Journal of Cerebral Blood Flow and Metabolism

.

11:388--397 © 1991 The International Society of Cerebral Blood Flow and Metabohsm Published by Raven Press, Ltd., New York

The Influence of a Cryogenic Brain Injury on the Cerebrovascular Response to Isoflurane in the Rabbit R. Ramani, Michael M. Todd, and David

S. Warner

Neuroanesthesia Research Group, Department of Anesthesia, University of Iowa College of Medicine, Iowa City, Iowa, U.S.A.

Summary: To determine if an acute neurologic injury al­ ters the cerebrovascular response to isoflurane, rabbits were anesthetized with morphine/N20, mechanically ventilated, surgically instrumented, and assigned to one of three groups. Group 1 animals (n 8) served as con­ trols and received no injury. In Groups 2 (n 9) and 3 (n 8), a 30-s cryogenic injury was produced in the left parietal region using liquid N2 poured into a funnel affixed to the surface of the skull. Regional CBF was measured using microspheres. In Groups 2 and 3, flow was deter­ mined before and then 30 and 90 min after injury or at equivalent times in Group 1. After the 90-min data were collected, 1 % [=0.5 minimal alveolar concentration (MAC)] and then 2% (=1.0 MAC) isoflurane was admin­ istered to uninjured rabbits in Groups 1 and to lesioned rabbits in Group 3. A mean arterial pressure of �80 mm Hg was maintained during isoflurane administration by an infusion of angiotensin II. In uninjured rabbits (Group 1), 2% isoflurane produced bilaterally symmetrical increases

in hemispheric CBF, from 76 ± 21 (mean ± SD) to 150 ± 48 ml 100 g 1 min I. CBF in the hindbrain increased from 91 ± 25 to 248 ± 102 ml l00 g -I min I. By contrast, in the lesioned rabbits of Group 3, 2% isoflurane resulted in CBF in the lesioned hemisphere changing from 56 to only 77 ml l00 g I min -I (NS), while in the contralateral hemisphere, CBF rose from 68 to 97 ml 100 g 1 min 1 (also NS). These results indicate that a cryogenic injury attenuates the normal CBF response to a volatile anes­ thetic, both in the damaged hemisphere as well as in ap­ parently uninjured regions distant from the injury focus. In a separate group of animals, a similar cryogenic injury abolished the CBF response to changing Pac02 in the in­ jured hemisphere, but not in either the contralateral hemi­ sphere or the cerebellum. It is possible that the CBF ef­ fects of isoflurane may be mediated via some intermedi­ ary neurogenic and/or biochemical process. Key Words: Cerebral blood flow-Diaschisis-Head injury­ Isoflurane-Volatile anesthetics.

The influence of volatile anesthetics on CBF has been the subject of many investigations (for re­ views, see Michenfelder, 1990). However, the ma­ jority of the available studies have been performed in normal animals or in humans with relatively small (and chronic) intracranial mass lesions. Very little is known about the actions of these drugs when given in the presence of an acute neurologic process such as trauma. Trauma can alter normal cerebrovascu-

lar responsiveness, both in animals and in humans. For example, Reilly et al. ( 1977) showed that the response to CO2 was blunted in primates subjected to a cryogenic injury, while both Saunders et al. ( 1979) and Lewelt et al. ( 1982) have shown alter­ ations in the responses to changing Pa02 and Paco2 in animals after fluid-percussion injuries. Only two experiments have looked at the influence of neuro­ logic injury on the response to anesthetic agents. Messeter et al. ( 1986) measured CBF before and after the administration of thiopental to a group of head-injured patients and observed either no re­ sponse or a paradoxical vasodilation in a subgroup of severely injured individuals. Yuan et al. ( 1988a) anesthetized rats with halothane and then subjected one group to a fluid-percussion irijury. They noted that while CBF rose in the normal animals given halothane, it decreased progressively in injured rats. They suggested that the injury had counter­ acted the normal vasodilative effects of the drug,

-

-

-

=

-

=

-

=

Received July 13, 1990; revised September 3, 1990; accepted September 10, 1990. Address correspondence and reprint requests to Dr. M. M. Todd at Department of Anesthesia, University of Iowa Hospitals and Clinics, GH6SE, Iowa City, IA 52242, U.S.A. Dr. R. Ramani's current address is Department of Neuroan­ esthesia, National Institute of Mental Health and Neuro Sci­ ences, Bangalore, India 560029. Abbreviations used: CVP, central venous pressure; ICP, in­ tracranial pressure; MAC, minimal alveolar concentration; MAP, mean arterial pressure.

