous oxide, a weak inhalation anesthetic, together with minimal surgery and local anesthesia of exposed sites. Even so, we observed that the lidocaine (SyloCaine) seizure threshold is elevated by nearly 50% above the value obtained in awake unanesthetized animals. Animals bearing chronically implanted electrodes, conversely. respond just like normal unoperated cats to the lidocaine challenge. Methods

Cats were studied under three different conditions : awake intact ; awake with chronically implanted electrode assembly ; and lightly anesthetized for acute experiments. The procedures for the first two categories have been described in detail (3) and will be mentioned only briefly. Those for the third group, however, yielded results which varied considerably from the results obtained on the former two and are presented more fully. A-lzc~ahc Intact. Eleven healthy adult cats were injected intravenously with lidocaine 2 (1 mg/kg/ set ) at weekly intervals. Weekly injection with a smaller or larger dose of lidocaine was continued until the dosesof lidoCaine that just did and just did not induce a tonic-clonic generalized convulsion were bracketed. Subsequently the animals were used for one of the other esperiments. 2.4~~akcZul@ntcd. Fifteen healthy adult cats were anesthetized with halothane. Limbic (amygdala and hippocampus) recording electrode pairs were placed stereotaxically through small burr holes. Stainless-steel cortical electrodes were tapped through the skull with the tips resting extradurally over the frontal, midsylvian, and occipital regions of the brain. Leads from the electrodes were soldered to a 14-pin plug and the entire assembly was cemented to the skull. The animals recovered quickly from the implantation procedure and thrived for many months. The seizure threshold was determined in these animals as described above for intact awake cats, except that brain electrical activity was recorded on a polygraph during and after lidocaine injection. When the experiments were finished, we gave a lethal dose of pentobarbital, perfused the brain with 10% formalin in situ, and removed it the next day. Brain sections were stained with H&E and examined microscopically for lesions and electrode sites. .‘lclrfc Experimrnts. Healthy adult cats were anesthetized with halothane and nitrous oxide (N,O). To guard against hypercarbia, mechanical venti2Made from lidocainehydrochloridecrystals dissolvedin sterile saline,with the adjustedto 7 with sodiumhydroxide (courtesy of Dr. Vinton Hallock, Astra Pharmaceutical Products). pH

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HEAVNER,

AND

DE

OLIVEIRA

lation was instituted to yield an end-expiratory CO, tension of about 4.5?. Apnea was maintained with intermittent injections of decamethonium. A femoral artery and vein were cannulated and the bladder was catheterized. The leg wound was flushed with lidocaine, then closed. Arterial pH and Pc,, were measured frequently, and minute volume was adjusted or sodium bicarbonate was injected as needed to maintain pH near 7.40 and PC,, near 30 mm Hg. Fluid and electrolyte losses were replaced with 5% glucose in lactated Ringer’s solution. Core (esophageal) temperature was maintained in the 38-39 C range with a warm water blanket, and a lead II electrocardiogram was monitored. A bilateral trigeminal ganglion block was performed to minimize pressure discomfort. A needle was passed transorbitally via the infraorbital foramen to a depth of 30-35 mm and 1 ml of 1% bupivacaine (a long-lasting local anesthetic) was injected. The head was then mounted in a stereotaxic frame. Cortical electrodes, made from stainless-steel sewing machine needles, were tapped through the skull to rest extradurally over the frontal, midsylvian, and occipital regions of the brain. In 15 of these cats, insulated coaxial electrodes were guided stereotaxically into limbic recording sites (usually left cortico-basal amygdala and right ventral hippocampus). After electrode placement, the ear and eye bars were loosened to reduce pressure discomfort and halothane was discontinued. Systemic analgesia was maintained for the remainder of the experiment with 70% nitrous oxide and oxygen (2.5 liters/min N,O and 1 liter/min 0,). Bracketing of the lidocaine seizure threshold was started 1 hr or more after halothane had been discontinued. Presence or absence of seizure patterns was determined by the characteristic high-voltage synchronous epileptiform bursts in cortical and depth leads. Lidocaine injection was repeated at hourly intervals until the doses that just did and just did not induce convulsions were determined in two or three tries. After the animal was killed its brain was perfused with formalin. then removed for sectioning and staining. Animals with intracranial hemorrhage, with cardiac arrest after lidocaine, or in which the lidocaine threshold could not be found in three tries were deleted from the series. The cumulative number of animals in a group that convulsed after a given dose of lidocaine was tabulated and the data were analyzed by the method of weighted probits (9) on a PDP-15 computer. The median convulsant dose (CD,,) of lidocaine was computed from the probit-log close equation. Differences between CD,,‘s were evaluated with the t-test. Results

