Relationships Between High Oxygen Extraction Fraction in the Acute Stage and Final Infarction in Reversible Middle Cerebral Artery Occlusion

Journal of Cerebral Blood Flow and Metabolism 16: 1176-1188 © 1996 The International Society of Cerebral Blood Flow and Metabolism Published by Lippincott-Raven Publishers, Philadelphia

Relationships Between High Oxygen Extraction Fraction in the

Acute Stage and Final Infarction in Reversible Middle Cerebral

Artery Occlusion: An Investigation in Anesthetized Baboons

with Positron Emission Tomography

*t Alan R. Young, IIGiuliano Sette, t:j:Omar Touzani, *Patrice Rioux, t§Jean Michel Derlon,

t:J:Eric T. MacKenzie, and *tJean Claude Baron

*INSERM U. 320, tCyceron Biomedical Cyclotron Unit of Caen, :f:University of Caen, CNRS URA 1829, and §University

Hospital of Caen, Caen, France; and I!Dipartimento di Scienze Neurologiche, 1° Clinica Neurologica, Universita "La

Sapienza", Rome, Italy

Summary: Studies in humans suggest that regions that show maximal increases in brain oxygen extraction fraction (OEF) in the hours following an ischemic episode are those most vul­nerable for infarction and are often, although not always, as­sociated with the final site of infarction. To clarify this issue, we followed the hemodynamic and metabolic characteristics of regions with an initially maximally increased OEF and com­pared them with the ultimately infarcted region in an experi­mental stroke model. Positron emission tomography (PET) was used to obtain functional images of the brain prior to and fol­lowing reversible unilateral middle cerebral artery occlusion (MCAO) in 1 1 anesthetized baboons. To model early reperfu­sion, the clips were removed 6 h after occlusion. Successive measurements of regional CBF (rCBF), regional CMR02 (rCMR02), regional cerebral blood volume, and regional OEF (rOEF) were performed during the acute (up to 2 days) and chronic (> 15 days) stage. Late magnetic resonance imaging (MRI) scans (coregistered with PET) were obtained to identify infarction. Reversible MCAO produced an MRI-measurable infarction in 6 of II baboons; the others had no evidence of ischemic damage. Histological analysis confirmed the results of the MRI investigation but failed to show any evidence of cor­tical ischemic damage. The lesion was restricted to the head of the caudate nucleus, internal capsule, and putamen. The infarct volume obtained was 0.58 ± 0.31 cm3. The infarcts were situ­ated in the deep MCA territory, while the area of initially

Received August 3, 1995; final revision received March 18, 1996; accepted March 18, 1996.

Address correspondence and reprint requests to Dr. J.-C. Baron at INSERM U.320, Cyceron Biomedical Cyclotron Unit of Caen, Boule­vard Henri Becquerel, BP. 5229 , 14074 Caen Cede x, France.

Abbreviations used: AN OVA, analysis of variance; CBV, cerebral blood volume; CM, canthomeatal; MCA, middle cerebral artery; MCAO, MCA occlusion; MRI, magnetic resonance imaging; OEF, oxygen extraction fraction; PET, positron emission tomography; rCBF, regional CBF; rCBV, regional CBV; rCMR02, regional CMR02; rOEF, regional OEF; Ro!, region of interest.

1176

maximally increased OEF was within the cortical mantle. The mean absolute rCBF value in the infarct region of interest (ROI) was not significantly lower than in the highest-OEF ROI until 1-2 days post-MCAO. Cerebral metabolism in the deep MCA territory was always significantly lower than that of the cortical mantle; decreases in CMR02 in the former region were evident as early as I h post-MCAO. In the cortical mantle, the rOEF was initially significantly higher than in the infarct-to-be zone. Subsequently, the OEF declined in both regions. The differences in the time course of changes in CMR02 and OEF between these two regions, with the eventually infarcted area showing earlier metabolic degradation and in tum decline in OEF, presumably underlie their different final outcomes. In conclusion, following MCAO, the region that shows an early maximal increase in the OEF is both topographically and physi­ologically distinct from the region with final consolidated in­farction if reperfusion is allowed at 6 h. This high OEF, al­though indicative of a threatened condition, is not an indicator of inescapable consolidated infarction and is thus a situation in which therapy could be envisaged. Whether or not it is at risk of infarction and thus constitutes one target for therapy remains to be seen. Key Words: Baboon-Cerebral blood flow and metabolism-Focal cerebral ischemia-Magnetic resonance imaging-Oxygen extraction fraction-Positron emission to­mography-Stroke-Temporary middle cerebral artery occlu­sion.

In clinical studies of stroke, positron emission tomog­

raphy (PET) has been of invaluable use in mapping both

hemodynamic and metabolic parameters (Lenzi et al.,

1982; Wise et al., 1983; Frackowiak and Lammertsma,

1985; Baron, 1991). One major finding in these studies

has been the frequent observation of an area in which

there is a high oxygen extraction fraction (OEF) that

persists up to 4 days following the ictus in some cases (Wise et al. , 1983; Frackowiak and Lammertsma, 1985;

TRANSIENT MeA 0 IN ANESTHETIZED BABOONS lI77

Ackerman et aI., 1989; Baron, 1991; Marchal et aL

1993). Because a high OEF reflects an inadequate oxy­

gen supply in relation to underlying metabolic needs, it is

implicit that the higher the OEF, the greater the risk of hypoxia and the tissue becomes more and more in a

situation of increased jeopardy. Wise et aI. (1983) re­

ported that brain regions in which the OEF was initially elevated were consistently associated with an eventual

infarction that in turn caused a secondary fall in OEF to abnormally low values. There are, however, certain ex­

ceptions to these observations and some brain regions that show an initially elevated OEF may not inevitably

progress to infarction (Powers et aI., 1985; Ackerman et

aI., 1989; Baron et aI., 1989; Furlan et aI., 1994). Ac­

cordingly, regions of high OEF most probably represent

tissue that is at risk, but concrete sequential data to sup­port this hypothesis are lacking. Thus, it is important to

clarify the relationship between OEF and final consoli­

dated infarction, especially if PET is ever to be used as a tool for patient selection in therapeutic trials. As of to­

day, it is not certain whether the area with maximal OEF

increase early after stroke is ineludibly destined to be

infarcted, or if it is only at risk, i.e., still amenable to

therapy.

In contrast to human studies that provide only discrete

time points of the highly dynamic changes in ischemic

brain physiology, studies in nonhuman primates are op­

timal to address these issues because they allow both

acute and sequential studies (Yon as et aI., �988; Tenjin

et aI., 1992; Monsein et aI., 1993; Pappata et aI., 1993;

Touzani et aI., 1995) with controlled reperfusion and

computed tomography/magnetic resonance imaging

(MRI) infarct mapping in the chronic stage (Spetzler et

aI., 1983; Sette et aI., 1993).