388

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NEUROLOGICINJUR�CBF,ANDISOFLURANE

but did not specifically examine the CBP response to halothane in a "before-and-after" fashion. Given these findings, it seemed reasonable to postulate that preexistent neurologic damage might alter the CBP response to a volatile anesthetic. To test this hypothesis, we subjected rabbits to a mild cryogenic brain injury and examined the regional CBP changes produced by graded doses of isoflu­ rane, the most commonly used modern volatile agent. MATERIALS AND METHODS

These experiments were approved by the University of Iowa Animal Care and Use Committee. The study was carried out in 30 male New Zealand White rabbits weighing 3.4-3.7 kg. Anesthesia was in­ duced with 5% halothane in O2 in a plastic box. An ear vein was cannulated and the trachea intubated with a 3.0mm-ID cuffed tube. Succinylcholine 1 mg kg-1 was in­ jected intravenously and mechanical ventilation begun using a tidal volume of 10-12 ml kg-1 and a rate of 30-35 min -I, with an inspired gas mixture of 0.5-1% halothane + 65-70% N20/baiance 02' Normocarbia was main­ tained (as assessed initially by expired CO2 measure­ ments and later confirmed by blood gas analysis) by ad­ justing the ventilator settings. An infusion of lactated Ringer's solution was started and continued throughout the study at the rate of 4.0 ml kg - 1 h-1. Succinylcholine was added to the infusion in amounts sufficient to main­ tain adequate paralysis (3 mg kg - 1 h-1). Esophageal tem­ perature was kept constant at 38°C using a servo-con­ trolled heating pad. Following initial preparation, the left femoral artery and vein were catheterized with PE-90 tubing. The cath­ eters were advanced into the abdominal aorta and right atrium for continuous monitoring of arterial (MAP) and central venous (CVP) pressures. These pressures were subsequently expressed as electronically determined means. The right femoral and left brachial arteries were cannulated with PE-160 tubing for the withdrawal of ar­ terial reference samples during microsphere injections. A small thoracotomy was then performed in the left third intercostal space, the left atrium was exposed, and a flanged PE-90 cannula was introduced into the left atrial appendage for subsequent injection of radioactive micro­ spheres. The margins of all surgical incisions were infil­ trated with 0.25% bupivacaine. After cannulations were completed, the rabbit was turned prone and its head fixed in a stereotaxic frame with the interaural line located =10 cm above the mid­ chest. The scalp was infiltrated with 0.25% bupivacaine, incised in the midline, and reflected to either side. An elliptical burr hole was drilled over the right frontal area and a saline-filled 5-Fr pediatric feeding tube was intro­ duced into the epidural space for monitoring intracranial pressure (ICP). The hole was sealed with cyanoacrylate adhesive. To prevent damping, a cotton wick was placed into the distal end of the feeding tube. Finally, a steel funnel with a lO-mm-diameter mouth was fixed to the left side of the skull, also with cyanoacrylate. The anterior margin of the funnel orifice was located =1 mm caudal to the coronal suture, while the medial margin was =1 mm lateral to the sagittal suture.

389

Protocol

After surgical preparation was complete, halothane was discontinued. Morphine 10 mg kg-1 injected intra­ venously over 1-2 min and a morphine infusion was started at the rate of 2 mg kg-1 h-I. Animals were there­ after ventilated with 70% N20lbaiance 02' A I-h waiting period was then permitted to allow the effects of halo­ thane to dissipate. End-tidal halothane concentrations at the end of this waiting period were 10%. Statistical analysis CBF. Since there were 95 CBF values for each rabbit [5 time points x [(2 hemispheres x 8 section levels/ hemisphere) + (3 hindbrain section levels)], some data simplification was required. As an initial step, group means were plotted against their respective standard de­ viations. This revealed a pattern of increasing standard deviation with increasing mean CBF. Data were hence