The log dose-response lines of the four treatment groups are shown in Fig. 1 and the corresponding CD,,‘s in Table 1. Koteworthy there is the es-

SElZL-RE

4

5

561

‘TIIKESFIOLD

6.3 Lldocalne

0

IO dose

‘mg

12.5 /kg)

I6

20

FIG. 1. Dose-response lines for the four treatment groups. Logarithm of lidocaine dose on the abscissa; cumulative frequency of convulsions, scaled to probits, on the ordinate. The two left lines, representing awake intact and awake cats bearing implanted electrodes, are statistically indistinguishable. So are the two right lines for nitrous oxide-ventilated cats with and without depth electrodes. The left and right pairs of lines though were statistically separable. The respective CD,,‘s can be read from the graphs by interpolation.

sential overlap of the response of awake intact and awake implanted animals. The 0.5 mg/kg (6.7%) difference between their CD,, was insignificant. The 1.1 mg/kg (10%) difference in CD,, between N,O-ventilated acute cats with or without limbic electrodes likewise was insignificant. DifTABLE MEDIAN

CONVULSANT

1 Doses

OF IJDOCAINIT

Fiducial range (9) img!kg) ~._-~

CD,, Preparation

,L

(mg/kg)

Awake. intact

11

7.65

7.27-8.04

Awake, implanted p

15

8.16

1.74-8.62

N20, cortical electrodes only 6

17

11.43

10.56-12.37

N20, cortical and depth electrodes b

10

12.55

11 .50-13.68

a Includes previously reported results (3) from 10 animals. b Excludes animals with intracranial bleeding or circulatory

arrest

ior both)

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ferences between the corresponding awake and N?O-ventilated groups of cats, on the contrary, were statistically significant (P < 0.01). Blood pressure and arterial blood gases could be measured only in the N,O-ventilated group of cats. Arterial pressure prior to injection always was 120 mm Hg systolic or greater. Mean arterial pH at or near the time of lidocaine injection was 7.39 2 0.05 and the Pcoz 28.Sk3.7 mm Hg, which are well within the range reported for normal awake cats (6). The importance of routine postmortem brain examinations was apparent, for we observed five instances of intracerebral or extradural hemorrhage after stereotaxic electrode placement. In three of these the electrode tip hit bone before reaching its stereotaxic destination. Signs of difficulty were noticeable prior to death, varying from nearly isoelectric EEG, to cardiac arrest after low doses of lidocaine, or to inability to induce seizures within a reasonable number of tries. Data from these experiments were excluded. None of the cats with cortical electrodes only showed gross or microscopic evidence of intracranial hemorrhage. One cat bearing implanted electrodes died from uremia secondary to obstruction of the urinary bladder neck by a stone. Numerous petechial hemorrhages were scattered through its brain. The brains of the other cats bearing implanted electrodes showed no hemorrhage, though all had various amounts of scarring surrounding the electrode tracts, and one had a small localized extradural abscess. Seizure-producing doses of lidocaine could not be determined in three cats because circulatory arrest followed a nonconvulsant dose of lidocaine. Though these animals were resuscitated, their blood pressures were too labile to warrant further testing and they were deleted from consideration. Discussion