We have calTied out one such a study in the acute (first

hour) and chronic stages following a 6-h temporary oc­clusion of the middle cerebral artery (MCAO) in the

anesthetized baboon. We chose a reversible occlusion model because it mimics the clinical situation in which,

ideally, early PET measurements may be made and con­tinued during therapeutic or occasionally spontaneous

MCA recanalization (Fieschi et al., 1989). The experi­

mental data obtained by Tenjin et aI. (1992), Pappata et

aI. (1993), and Heiss et aI. (1994) suggested that remark­

able dynamic changes in the OEF in the acute stage of an

ischemic episode may be region dependent. Therefore, in

the present investigation, the following question was

asked: Is the area with maximal OEF ilTeversibly des­

tined to consolidated infarction, or does it represent only

vulnerable tissue? To this end, we compared the topog­

raphy of the region with early maximally elevated re­

gional OEF (rOEF) with that which represents the final

consolidated infarct and compared the time course of

hemodynamic and metabolic changes in these two re­

gions.

METHODS

Experiments were performed in 1 1 adolescent male baboons (Papio anubis) with body weights ranging from 7 to 16 kg. The baboons were housed in individual cages maintained at 24°C with 50% relative humidity on a 12-hiI2-h light/dark cycle and were fed commercial chow supplemented with fresh fruits and water ad libitum. The evening prior to investigation, all solid foods were withdrawn.

Anesthesia The baboons were tranquilized initially with a short-acting

barbiturate (methohexital 20 mg/kg i.m.; B rietal, Lilly), and subsequent to the placement of catheters in the external saphe­nous veins, anesthesia was induced by the administration of etomidate (3 mg/kg i.v.; Hypnomidate, 125 mg/ml, Janssen). The hypnotic effects of etomidate were potentiated by cloni­dine (80 f.Lg, infused intravenously over LO min; Catapressan, Boehringer Ingelheim). Following the administration of atra­curium (0.5 mg/kg i.v.; Tracrium, Wellcome) so as to achieve adequate muscuLar relaxation, endotracheal intubation was per­formed. All baboons were placed on intermittent positive­pressure ventilation with periodic hyperinflation to minimize atelectasis. A fixed respiratory rate (20 min-I) was employed and the tidal volume adjusted so as to maintain normocapnia (p "co2 39-4 1 mm Hg). In every case, anesthesia was main­tained with 67% nitrous oxide in oxygen (humidified at 38°C) along with continuous intravenous infusions of etomidate (0.3 mg/kg-h) and atracurium (0.75 mg/kg-h). Under this mainte­nance anesthetic regimen, MABP remained remarkably stable throughout the entire duration of the studies. During surgical interventions, anesthesia was supplemented with isoflurane (0.5�1.5%; Forene, Abbott); isoflurane was discontinued at least 90 min before any PET measurement. Prior to the place­ment and removal of the stereotaxic ear bars, atropine sulfate (0.25 mg i.v.) was administered. All baboons received an in­travenous perfusion of heparin (50 Iu/h at 3 ml/h) starting 2 h before the occlusion and continued until 1 h after the reperfu­sion period.

MeAO The transorbital approach to the right MCA described by

Hudgins and Garcia (1970) was employed. Following enucle­ation, a small craniectomy was performed using a high-speed saline-cooled dental drill to expose the right MCA. The dura was opened and the arachnoid dissected to allow placement of two microvascular clips: one on the proximal part of the main MCA trunk and the other on the orbitofrontal branch. During this procedure, warmed saline was used to reduce the possibil­ity of vascular spasm. The occlusion was temporary (6 h), and both the occlusion and the reperfusion phases were verified by Doppler sonography. Reconstruction of the orbit under aseptic conditions allowed a complete postoperative recovery in all baboons and permitted long-term survival, with the exception of one baboon that died at day 3 due to postoperative compli­cations.

Postoperative care Before recovery from the final surgical intervention, a blood

transfusion was given (250 ml of concentrated erythrocytes; hematocrit > 80%; hemoglobin 20 gIL, preceded by 2 mg dexa­methasone i.m.; Soludecadron). In all instances, neostigmine (0.5 mg i.v.; Prostigmine) was administered to reverse the ef­fects of atracurium before the baboons were weaned from the ventilator. Antibiotic treatment with cephamandole was contin­ued over 5 days (15 mg/kg Lm. daily; Kefandol).

J Cereb Blood Flow Metab, Vol. 16, No.6. 1996

1178 A. R. YO UNG ET AL.

Physiological monitoring End-tidal CO2 and N20 concentrations were monitored con­

tinuously by an infrared adsorption analyzer (5200 CO2 moni­tor; Ohmeda). Inspired oxygen concentrations were continu­ously measured by an electrochemical system (Ohmeda). Fol­lowing the percutaneous insertion of a catheter into each femoral artery, blood samples were withdrawn periodically for the measurement of Paco2, Pao2, pH, and hemoglobin concen­trations (ABL 300; Radiometer). Hematocrit and blood glucose concentrations (glucose oxidase method; Beckman) were also measured frequently. Total blood loss due to sampling and the surgical intervention was estimated to be of the order of 70 ml. As a vehicle for intravenous anesthetic agents, physiological fluids (saline or Ringer lactate) were infused intravenously (-50 mllh, over 14 h). A urethral catheter was inserted and urine output measured (-500-800 ml over 14 h). Metabolic acidosis, defined as a base deficit of >3 mEq/L, was corrected by NaHC03 administration. A physiological recording system (Hewlett Packard) was used to monitor the ECG, heart rate, and arterial pressure both by a plethysmograph and directly by a strain gauge system. Body temperature was maintained within normal limits (37-38°C) by heating blankets. Following the end of each PET session, the baboons were allowed to recover fully and were returned to their cage.