Left

Right

+5 +4 +3 +2 +1

;""'+-1

FIG. 1. Diagrammatic summary of the experimental protocol. CBF was recorded in ali groups under baseline conditions. Approximately 15 min thereafter, a 30-s cryogenic lesion was produced in Group 2 and 3 animals, with CBF measured 30 and gO min later (and at equivalent times in Group 1). Immediately after the gO-min flow measurement, 1% (=0.5 MAC) isoflurane was added to the inspired gas mixture in Groups 1 and 3. After 20 min, CBF was again determined, and isoflurane was increased to 2% (=1.0 MAC).

x

reference withdrawal rate tissue weight

x

100

(1)

between section levels within a major region, 1 there were no unique interactions between section level, group, and/ or measurement interval. Section levels were therefore combined to yield weighted means for the three major regions (left hemisphere, right hemisphere, and hind­ brain). This simplified data set was again examined by analysis of variance, with group, major region, and mea­ surement interval as factors. When a pairwise compari­ son between groups was required at a single measurement interval, Fisher's LSD testing was performed. Since this does not adequately adjust for overall variance or the number of mUltiple comparisons possible, significance was accepted only for p values of �0.01. Notations have been made for intergroup comparisons where calculated p values lie between 0.01 and 0.02. These criteria were es­ tablished to minimize the chances of a multiple compar­ ison error. Similarly, a repeated measures analysis within a single group (to examine changes over time) required a p value of �0.01 to be accepted as significant. Other variables. Differences in MAP, CVP, ICP, blood gases, hematocrit, and angiotensin doses were examined using a two-factor analysis of variance, with the five mea-

-1

-2 -3 FIG. 2. Diagrammatic summary of tissue sections used to determine forebrain regional CBF. Hindbrain sections are not shown. The shaded circle in the left hemisphere repre­ sents the cryogenic injury in Groups 2 and 3.

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1 In the forebrain, far anterior and posterior sections (sections 5, 4, -2, and -3; see Fig. 2) had higher flows than more centrally located sections. This may be a sectioning "artifact," since cen­ trally located coronal sections contain substantially more white and deep gray matter than do those located at the poles of the brain. The cryogenic lesion did not alter this sectional flow pat­ tern.

NEUROLOGICINJUR�CBF,ANDISOFLURANE

surement intervals treated as "within-group" variables (repeated). The initial null hypothesis was that there were no group x time interactions. If this was rejected, the null hypothesis of no group or no time differences was exam­ ined. Finally, when indicated, pairwise testing was per­ formed using the Fisher's LSD test, accepting p < 0.05 as significant. All statistical procedures were performed on a Macin­ tosh SE/30 computer, using SuperAnova (Abacus Con­ cepts, Berkeley, CA, U.S.A.).

391

20 min after increasing Pac02 by 20 mm Hg. Animals were killed, and the brains were removed, fixed in formalin, and then divided into left hemisphere, right hemisphere, and hindbrain (no additional sectioning was carried out). CBF in each major region was calculated as described above. RESULTS

Data from five animals (two in Group 1, one in Group 2, two in Group 3) were discarded. In three (two in Group 1, one in Group 3) radioactivity in the brachial and femoral arterial reference samples in­ dicated inadequate mixing of microspheres. In one Group 2 animal, a catheter was dislodged prior to the first flow measurement, and the fifth animal (in Group 3) was discarded because MAP was too low «80 mm Hg without angiotensin support) during the baseline period. This left eight animals in Group 1, nine in Group 2, and eight in Group 3. There were no intergroup differences or changes over time in Pao2, Paco2, pH, hematocrit, or CVP (Table 1). There were also no intergroup differences

Supplemental studies

When data analysis from the above experiment was completed, it was decided to determine whether the le­ sioning procedure resulted in any alterations in the re­ gional CBF response to Pac02' since this is one of the most widely studied and most potent cerebral vasodila­ tors. Five rabbits were prepared in a fashion identical to that described above, except that no epidural ICP moni­ tor was placed. CBF was measured during normocapnia and again 20 min after Pac02 had been increased 20 mm Hg by adding CO2 to the inspired gas mixture. Pac02 was then returned to normal and a cryogenic lesion performed as above. Ninety minutes later, a second pair of CBF measurements were made, first at normocapnia and then