We observed that the lidocaine seizure threshold is elevated about 50% in cats ventilated with N,O. Evidently, one or more factors unique to acute experiments raise the lidocaine seizure threshold above that in awake cats. Perhaps the most important difference between the two groups is the use of N,O in the former. To our knowledge, N,O has not previously been incriminated as altering the convulsant threshold; we had indeed chosen it for that very reason. Decamethonium, the neuromuscular blocking agent used here, was selected over the more commonly used gallamine because the latter prolongs electrical afterdischarges in isolated cortex (10) whereas decamethonium and succinylcholine do not modify cortical afterdischarge. This conclusion is consistent with the observation by Munson and Wagman (11) that gallamine and curare alter the lidocaine seizure threshold but succinylcholine and decamethonium do not. As an earlier report ( 1) described a nro-

SEIZURE

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THRESHOLD

nounced effect of succinylcholine on lidocaine seizures, decamethonium seemed to be the only muscle relasant without effect on the lidocaine threshold. In view of the well-known sensitivity of the local anesthetic seizure threshold to variations in arterial PcoC and pH (5. S, 1 l), we took pains to keep these parameters within the normal range for cats (6), i.e., pH 7.39 and P,,, 31.5 mm Hg. The nearly identical threshoIds of awake intact cats and awake cats bearing implanted electrodes argue against attributing changes in threshold to the destructive presence of brain electrodes. AUl the more so as most chronically prepared cats carried four implanted electrode pairs, whereas acutely operated cats (for reason of space limitation) usually had only two depth electrode pairs. An acute edematous response to the electrodes conceivably may have developed, but that still cannot explain why cats with noninvasive electrodes had thresholds similar to those of cats with depth electrodes. These other factors being thus eliminated, N,O stands as the most plausible cause of the elevated seizure threshold encountered in acute experiments. (We did not, for humane reasons, check the lidocaine threshold in the absence of N,O ‘J. In addition to showing the anticonvulsant properties of nitrous oxide, this study illustrates that even light anesthesia and minor surgery introduce fresh experimental variables. Such variables further complicate interpretation of results and their extrapolation to normal intact animals. When circumstances permit, chronically prepared animals provide the least biased means of studying normal CNS responses. References 1.

F.. -4. B. BULL, and P. GLEES. 1956. Electroencephalogram of the cat after intravenous injection of lidocaine and succinylcholine. A~tcstltrsiologg 17 :

ACHESON,

802-808.

R. S., and R. G. ORISCELLO. 1968. Petit and grand ma1 convulsions lidocaine hydrochloride treatment of ventricular tachycardia. J. :1rrrev. Med. Ass. 204 : 201-204. 3. DE JONG, R. H., and J. E. HEAVNCR. 1971. Diazepam prevents local anesthetic seizures. .-lmsthcsiology 34 : 523-531. 4. DE JONG, R. H., and L. F. WALTS. 1966. Lidocaine-induced psychomotor seizures in man. L-lrta A~tacstkesiol. Srarld. Suppl. 23 : 598-604. 5. DE JONG, R. H., I. H. WAGBIAN, and D. -4. PRINCE. 1967. Effect of carbon dioxide on the cortical seizure threshold to lidocaine. Esp. Neuvol. 17: 221-232. 6. DEJOURS, P.. and il. LACAISSE. 1971. Le pH et les pressions partielles de l’oxygcne et du gaz carbonique du sang arterial chez le chat normal et eveill& J. l~A~~sio/.

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ENGLESSON, S., N. J. PAYMASTER, and T. R. HILL. 1965. Electrical seilzure activity produced by Xylocaine and Citanest. Acta Anaesthesiol., Scat& Suppl. 16: 47-s-I.

FINNEY, D. J. 1964. “Probit Analysis,” 2nd ed., Cambridge Univ. Press, London. 10. HALPERN, L. M., and R. G. BLACK. 1968. Gallamine triethiodide facilitation of local cortical excitability compared with other neuromuscular blocking agents. J. Pharwacol. Exp. Ther. 162 : 166173. 11. MUNSON, E. S., and I. H. WAGMAN. 1969. Acid-base changes during lidocaine induced seizures in Macaca mulatta. Arch. Ncwol. 26: 406112. 9.