PET procedures A control PET measurement was performed -2 weeks prior

to the occlusion. Two further measurements were made during the occlusion (MCAO + 1 h and MCAO + 4 h) and another at 1 h following the reperfusion phase (MCAO + 7 h). Additional PET studies were performed at 1-2 days post-MCAO and again between 15 and 50 days postocclusion (Fig. 1). We used the four-ring, seven-slice LET! TTV03 PET device (CEN, Grenoble, France) with an intrinsic spatial resolution of 5.5 x 5.5 x 9 mm [x, y, zl (Mazoyer et aI., 1990). To obtain repro­ducible head positioning (both inter- and intraanimal), the ba­boon's head was fixed in a specially designed frame with ear bars being placed in the external bony auditory canal and checked by a radiogram. External laser beams allowed us to select seven planes [-27 to +45 mm relative and parallel to the canthomeatal (CM) line] for imaging, according to an anatomi­cal PET atlas (Riche et aI., 1988). A 68Ga_ 68Ge transmission scan was performed prior to each PET session. With use of the 1502 steady-state technique (with measured attenuation correc­tion), successive inhalations of 1502-labeled CO, O2, and CO2 allowed parametric imaging of the regional cerebral blood vol­ume (rCBV), blood volume-corrected regional cerebral oxygen metabolism (rCMR02), regional CBF (rCBF) and the rOEF to be obtained (Frackowiak et aI., 1980; Sette et aI., 1989). A calculation was performed to obtain the rCBF/rCBV image.

Morphological imaging In the chronic stage (> 16 days after MCAO), the baboons

underwent an MRI procedure in vivo (slice thickness 3 mm, 1'2 scan; Signa 1.5 T, General Electric, Milwaukee, WI, U.S.A.) to establish the presence, topography, and size of a hyperintense

signal, taken to represent approximately the final infarct (Sette et aI., 1993). The same positioning procedure as that described for the PET study was employed during these examinations, which allowed us to obtain superimposable images for both procedures.

Regions of interest Region of interest based on highest OEF at MeAO + 1 h.

In accordance with the literature (Pappata et aI., 1993), we chose a region of interest (ROI) based on a computer-generated isocontour (88% of the maximal pixel value) that delineated the highest-OEF area in the I -h post-MCAO PET image. In this objective ROI procedure, the threshold (88%) was adjusted from that used by Pappata et a!. ( 1993) to allow for improved spatial resolution of the PET camera and so as to result in similar surface areas of interest (see Results). This ROI was constantly most prominent on the basal ganglia plane situated parallel to and +2 1 mm above the CM line (Fig. 2). The plane chosen represents most of the territory supplied by the MCA and showed the highest OEF with the most typical and striking changes as well as the area of infarcted tissue (see following). This highest-OEF ROI, where the phenomenon of misery per­fusion was maximal, was copied by a computer mirror function onto the contralateral hemisphere. Subsequently, both these ROIs (on the ipsilateral and contralateral hemispheres) were transposed by an image analysis system to the PET images of all other parameters (i.e., CBF, CMR02, CBV, and CBF/CBV images) obtained on the same plane and at all times studied.

ROJ ba�ed on consolidated infarct. The MR images (coreg­istered with PET) were used to select the plane on which a hyperintense area was readily demonstrable. Again, this region was located on the basal ganglia plane (CM + 2 1 mm). These images, standardized to the same pixel size as the PET image, were displayed on a video device to determine an irregularly shaped ROJ (presumably representative of the infarct) that fol­lowed a computer-generated isocontour determination of den­sity changes in the MRI hyperintense area and for which the computer provided the area in square centimeters. The infarct ROI was then copied by a computer mirror function procedure onto the contralateral hemisphere with respect to the midline (Fig. 2). As before, following computer-assisted superimposi­tion of the MRI and PET images, the ROls were transposed onto the corresponding PET parametric images. The values for these functional parameters were then calculated for the infarct ROl on these images for both hemispheres and for all PET studies.

Neuropathology Approximately 4 weeks after MCAO, the baboons were cu­

rarized, ventilated, and deeply anesthetized with 2-3% isot1u­rane. Heparin (5,000 IU) was administered intravenously to facilitate exsanguination. The baboons were placed in the su­pine position, the thorax opened through a midline incision, and a cannula inserted into the ascending aorta via the left ventricle. After incising the right atrium and clamping the descending aorta, heparinized saline (5 L) was perfused at the baboon's

clip on clip off Rcperfusion phase MRI

r <:

rt· �l (in vivo) FIG. 1. Timing of the positron emis--;-:,," t-14 days y sion tomography (PET) studies (ar-

t f t f t rows) and magnetic resonance imag-ing (MRI) measurements. MCAO, middle cerebral artery occlusion.

PRE-MCAO t=O t+l h t+4h t +7 h t +1-2 days t >15 days t-28 days

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TRANSIENT MeAO IN ANESTHETIZED BABOONS 1I79

FIG. 2. Illustration of the region-of­interest (ROI) placements in baboon no. 10, showing the positron emis­sion tomography image of the oxy­gen extraction fraction (OEF) ob­tained at middle cerebral artery occlu­sion (MCAO) + 1 h and the chronic stage_magnetic resonance image (MRI T 2) obtained (in vivo) at MCAO + 23 days. The axial cuts were taken parallel to and + 21 mm above the canthomeatal plane. The OEF image is represented with respect to a pseu­docolor scale for pixel values be­tween ° and 1.0. The highest-OEF ROI (identified by a computer­generated isocontour equal to 88% of the maximal pixel value) is shown to­gether with the contralateral mirror image counterpart. The contrast for the infarct ROI shown on the MR im­age was c�osen to highlight the hy­perintense T 2 signal; note also a rela­tive dilatation of the adjacent lateral ventricle. The highest-OEF ROI is es­sentially restricted to the cortical mantle, while the infarct was always located in the deep MCA territory (right basal ganglia) .

mean arterial pressure until the perfusate from the right atrium was bloodless. Thereafter, 8 L of FAM (formaldehyde 40%/ glacial acetic acid/absolute methanol I: I :8) was perfused at the same pressure to fix the brain in situ. Following decapitation, the head was placed in the FAM fixative at 4°C for a minimum of 24 h. The brain was removed from the skull and immersion­fixed in FAM for a further 7 days minimum before being trans­ferred to a 70% solution of alcohol.

Based on the MRI data. a single block of tissue (in which the infarct was located) was cut and embedded in paraffin wax. Coronal sections 15 f.Lm thick were taken with reference to a stereotaxic atlas of the baboon's brain (Riche et aI., 1968) and stained with hematoxylin and eosin for examination of infarct topography by conventional light microscopy. In addition, a standard image analyzer was used to map the contour of healthy tissue. The surface area of the lesion was subsequently calculated by subtraction from the contralateral hemisphere (Touzani et aI., 1995; Young et aI., 1995). Infarct volume was calculated by integration over 10 equidistant slices that encom­passed the whole lesion. In this study, the histological results were intended to confirm the infarct seen on the MRI data; a three-dimensional reconstruction and matching of histological and MRI data were not possible because of the different ori­entations of the cuts as well as the shrinkage and collapse effect related to the histological procedure.