TABLE 1. Blood gases, hematocrit, and hemodynamic data

p.02 (mm Hg) Group I (isoflurane) Group 2 (lesion) Group 3 (lesion + isoflurane) Pac02 (mm Hg) Group I Group 2 Group 3 pH (units) Group I Group 2 Group 3 Hematocrit (%) Group I Group 2 Group 3 MAP (mm Hg) Group I (isoflurane) Group 2 (lesion) Group 3 (lesion + isoflurane) CVP (mm Hg) Group I Group 2 Group 3 ICP (mm Hg) Group I Group 2 Group 3 Angiotensin dose (f.Lg min -1 ) Group I Group 2 Group 3

Baseline

30 min

90 min

120 min

150 min

146 ± 10 152 ± 22 158 ± 6

151 ± 11 152 ± 21 161 ± 6

149 ± 9 151 ± 17 156 ± 14

146 ± 9 153 ± 17 157 ± 17

147 ± 11 153 ± 17 160 ± 13

38 ± 2 37 ± 3 37 ± 2

38 ± 3 36 ± 2 37 ± 2

37 ± 2 36 ± 2 38 ± I

38 ± 2 36 ± 3 36 ± 2

38 ± 2 37 ± 2 38 ± 2

7. 39 ± 0.02 7.40 ± 0.03 7.40 ± 0.03

7.40 ± 0.04 7.41 ± 0.04 7.40 ± 0.04

7.39 ± 0. 03 7. 42 ± 0.04 7.39 ± 0.03

7.38 ± 0.04 7.41 ± 0.04 7.38 ± 0.03

7.37 ± 0.03 7.40 ± 0.03 7.37 ± 0.03

34 ± 2 35 ± 2 35 ± 2

33 ± 3 33 ± 2 34 ± 2

33 ± 3 33 ± 2 34 ± 2

33 ± 2 33 ± 2 34 ± 2

33 ± 3 32 ± 3 34 ± 2

86 ± 4 87 ± 6 85 ± 4

88 ± 4 90 ± 7 88 ± 5

92 ± 7 94 ± 7 93 ± 6

86 ± 6 91 ± 8 84 ± 4

89 ± 5 94 ± 9 85 ± 9

3 ± 1 3 ± I 3 ± I

3 ± I 3 ± 1 3 ± I

3 ± 1 3 ± I 3 ± I

3 ± I 3 ± I 3 ± I

3 ± I 3 ± I 3 ± I

6 ± I 5 ± 2 3 ± 2

5 ± 2 5 ± 3 4 ± 2

5 ± I 6 ± 4 4 ± 2

6 ± I 6 ± 4 5 ± 3

7 ± I 7 ± 4 6 ± 3a

0 0 0

0 0 0

0 0 0

0.06 ± 0.07 0 0.04 ± 0.07

0.26 ± 0.23 0 0.33 ± 0.22

All values are means ± SD. Note that animals in Group I (n 8) did not receive a cryogenic lesion, but were given isoflurane, while animals in Group 2 (n 9) underwent lesioning, but did not receive isoflurane. Animals in Group 3 (n 8) both had a lesion and received isoflurane. There were no changes or intergroup differences at any measurement interval for Pa02, P.c02, pH, hematocrit, or CVP. There was a small increase in MAP at the 9O-min measurement interval, but with no intergroup differences. ICP did not change in Group I, while there was a small but significant rise over time in ICP in both Groups 2 and 3. There were no differences between the angiotensin doses given to Groups I and 3. For abbreviations see the text. Significantly different from the 9O-min (preisoflurane) value. =

=

=

a

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in MAP at any time, although a small increase was observed in all groups at 90 min after baseline (be­ fore isoflurane exposure). In the two groups of an­ imals given isoflurane (Groups 1 and 3), the admin­ istration of angiotensin was required to maintain MAP >80 mm Hg. There were no differences be­ tween these two groups in the angiotensin doses required. Because of the brief duration of the initial freeze and the relatively acute nature of the study, the ICP changes were small. As seen in Table 1, the maxi­ mum ICP in any group was 6.6 ± 4. 1 mm Hg in Group 2 animals at 150 min. In fact, ICP exceeded 12 mm Hg at any time in only one animal (from Group 2). Flow data are presented in Table 2. Prior to in­ jury, there were no intergroup differences in CBF for any major region. At 90 min post injury, CBF in the lesioned hemisphere (Groups 2 and 3) had de­ creased from =70 to =53 ml 100 g-I min-I (both groups combined), with a lesser decrease in the contralateral hemisphere and no changes in hind­ brain. The CBF changes produced by isoflurane (and the changes in the unexposed group) are shown in Fig. 3. When isoflurane was given to Group 1 (unlesioned) rabbits, CBF in all regions increased in a dose-related fashion, reaching a max­ imum during 1 MAC agent exposure of 150 ± 48 ml 100 g-1 min 1 in the two hemispheres and 248 ± 102 ml 100 g-I min -I in the hindbrain. These changes were highly significant (p 0.0004). By contrast, in Group 3, CBF in the lesioned hemi­ sphere did not change with 0.5 MAC agent (55 ± 25 to 59 ± 2 1 ml 100 g-I min-I and then increased to 77 ± 37 ml 100 g-I min-I at 1.0 MAC. These -

changes did not achieve statistical significance. In the contralateral hemisphere, CBF increased from 68 ± 3 1 ml 100 g-1 min- 1 at t 90min to a maxi­ mum of 97 ± 52 ml 100 g- 1 min-1 in the presence of = 1.0 MAC volatile agent (also NS by established criteria, although p 0.06). Intergroup compari­ sons indicated that flows in both hemispheres were significantly lower (p < 0.0 1) in Group 3 as com­ pared with Group 1. While hindbrain CBF in the lesioned animals increased to 16 1 ± 79 ml 100 g-l min-I, the difference between this and CBF in Group I achieved a p value of only 0.02. During the same time period (i.e., between 90 and 150 min after injury), there were no significant CBF changes in either hemisphere or hindbrain of lesioned animals not exposed to isoflurane (Group 2). Since the cryogenic injury appeared to abolish the response to isoflurane administration in the in­ jured hemisphere and attenuated the response in the contralateral hemisphere, we entertained the hy­ pothesis that the lesion was so severe as to abolish all cerebrovascular responsiveness throughout the brain. This led to the performance of the supple­ mentary study examining the regional CBF re­ sponse to changing Paco2 before and after lesioning. As seen in Table 3, CO2 responsiveness was intact in all regions prior to injury. Ninety minutes follow­ ing the cryogenic lesion, this response was abol­ ished in the left hemisphere, but remained unim­ paired in the right hemisphere and hindbrain. =

=

=

DISCUSSION

The modern volatile anesthetics (halothane, en­ flurane, isoflurane) are known cerebral vasodila-

TABLE 2. CBF values

Group 1 (n 8): no lesion, isoflurane Left hemi. Right hemi. Hindbrain Group 2 (n 9): lesion, no isoflurane Left hemi. Right hemi. Hindbrain Group 3 (n 8): lesion, isoflurane Left hemi. Right hemi. Hindbrain

Baseline

30 min

90 min

120 min

150 min

81 ± 24 81 ± 27 87 ± 28

80 ± 23 81 ± 24 91 ± 25

76 ± 21 76 ± 21 91 ± 25

(=0.5 MAC isoflurane) 115 ± 53 120 ± 50 159 ± 68

(=1.0 MAC isoflurane) 150 ± 48 150 ± 47 248 ± 102

n ± 18 76 ± 23 89 ± 30

58 ± 19 65 ± 18 79 ± 23

52 ± 12 57 ± 10 74 ± 14

64 ± 15 74 ± 22 94 ± 25

62 ± 21 71 ± 18 88 ± 24

69 ± 19 71 ± 17 88 ± 26

53 ± 13 59 ± 16 70 ± 17

55 ± 25 68 ± 31 84 ± 41

(=0.5 MAC isoflurane) 59 ± 21 75 ± 25 104 ± 30

(=1.0 MAC isoflurane) 77 ± 37 97 ± 52 161 ± 79

=

=

=

All values are means ± SD, expressed as ml 100 g-I min -I. In Groups 2 and 3, a left parietal cryogenic injury was produced immediately after obtaining baseline data. In Groups 1 and 3, =1% isoflurane was added to the inspired gas mixture immediately after recording the 90-min data. A further increase to 2% took place after the 120-min point. Data obtained at 120 and 150 min in Groups 1 and 3 therefore represented a 30-min exposure to isoflurane, with at least 20 min of a stable end-tidal concentration equal to =0.5 and =1.0 MAC, respectively. Statistical notations can be found in Fig. 3. MAC, minimal alveolar concentration; hemi., hemisphere.