Data analyses Physiological and biochemical data were evaluated by analy­

sis of variance (ANOYA) followed by Tukey's test. ANOYA with repeated measurements was also used to identify any in­teraction between the functional parameters measured with re­spect to time, region, and/or ipsilateral/contralateral side dif­ferences. A Student paired t test (without Bonferroni correction in this descriptive approach) was employed to analyze the side­to-side differences and to compare the two ROIs. Mean ± SD values are given throughout the text and figures. In addition, changes in the asymmetry ratios for individual PET parameters at a given lime postocclusion were assessed for their statistical

significance with respect to the preocclusion 95% confidence limits.

RESULTS

Group characteristics

Six-hour reversible MeAO in healthy young anesthe­

tized baboons produced readily demonstrable MRI

changes suggestive of infarction (confirmed by subse­

quent neuropathology in each animal) in only 6 of 11

baboons (Table 1). Of these six baboons, one died 3 days

after the occlusion due to postoperative complications

and one other failed to complete the full PET protocol

TABLE 1. Inclusion criteria for the study

Baboon MRI Full PET Consolidated Group no. changes protocol infarction no.

I NRD + +/- 2 2 0 + 2 3 + + + 1 4" + +b + Ex 5 + 0 + Ex 6 + + + I 7 0 + 2 8 + + + I 9 0 0 Ex

iO + + + 1 II 0 +c 2

MRI, magnetic reasonance imaging; PET, positron emission to­mography; NRD, not readily demonstrable; Ex, excluded from study; Group I, readily demonstrable infarct; Group 2, no readily demon­strable infarct.

a Baboon died after postoperative complications. h Day 1-2 PET study missing. c Day 1-2 and day >15 PET studies missing.

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1180 A. R. YOUNG ET AL.

(due to technical difficulties) and was therefore excluded

from the final PET analysis. Thus, overall, four baboons

with both readily delineated changes in the MRI and

complete PET protocol were available for this study

(Group 1). The remaining baboons showed no evidence

of infarction in four (one of which was excluded for PET

device failure just after MCAO) and questionable MRI

changes in one; this set of four "control" baboons will

be referred to as Group 2 (Table 1).

Neurological deficit and pathology

All baboons recovered consciousness within 20 min

after etomidate anesthesia was discontinued. There was

only slight contralateral hemiparesis in the upper limb

and visual field defeCts in the remaining eye. Turning of head, shoulders, and eye to the right ("neglect") was

commonly observed. All baboons showed prompt neu­

rological improvement readily discernible within a 24- to

48-h recovery period. During the days after MCAO, the

baboons were alert and exhibited normal feeding and

grooming behavior.

To allow confirmation of adequate infarct imaging by

MRI procedures, we used light microscopy on brain sec­

tions stained with hematoxylin and eosin. Of the 11 ba­boons, 4 showed no evidence of gross macroscopic dam­

age and readily demonstrable changes on MRI (Table I). The remaining seven animals all showed a consolidated

infarct including the animal that died due to postopera­

tive complications. The six animals with chronic infarc­

tion had only small consolidated infarcts (mean ± SD

volume = 0.58 + 0.3 \ cm3) located in the head of the

caudate nucleus, internal capsule, and putamen (see Fig.

3). The baboon with the smallest infarct (0.2 cm3) is the

one with questionable MRJ findings. In no instance was

there evidence of a consolidated infarct in cortical re­

gions (i.e., the region associated with the highest changes

(6 h TEMPORARY MeAO IN THE BABOON )

No. 3 6 8 10

FIG. 3. Top: Schematic topographical distribution of the regions of interest used in the investigation for the four baboons in Group 1. The hatched area includes values corresponding to an isocontour of �88% of the maximal pixel value obtained for the oxygen extraction fraction (OEF) at postocclusion. The filled area represents the region of final infarction as determined by magnetic resonance imaging (MRI) techniques. On these axial cuts (obtained at the level of the lateral ventricles and basal ganglia; see Fig. 2 for orientation) that are representative of the positron emission tomography images and coregistered MRI scans, there was no measurable overlap of the two regions. Bottom: Representative macrohistological coronal sections showing consolidated infarction in baboons -4 weeks following temporary middle cerebral artery occlusion (MCAO) for 6-h duration (Group 1). p, putamen; ic, internal capsule; c, caudate nucleus. Right hemisphere of the brain is shown on the left. Infarction is restricted to the basal ganglia region with sparing of the parasylvian cortex (i.e., the region in which the highest OEF was essentially located). Dilatation of the right ventricle is apparent. These digitalized coronal sections were also taken at the level of the lateral ventricles. There was no evidence of a hemorrhagic infarct. Calibration bar = 1 cm.

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TRANSIENT MeAO IN ANESTHETIZED BABOONS fI8I

in OEF noted in the PET studies). Figure 3 illustrates the

topography of the MRI and histological mapping of the

consolidated infarct, showing good correspondence for

the two methods employed.

Physiological and biochemical data

The physiological and biochemical data for the four

baboons in Group I are presented in Table 2. No signifi­

cant differences were noted for any of these measured

parameters with respect to time (ANOV A).

Size and topographical distribution of ROIs

In all baboons, high OEF was prominent in the MCAO

+ I h image. In both groups, the highest-OEF ROI was

essentially located on the cortical mantle. In Group I, it

was topographically distinct from the infarct ROI that

was located in the deep MCA territory (see Figs. 2 and

3). At MCAO + I h, there was no significant difference

in the area of highest OEF between Group I and Group 2 (3.29 ± 1.21 and 3.47 ± 1.41 cm

2, respectively). The

area of the infarct ROI was 1.28 ± 0.54 cmz.

Group 1 Absolute and side-to-side asymmetry indexes for ROIs.

The data are shown in Tables 3-5. Figure 4 illustrates the

PET functional images of CBF, CMR02, and OEF, ob­

tained in the anesthetized baboon before and following 6

h of reversible MCAO. For clarity, the ROIs have been

omitted from the color figures. Figures 5 and 6 show the

time course of the individual values (including the CBFI

CBV ratio) for each baboon. ROI based on highest OEF. Tables 3 and 4 show the

absolute values (mean ± SD) obtained in the highest­

OEF ROI for the ipsilateral and contralateral hemi­

spheres, respectively, before and after MCAO. Because

of large variance due to the small sample number and

interanimal variability, we decided to normalize the data

obtained with respect to contralateral homologous values

since no significant contralateral effects could be evi­denced by repeated-measures ANOV A on time effect

(except for a significant widespread reduction of CBV at

the latest measurement, indicating the side-to-side ratios

for CBV for that specific time should be interpreted with

caution). As such, the data will be discussed in terms of

the asymmetry index values (i.e., the ipsilateral!

contralateral side ratios; see Table 5 and Fig. 5). By this

approach we were able to evidence more clearly the ef­

fects due to MCA occlusion, which were assessed sta­

tistically by comparing the values at each time post­

MCAO with control values (paired t tests).

rOEF indexes were maximal at MCAO + 1 h. This

elevated rOEF was still present at MCAO + 4 h (though

to a lesser extent) and then tended to remain at control

levels or slightly below following reperfusion until the

latest PET study (see Fig. 5c).

rCBF indexes were significantly reduced at MCAO + 1 h and + 4 h. Following removal of the clips, rCBF

values returned to, or were slightly above, normal and

remained stable over the next 24-48 h (Fig. Sa). The

final PET study, however, revealed a moderate but sig­

nificant reduction in rCBF.