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NEUROLOGICINJUR�CBF,ANDISOFLURANE

200

Left Hemisphere

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Right Hemisphere

180

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120

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400

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FIG, 3, Regional CBF responses to isoflurane (Isof). CBF values for the left and right hemispheres and hindbrain are shown for the measurement intervals before (90 min) and during exposure to 1% [=0.5(MAC); 120 min] and 2% (=1.0 MAC; 150 min) isoflurane. Values are means ±SD, although for clarity error bars are shown only for Groups 1 and 2. *p < 0.01 for Group 1 vs. Group 3 within any given measurement interval; # p < 0.01 for Group 1 vs, Group 2; t comparison between Groups 1 and 3 where significance achieved a value of 0.01 '" P '" 0.02.

tors, and all have been shown to increase CBP un­ der many conditions (e.g., Todd and Drummond, 1984; Eintrei et al., 1985), Their relative vasodilat­ ing potencies have been the subject of some debate, although halothane has been viewed as the most potent dilator (Todd and Drummond, 1984; Eintrei et al., 1985). More recent work suggests that the

global CBP effects of halothane and isoflurane are similar (Drummond et al., 1986; Madsen et al., 1987a,b, Hansen et al., 1988), but the two agents may produce different CBP distribution patterns (Hansen et al., 1988). To date, most studies dealing with the cerebro­ vascular effects of these drugs have been carried

TABLE 3. Regional responses to PaC02 before and after cryogenic injury

Before lesion

PaCOZ (mm Hg) CBF (mIIOO g-I min -I) Left hemisphere Right hemisphere Hindbrain

90 min post lesion

Normocapnia

Hypercapnia

Normocapnia

Hypercapnia

40 ± 2

61 ± 2

38 ± 2

62 ± 2

52 ± 11 56 ± 15 64 ± 17

45 ± 6 60 ± 14 65 ± 18

87 ± 14 98 ± 21 147 ± 55

49 ± 15 94 ± 39 121 ± 60

All values are means ± SD. Hypercapnia resulted in significant increases in CBF in all cases except in the left (lesioned) hemisphere 90 min post lesion. In the right hemisphere and hindbrain, the magnitude of the response to changing Paco2 did not differ between pre­ and postlesion periods.

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out in either normal animals or humans or in hu­ mans with small supratentorial tumors. These re­ sults may not apply in the presence of an acute brain injury and the current experiments were un­ dertaken to evaluate the possibility that injury would alter the CBP responses to a commonly used volatile anesthetic. Our findings indicate that a freeze lesion alters baseline blood flow and also dra­ matically attenuates the increase in CBP produced by this drug. However, while this is a new finding, it is not entirely surprising. Plow changes after ex­ perimental brain injuries are well known (Reilly et aI., 1977; Martins and Doyle, 1978; Saunders et aI., 1979; Pappius, 198 1; Lewelt et aI., 1982; Andersen et aI., 1988; Yuan et aI., 1988a,b). Several groups have also shown alterations in normal cerebrovas­ cular responsiveness to CO2 and O2, Reilly et aI. ( 1977), Saunders et aI. ( 1979), and Andersen et aI. ( 1988) have demonstrated a blunted response to Paco2 following cryogenic or fluid-percussion inju­ ries, while Lewelt et aI. ( 1982) and Andersen et aI. ( 1988) have shown a reduction in the degree of hyp­ oxic vasodilation. Changes in cerebral metabolic rate are also common following trauma (Obrist et aI., 1984; Cold, 1986; Muizellar et aI., 1989), and several authors have suggested that normal cou­ pling is disrupted both in humans (Obrist et aI., 1984; Cold, 1986; Messeter et aI., 1986) and in ani­ mals (Pappius, 1981). This is of particular relevance since Drummond et aI. ( 1986) and Hansen et aI. ( 1989a) have shown that CBP during exposure to halothane and isoflurane is in part dependent on cerebral metabolic rate. Our findings are also indi­ rectly supported by two other reports. As noted in the introductory section, Messeter et aI. ( 1986) demonstrated that in severely injured head trauma patients, thiopental administration resulted in either no change in CBP or a paradoxical increase. Yuan et aI. ( 1988a), in the only laboratory study dealing directly with this subject, showed that in normal rats anesthetized with halothane, CBP gradually in­ creased over time. When a fluid-percussion injury was performed, this pattern was not seen. CBP pro­ gressively fell and the authors concluded that the injury blocked the normal vasodilation produced by the volatile agent. While we found that a cryogenic injury obliter­ ated the normal CBP response to isoflurane in the injured hemisphere, of even greater interest is the fact that an altered response was seen in tissue re­ gions distant from the lesion focus. Por example, in the frontal pole of the injured hemisphere (i.e., that ipsilateral region most distant from the lesion cen­ ter), CBP did not change with the maximum dose of isoflurane (i.e., 59 ± 19 to 69 ± 29 ml 100 g� 1 J

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min� I, a change identical to that seen over a com­ parable time period in lesioned animals not given isoflurane). In normal animals, CBP in this same region increased from 84 ± 2 1 to 148 ± 52 ml 100 g� 1 min�I. Even in the contralateral frontal pole (perhaps the forebrain area most distant from the injury focus), CBP in lesioned animals increased only from 66 ± 39 to 99 ± 58 mll00 g� 1 min� I. This alteration in responsiveness in distant regions does not appear to be the result of direct injury to tissues outside of the lesioned hemisphere. Several pieces of evidence support this contention. Evans blue staining was never seen outside of the injured hemi­ sphere. In addition, earlier studies from our labora­ tory have repeatedly shown normal tissue water contents in both the ipsilateral frontal pole and the contralateral hemisphere (Kaieda et aI., 1989a,b). CO2 responsiveness was also intact in most of those brain regions where isoflurane responsiveness was diminished (e.g., contralateral hemisphere). Pi­ nally, other unpublished work in our laboratory in­ dicates that while EEG slowing persists over the injured hemisphere for the duration of the study as described, similar slowing is not seen over the con­ tralateral hemisphere. We realize that this does not rule out the possibility of histopathologic damage, but we believe this to be unlikely. This finding may cast light on the mechanisms by which volatile anesthetics dilate the cerebral vascu­ lature. Most in vitro data suggest that these drugs act at a very local level, i.e., on either the cell mem­ brane, the sarcoplasmic reticulum, or the contrac­ tile apparatus of vascular smooth muscle. Harder et aI. ( 1985) showed that relatively high concentra­ tions of halothane depolarized muscle cell mem­ branes from cat middle cerebral arteries at the same time these vessels were dilating, suggesting that electromechanical coupling was disrupted and indi­ rectly suggesting an intracellular site for its relaxant properties. Work in our own laboratory indicates that isoflurane relaxes rabbit basilar arteries in a dose-dependent manner, regardless of whether the vessel is constricted with potassium, norepineph­ rine, or serotonin (Jensen et aI., 1989). Removal of the endothelium does not substantially alter this re­ sponse (N. P. Jensen, personal communication). However, other studies using extracerebral vessels have shown that halothane can selectively alter the constriction produced by az-agonists (Larach et aI., 1987) and that the endothelium may play some role (Blaise et aI., 1987). We question the applicability of these results to the CBP changes noted in vivo. If isoflurane increased CBP via such direct effects, one would expect to see a normal response in undamaged vessels distant from the focus of a

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cryogenic lesion, just as the response to Paco2 is preserved in distant areas. We therefore believe that isoflurane may be acting at least in part via some intermediary mechanism that is susceptible to alteration by the cryogenic injury. At this point ex­ tensive speculation would be inappropriate. Possi­ bilities obviously include neurogenic pathways and/ or various vasoactive substances. Unfortunately, few data exist to suggest which pathway(s) might be involved. It seems probable that the intermediary mecha­ nism(s) that plays a role in isoflurane-mediated va­ sodilation may also be involved in the distant CBF changes produced by the lesion itself. Focal neuro­ logic lesions, particularly in the cortex, have repeat­ edly been shown to produce widespread vascular and biochemical changes. Perhaps the best known is the phenomenon of diaschisis (Feeney and Baron, 1986), wherein even small cortical lesions result in functional and physiologic alterations in the contralateral cerebral or cerebellar hemi­ spheres. The pathways responsible for such changes are incompletely understood, but involve­ ment of transcallosal conduction has been sug­ gested. Of interest is the fact that Evans blue stain­ ing of the corpus callosum underlying the cryogenic injury is common in our model. We did not detect any differences between left and right cerebellar hemispheres in lesioned animals, or any differences in the response to isoflurane between the cerebellar hemispheres. With respect to changes in vasoactive compounds, Pappius and Wolfe ( 1986) have indi­ cated that tissue concentrations of both serotonin and certain prostaglandins appear to change follow­ ing injury, particularly in the lesioned hemisphere. They have also shown that blockade of serotonin synthesis can alter the cerebral metabolic rate pat­ terns after injury (Pappius et aI., 1988). Unfortu­ nately, these studies were carried out after a much longer delay than in our animals, and few CBF changes were noted. Of potential interest is the fact that Martins and Doyle ( 1978) failed to detect dis­ tant changes in CBF following a freeze lesion. Their study was performed in ketamine-anesthetized pri­ mates. Both cryogenic and impact injuries are fol­ lowed by increases in interstitial glutamate (Baeth­ mann et aI., 1989; Faden et aI., 1989), and ketamine is an antagonist of glutamate action at the N­ methyl-D-aspartate receptor (Weiss et aI., 1986; Choi et aI., 1988). In addition, ketamine will block the occurrence of spreading depression (Gorelova et aI., 1987; Hernandez-Caceres et aI., 1987), a phe­ nomenon that is associated with widespread flow changes. There are methodologic factors that impact on