No significant change in rCMR02 indexes was noted

either during the acute stage or at any time thereafter

except for a small but significant decrease at the last PET

study. One notes, however, that this trend, though not

significant, was already present at 24-48 h (Fig. 5b).

rCBV indexes were elevated during the period of oc­

clusion (though significantly so for MCAO + 4 h only); at the terminal study, they were mildly but significantly

lower than pre-MCAO values. The index for the CBFI CBV ratio fell markedly during the period of occlusion,

but returned to near normal after MCA reopening (Fig.

5d).

ROI based on MRI infarct. Tables 3 and 4 show the

absolute values obtained by PET in the ipsilateral and

contralateral hemispheres, respectively. Again, we have

presented the data in terms of the ipsilateral/contralateral

ratios since no global effects were noted in the contra­

lateral hemisphere (see Table 5 and Fig. 6), except for the final measurements of rCBV.

In the infarct ROI, the ratio for the rOEF showed a

trend for an increase at MCAO + 1 h, and in one baboon

TABLE 2. Physiological and biochemical parameters measured before and after 6 h of temporary middle cerebral artery occlusion (MeAD) in anesthetized baboons: Group 1

MCAO MCAO Reperfusion MCAO MCAO Condition Pre-MCAO +1 h +4 h +1 h +1-2 days >15 days

�----Heart rate (beat/min) 135 ± 15 118 ± 16 133 ± 18 144± 31 145 ± 12 125 ± 14 MABP (mm Hg) 1 1 7± 10 1 02 ± 8 100± 8 104±6 105 ± 5 107 ± 20 P,co2 (mm Hg) 39.8 ± 0.9 39.7 ± 0.4 40.1 ± 1.0 39.8 ± 0.5 40.0 ± 1.0 39.6 ± 0.7 Pao2 (mm Hg) 104± 1 1 1 15 ±23 114± 15 113±13 113 ±21 123 ± 21 pH 7.409 ± 0.038 7.404 ± 0.022 7.394 ± 0.027 7.378 ± 0.018 7.418 ± 0.010 7.411 ± 0.003 Hb (gil 00 m]) 13.2 ± 0.5 1 1.8 ± 1.5 11.4 ± 1.6 12.9 ± 3.6 11.0±3.3 11.3 ± 0.5 Ht (%) 38.4 ± 2. 1 36.3 ± 4.3 34.1 ± 4.4 37.0 ± 10.1 31.6 ± 9.6 37.0 ± 3.0 Gl (gIL) 1.01 ±0.12 I.J9 ±0.31 1.09 ± 0.26 1.09 ± 0.17 1.38 ±0.90 0.80 ±0.04 Temp. (OC) 36.9 ± 0.5 37.7 ± 1.1 38.2 ± 0.6 38.4 ± 0.5 37.0 ± 0.8 37.2 ± 0.9

Hb. hemoglobin; Ht. hematocrit; GI. plasma glucose concentration; Temp .. body temperature.

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1182 A. R. YOUNG ET AL.

TABLE 3. Functional parameters measured betc)re and after 6 h of temporary middle cerebral artery occlusion (MCAO): Group I-ipsilateral hemisphere

Ipsilateral MCAO MCAO Reperfusion MCAO MCAO hemisphere Pre-MCAO +1 h +4 h +1 h +1-2days >15 days

��-----------------�-------���--'--------'---.

ROl based on high OEF 26.7 ± 4.3"

CBF 36.6 ± 1 0.2 29.0 ± 8.7 2 8.9 ± 4.5 38.2 ± 6.4 34.9 ± 2.0g CMR02 3.30 ± 0.94 3.32 ± 0.20" 3.06 ± 0.59' 2.98 ± 0.621 2.22 ± 0.7yf 2.56 ± 0.62-'

OEF 0.53 ± 0. 1 4 0.78±0.IY 0.75 ± 0.2(Ji 0.50 ± O.22g 0.46 ± 0. 1 2 0.63 ± O.O&'

CBVa 2.7 ± 0.5 3.8 ± 0.51 3.7 ± 0.7 3.5 ± 0.6 3.3 ± 0.9 2.1 ± 0.4"

CBF/CBV" 14. 1 ± 5.4 7.7 ± 2.6 8.0 ± 2.0 1 J.3 ± 3.5 11.3 ± 2.7 13.0 ± 2.1

ROI based on infarct 21.1 ± 3.7" 21.6 ± 3.9"

CBF 3 8.2 ± 11.0 24. 8 ± 7.6 24.4 ± 6.0 34.7 ± 8.3

CMR02a 3.44 ± 1.28 2.33 ± 0.14 1.98 ± 0.26" 2.02 ± 0.46" 1.02 ± 0.24" 1.62 ± 0.31"

OEF 0.53 ± 0.12 0.65 ± 0.13 0.57 ± 0.13 0.39 ± 0.20 0.35 ± 0.08' 0.51 ± 0.05

CBV" 2.4 ± 0.4 2.9 ± 0.4 2.8 ± 0.4 3.1 ± 0.9 2.0 ± 0.5 1.7 ± O.Sd

CBF/CBV 16.3 ±6. 1 8.9 ± 3. 1 8.6 ± 1.9 11.7 ± 3.0 10.6 ± 1.9 13.2 ± 4.0

CMR02 and CBF are expressed in ml 100 g-' min-', cerebral blood volume (CBV) in ml 100 ml-', and the CBF/CBV ratio as min-i; OEF, oxygen

extraction fraction (dimensionless). Mean ± SD values are given, n = 4 baboons. " Significant (p < 0.0 1 ) time effect b y one-way analysis o f variance (ANOVA) with repeated measures.

b Significantly different from pre-MCAO. e Different from MCAO + I h. d Different from reperfusion + 1 h. e Different from MCAO + I h, + 4 h, and reperfusion + 1 h by ANOVA with Tukey's test. Student's paired t test comparing the highest-OEF and infarct regions of interest (ROIs) as a function of time; Jp < 0.05; gp < 0.0 1 ; "p < 0.001.

only, the OEF value lay outside the 95% confidence limits for the pre-MCAO measurements (Fig. 6c). At

MCAO + 4 h, the OEF had returned to essentially normal values. During the reperfusion phase, the rOEF value declined significantly and remained at low levels until

the end of the study.