the interpretation of our results. The study duration was short, since the cryogenic injury was only 90 min old at the time of isoflurane administration. It is possible that differences might be seen with more chronic lesions, since Pappius ( 198 1) has shown that the maximal cerebral metabolic rate and CBF effects of a cryogenic injury in the rat do not occur until 24-72 h post injury. We also employed a back­ ground anesthetic consisting of morphine/N20. This combination was chosen because blood pres­ sure is well maintained, because of the relatively minor effects of morphine on CBF, and because it preserves the normal vascular responsiveness to CO2, However, the presence of N20 may magnify the CBF changes produced by isoflurane (Hansen et aI., 1989b), and hence the differences seen be­ tween normal and lesioned animals may not have been as great in the presence of an alternative back­ ground. We chose to determine flow in intact coro­ nal brain sections rather than to separate cortical and subcortical regions. This was done to allow us to examine regional changes without complicating our analysis by even more data points. We also feel that more detailed regional analysis is best exam­ ined by autoradiography. We do not believe that this severely limits our findings, since Pappius ( 1981), using such methods, has shown that cerebral metabolic rate and CBF changes occur throughout the injured hemisphere, not just in cortex. Never­ theless, we recognize that differences between cor­ tical and noncortical flow responsiveness may have been present. Our work is also influenced by the use of angiotensin II to support blood pressure dur­ ing isoflurane administration. We strongly believe that such blood pressure support is necessary. Both a cryogenic injury and isoflurane alter or abolish autoregulation (Miller et aI., 1975; Reilly et aI., 1977; McPherson and Traystman, 1988). If blood pressure had been allowed to fall during isoflurane administration, we might have seen much smaller CBF changes than demonstrated here. In normal rabbits, intravenous angiotensin in doses similar to those used here has relatively little effect on CBF (Maktabi et aI., 1990), although it may act as a mild vasoconstrictor in the isoflurane-anesthetized rab­ bit (Patel and Mutch, 1990). It is possible that the blood-brain barrier disruption produced by the freeze lesion may have increased the access of an­ giotensin to the extravascular space and enhanced its vasoconstrictive properties. However, this seems an unlikely explanation for the widespread effects of the lesion on vascular responsiveness since the blood-brain barrier was intact in regions outside of the lesion focus. In conclusion, we have shown that a relatively J

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mild cryogenic injury alters the normal increases in CBF produced by isoflurane. This response is oblit­ erated in the hemisphere ipsilateral to the injury and is markedly attenuated in the contralateral hemi­ sphere (and possibly in the hindbrain). This pattern of responsiveness differs from that seen for CO2, These observed changes in anesthetic responsive­ ness in tissues distant from the lesion may not be compatible with isoflurane acting directly on vascu­ lar smooth muscle. Acknowledgments: This work was supported by USPHS grants RO I-NS24517 (Dr. Todd) and R29GM39771 (Dr. Warner). The authors would like to thank the following people for their assistance: Mazen Maktabi, M.D., and Tom Smith, B.S., for help in implementing the microsphere technique for measuring cerebral blood flow; Edward Scalf, B.S., for his technical expertise; and Twyla Salisbury for her assistance with the final manu­ script. Ms. Cindy Long (Department of Preventive Med­ icine) assisted in the statistical analysis.

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