During occlusion, the index for rCBF remained sig­nificantly lower than pre-MCAO levels. Reperfusion re­

stored temporarily these values toward normality, but the

subsequent two PET studies again showed a marked and

significant fall in the rCBF index values (see Table 5 and

Fig. 6a).

When compared with the pre-MCAO index values, rCMR02 declined significantly as early as I h postoc­

clusion, and a significant and sustained reduction in oxy­

gen metabolism, unaffected by reperfusion, was noted up

until the final PET measurement (Fig. 6b). Of note is the

deterioration in the CMR02 ratio from reperfusion + 1 h to MCAO + 1-2 days ..

CBV indexes remained stable throughout most of the

investigation, although a significant reduction was noted at the last PET measurement. The CBF/CBV ratios were

markedly reduced during occlusion and showed a partial

return toward normal values after reperfusion (see Table

5 and Fig. 6d).

Comparison of highest-OEF and infarct ROls on ip­

silateral hemisphere. We compared the absolute values

for PET variables between the two ROIs (Table 3).

There was a significant difference in rCBF only for the

time periods MCAO + 1-2 days and MCAO > 15 days,

where rCBF was maintained in the highest-OEF ROI

relative to the infarct ROl. In contrast, the rCMR02

showed significant differences at all times after occlu­

sion, where rCMR02 was maintained in the highest-OEF

TABLE 4. Functional parameters measured before and (lfter 6 h of MCAO: Group i-contralateral hemisphere

Contralateral MCAO MCAO Reperfusion MCAO MCAO hemisphere Pre-MCAO +1 h +4 h +1 h +1-2 days >15 days

ROI based on high OEF CBF 37.3±8.3 40.8 ± 3.9 39.0 ± 2.0 36.0 ± 4.8 37.3 ± 3.6 35.3 ± 4.6 CMR02 3.36± 1. 1 4 3.33 ± 0.45 3.56 ± 1.17 3.61 ± 0.95 2.74 ± 0.60 3.26 ± 0.84 OEF 0.52 ± 0.12 0.53 ± 0.02 0.5 8 ± 0.11 0.62 ± 0.29 0.53 ± 0.07 0.61 ± 0.08 CBV 2.7 ± 0.6 3.2 ± 0.7 3.3 ± 0.7 3.3 ± 0.8 3.1 ± 0.4 2.4 ± 0.5 CBF/CBV 14.6 ± 4.9 13.2 ± 3.7 12.4 ± 2.4 11.6 ± 3.8 12.5 ± 2.8 15.1 ± 3.0

ROI based on infarct CBF 37.7 ± 9.1 4 1.2 ± 2.1 41.2 ± 3.8 38.6 ± 3.9 34.9 ± 1.2 33.0 ± 6.6 CMR02 3.44 ± 1.34 3.54 ± 0.43 3.53 ± 1.02 3.49 ± 0.8 8 2.54 ± 0.54 3.05 ± 0.89 OEF 0.53 ± 0.10 0.56 ± 0.04 0.57 ± 0.13 0.57 ± 0.26 0.53 ± 0.05 0.61 ± 0.09 CBV 2.5 ± 0.3 2.8 ± 0.6 2.8 ± 0.4 2.9 ± 0.5 2.7 ±O.I 2.1 ±0.3 CBF/CBV 15.4 ± 4.3 15.1 ± 3.1 15.0 ± 1.8 13.9 ± 2.7 12.8 ± 1.1 16.2 ± 4. 1

See Table 3 for abbreviations. CMR02 and CBF are expressed in ml 100 g-' min-', CBV in ml 100 ml-', and CBF/CBV ratio as min-'; OEF is dimensionless. Mean ± SD values

are given, n = 4 baboons. No significant difference for any time.

J Cereb Blood Flow Metab, Vol. 16, No.6, 1996

TRANSIENT MeAO IN ANESTHETIZED BABOONS

FIG. 4. a: Evolution of changes in CBF before and after 6-h, right middle cere­bral artery occlusion (MCAO) in the anesthetized baboon (no. 10). The im­ages are presented with respect to the pseudocolor scale shown on the right for pixel values between 0 and 60 ml 100 g-' min-', and the axial cuts were taken parallel to and +21 mm above the can­thomeatal plane. The control study was performed at least 2 weeks prior to the occlusion and other positron emission to­mography measurements made during and after MCAO (see Fig. 1 for timing protocol) . The arrow shows a reduction in CBF over the whole of the MCA territory in the MCAO + 1 h image with some spontaneous reperfusion (REP) occur­ring in the cortex even while the micro­vascular clips were still in place (MCAO + 4 h) . For this baboon, magnetic reso­nance imaging procedures revealed the presence of infarction only in the right basal ganglia region while the cortex re­mained unaffected. This area (indicated by an arrow on the MCAO + 21 day im­age) exhibited low flow at all times except in the early postreperfusion phase. b: Evolution of changes in CMR02 before and after 6-h, right MCAO in the anes­thetized baboon. Pixel values range from o to 5.0 ml 100 g-' min-'. At MCAO + 1 h and + 4 h, cortical values for oxygen metabolism remained unchanged (ar­rows) , while the deep MCA territory showed a significant reduction in CMR02 levels when compared with the contralat­eral side. Clip removal failed to restore this hypometabolic region to normal val­ues. At MCAO + 24 h, the value in the ischemic core (arrow) was 1.31 ml 100 g-' min-'. See (a) for further details. c: Evolution of changes in the oxygen ex­traction fraction (OEF) before and after 6-h, right MCAO in the anesthetized ba­boon. Pixel values range from 0 to 1.0 (dimensionless) . Note the area of in­creased OEF (arrow) at MCAO + 1 h, a phenomenon that was evident in all ba­boons studied and persisted until the ar­tery was reperfused. Head of arrow cor­responds to the region that showed ulti­mate infarction at MCAO + 21 days. See (a) for further details.

lI83

(A)

(8)

(C)

] Cereb Blood Flow Metab. Vol. 16. No.6, 1996

1184 A. R. YOUNG ET AL.

TABLE S. Mean asymmetry index values expressed as ipsilaterallcontralateral side ratio obtained before and after 6 h of temporary MeAD in anesthetized baboons-Group I

Condition! parameter Pre-MCAO

ROI based on high OEF CBF 0.98±0.10 CMR02 0.99 ± 0.05 OEF 1.01 ± 0.06 CBV 1.02 ± 0.05 CBF/CBV 0.95 ± 0.07

ROJ based on infarct CBF CMR02 OEF CBV CBF/CBV

1.01±0.12 1.00 ± 0.07 0.99 ± 0.06 1.06 ± 0.14 LOS ± 0.14

See Table 3 for abbreviations.

MCAO +1 h

0.71 ± O.IS,,·d 1.01 ±0.15" 1.46 ± 0.20"" 1.20 ± 0.16 0.59 ± 0.14"

0.60 ± 0.16" 0.67 ± 0.10" 1.16 ± 0.20 1.07 ± 0.08 0.59 ± O.IS"

Mean ± SD values are given, n = 4 baboons.

MCAO +4 h

0.74 ± 0.13a., 0.S9±0.IY 1.28 ± 0.11"" 1.14 ± 0.05"" 0.66 ± 0.13"

0.60 ± 0.18" 0.59 ± 0.13" 1.00 ± 0.13 1.03 ± 0.05 0.59 ± 0.17"

"p < 0.05, bp < 0.01, relative to control pre-MCAO values, by Student's paired t test.

Reperfusion MCAO MCAO +1 h +1-2 days >15 days

1.06 ± 0.06 0.94 ± 0.10' 0.75 ± 0.06b., 0.84 ± 0.14<1 0.80 ± 0.14" 0.79 ± 0.09"'<' 0.80 ± 0.14' 0.85 ± 0.14 1.04 ± 0.04" 1.09 ± 0.13 1.06 ± 0.20 0.S7 ± 0.04h 0.99±0.IS 0.90 ± 0.10 0.S7 ± O.OS

0.S9 ± 0.16 0.61 ± 0.1 zi' 0.66 ± 0.04" 0.60 ± 0.13" 0.40 ± O.OSb 0.54 ± 0.07" 0.66 ± 0.12" 0.66 ± 0.08h 0.83 ± 0.06" 1.04 ± 0.11 0.65 ± 0.17 0.82 ± 0.09" 0.84 ± 0.09" 0.83 ± 0.10" 0.81 ± 0.10"

c p < O.OS, dp < 0.01, "p < 0.001 by Student's paired t test comparing the highcst-OEF and infarct ROIs as a function of time.

ROI but was consistently reduced in the ROI that

evolved toward infarction. In terms of the rOEF values,

significant differences between the two regions existed at

MCAO + 1 h and MCAO + 4 h where the rOEF was

more elevated in the highest-OEF ROI than in the infarct

ROL After reperfusion, the OEF returned to normal in

the highest-OEF ROI, while in the infarct ROI, it dete­

riorated considerably in the acute stage and then returned to normal, although still at significantly lower values

than in the highest-OEF ROl. At MCAO + 1 h, rCBV

was significantly greater in the highest-OEF region when

compared with the infarct ROL The values for the CBFI

CBV ratio changed in parallel in the two regions at all

time intervals studied.

Comparison of the index values between the two ROls

is shown in Table 5. As compared with the infarct ROI,

o

the highest-OEF ROI was characterized by significantly

higher OEF, CBF, and CMR02 indexes at 1 h, 4 h, and

1-2 days after MCAO and higher CBF and CMR02 indexes in the chronic stage.

Group 2 There was no significant ditterence between the physi­

ological and biochemical parameters of the two groups

(data not shown). The profile of changes in the param­

eters measured at MCAO + I h in the highest-OEF ROI

(the only ROI available for analysis) was similar to that

of Group 1. The individual data are illustrated in Fig. 7.

Across the four baboons, the OEF index was signifi­

cantly increased (p < 0.01) and CBF values significantly

reduced (p < 0.05), while the CMR02 was not signifi­

cantly lower than the pre-MCAO levels. The time course

1.8 FIG. 5. Asymmetry ratios for the highest-oxygen extraction fraction (OEF) region of interest (ROI) [corti­

� 1.2 -� �--------- --------

-.,,��� cal middle cerebral artery (MCA) ter­ritory] expressed as the ipsilateral! contralateral mirror image before and after 6 h of temporary MCA occlusion (MCAO) in the anesthetized baboon (Group 1). Data are shown for indi­vidual baboons that had a magnetic resonance imaging-identifiable ne­crotic lesion. The highest-OEF region was obtained b y a computer­generated isocontour (88% of the maximal pixel value) in the 1-h post­

Eo.8 � � 0.4 <

1iI- .•....• �.-,:::."...J.�-.... �/ 1.2

...... --�/ -�--------- - -

0.6 (a) CBF (c) OEF

1.2

0.4 MCAO positron emission tomography

.....

image and subsequently transferred 1i! 0.4 to the other measured parameters i CMR02 and for each time interval. a: CBF; b: � O+----..---.----r---r---r----.

(b) (d) CBF/CBV

CMR02; c: OEF; d: CBF!cerebral .e­blood volume (CBV; reciprocal mean transit time). (e), baboon no. 3; (.), baboon no. 6; (0), baboon no. 8; (D),

PRE- MCAO MCAO REP MCAO MCAO MCAO + I h + 4 h + I h + 1-2 D > 15 J)

O�----..----�---r----.---,----, PRE- MCAO MCAO REP MCAO MCAO

MCAO + I h + 4 h + I h + 1-2 D > 15 D

baboon no. 10. REP + 1 h represents the reperfusion period at 1 h following removal of the microvascular clips (i.e., MCAO + 7 h). The 95% confidence limits for the pre-MCAO values are represented by the broken line. See Fig. 3 for an example of the topography of the ROls chosen.

J Cereb Blood Flow Metab, Vol. 16. No.6, 1996

TRANSIENT MeAO IN ANESTHETIZED BABOONS 1185

c � 1.2 � 1. --c� ::-=c=� .s0.8 FIG. 6. Asymmetry index ratios for the infarct region of interest (ROI) [deep middle cerebral artery (MCA) territory] expressed as the ipsilateral/

10.4

< (a) CBF 0.4 (c) OEF 04----r--�----r---�--_.--_,

contralateral mirror side image before ea 0 +----,-----,...----.----,----,...---, and after 6 h of temporary MCA oc- � elusion (MCAO) in the anesthetized � 1.2 baboon (Group 1). The infarct ROI "E �-------------------

1.2 �-------------------

was defined by an irregular contour c ... .. based upon the hyperinte'lse mag- U 0.8 --''''' '�:��'.��--------netic resonance imaging T 2 signal .....

�� (see legend to Fig. 5 for further details ea 0.4 and see also Fig. 3 for the topographi- � (b)

0.8

0.4

cal location of the infarcts). � CMR02 � 04---'-�""--�--'-��� (d) CBF/CBV

04---,---,...---,---.--,...---, c.. - PRE- MCAO MCAO REP MCAO MCAO

MCAO + I h + 4 h + I h + 1-2 j) > 15 j) PRE- MCAO MCAO REP MCAO MCAO

MCAO + 1 h + 4 h + I h + 1-2 D > 15 D

of changes in these parameters was also comparable in

both groups (see Fig. 6), with the notable exception that

there were no significant changes in the value of CMR02 at any time point studied, including the chronic stage, in

this "control" group (see Fig. 7).

DISCUSSION

The main message from this study is that the area with

maximally increased OEF observed I h after MCAO is

both topographically and physiologically distinct from

that region that will undergo consolidated infarction, if

reperfusion is allowed 5 h later. Thus, the highest-OEF

area is not inevitably bound to, but may only be "at

risk" for, consolidated infarction-the most important

determinant of clinical outcome after MCA territory

stroke (Brott et ai., 1989).

Q � � 1.2

.s0.8 � �0.4

<

i---� ��.�� ----- �

(a) CBF ea O�--.--.--.---,---.---, � �

'a 1.2 � 8 0.8 :=.=:=��

(b) CMR02

2

1.5

0.5 (c) 0

1.2

0.8

0.4

(d) 0

Topographically, the infarcted area corresponded to

the deep MCA territory (i.e., the striatocapsular area),

whose increased vulnerability is well known (Crowell et

ai., 1981; Plets, 1981; Marinkovic et aI., 1985; Meier­

Ruge et aI., 1992) and the highest-OEF area to the MCA

cortical territory (see Fig. 3). This observation was based

on comparison of coregistered PET and chronic MRI,

and we confirm here, histologically, the validity of as­

sessing consolidated infarction (if of sufficient size) with

chronic-stage MRI.

We documented the effects of MCAO based on side­

to-side ratios because no significant contralateral alter­

ations in any of the measured parameters were noted at

any time (except for a late change in CBV). The absence

of contralateral cerebral metabolic depression is in agree­

ment with the reports of Tenjin et al. (1992) and Pappata

et ai. (1993) with respect to early-stage MeA territory

OEF

CBF/CBV

FIG. 7. Asymmetry ratios for the highest-oxygen extraction fraction (OEF) region of interest [cortical middle cerebral artery (MCA) terri­tory] shown for individual baboons that had no readily demonstrable changes in the magnetic resonance imaging procedure (Group 2). (see Fig. 5 for further details) (e), baboon no. 1; (_), baboon no. 2; (0), baboon no. 7; (D), baboon no. 11. Note that the CMR02 values remain essentially stable throughout the entire study (el. Fig. 5b).

PRE- MCAO MCAO REP MCAO MCAO MCAO + 1 h + 4 h + 1 h + 1-2 D > 15 j)

PRE- MCAO MCAO REP MCAO MCAO MCAO + I h + 4 h + I h + 1-2 j) > 15 j)

J Cereb Blood Flow Metab, VoL 16, No.6, 1996

1186 A. R. YOUNG ET AL.

infarction, while a lack of delayed metabolic changes l as

proposed by Andrews (1991)] might be explained by the

relatively small infarcts obtained in the present study.

Although the early rOEF was maximal in the cortical

mantle and significantly higher there than in the deep

MCA territory (see Tables 3 and 5), it nevertheless ex­

hibited an overall trend (significant in one baboon) for a

transient increase in the latter region I h after MCAO.

Possibly, in the infarct-to-be area, there was, very early

in the course of postocclusion events, a severe and pro­

gressive metabolic deterioration, driving the rOEF back

toward normal values. This interpretation would concur

with the baboon studies of Pappata et al. ( 1993), though

tissue outcome was not evaluated in that study. Our finding that the highest-OEF area escaped infarc­

tion (provided reperfusion took place) is in apparent con­

flict with the Wise et al. (1983) study in which it was

concluded that an initially high rOEF always predicted

poor tissue outcome (as assessed by low rOEF and low

rCMR02 in the subacute stage). However, in none of

their patients did these investigators document the exis­

tence of reperfusion. Other clinical and experimental in­

vestigations are more consistent with our present find­

ings. Baron et al. (1987) and Powers et aI. (1985) each

reported anecdotally on one patient in whom areas with

extremely high rOEF escaped infarction at follow-up. In cats subjected to permanent MCAO, Heiss et aI. (1994) also mentioned one atypical animal in which the area

with initially increased OEF did not deteriorate metaboli­

cally at follow-up.

We found that despite efficient reperfusion in both the

highest-OEF and the ultimately infarcted areas, their out­

come was radically different. This suggests that, regard­

ing the latter, either reperfusion did not prevent further

deterioration in an already irreversibly damaged tissue,

or it actually induced additional tissue injury that led to

irreversible lesions. However, because this latter sce­

nario was not observed in the highest-OEF area, reper­

fusion injury would not appear to be universal (if it exists

at all), and quite to the contrary, reperfusion may well

have salvaged the area of highest-OEF tissue (Young et

aI. , 1995). The fall in the rOEF after reperfusion does not

appear to be predictive of tissue outcome since it was

observed in both types of tissues, but rather reflects the

changing balance between prevailing CBF and CMR02.

After the first postreperfusion study, the rCBF in the

eventually infarcted area exhibited a secondary reduc­

tion, together with a further decline in rCMR02, while

both variables remained essentially stable in the highest­

OEF area. This phenomenon of ' 'delayed hypoperfusion"

in partially reperfused but irreversibly injured tissue has

been reported before (Michenfelder and Milde, 1990), but its interpretation has been debated. Although it could reflect some deleterious after-effects of reperfusion, the fact that the rCMR02 was already significantly reduced

J Cereb Blood Flow Metab. Vol. 16. No.6. 1996

before reperfusion set in would suggest, to the contrary,

that following reperfusion there occurs a progressive re­

coupling of perfusion with the markedly lowered meta­

bolic needs. A similar situation has been noted previ­ously in macaque monkeys with temporary MCAO

where Crowell and Olsson (1972) showed a lack of car­

bon black filling of the vessels in the core of the infarct

despite a recanalized MCA. It is possible that in our

study the use of heparin may have contributed to im­

proved microcirculation by restricting capillary­

obstructing polymorphonuclear leukocytes, thereby lim­

iting a no-reflow phenomenon, and have thus resulted in

small infarcts (del Zoppo et aI. , 1991; Okada et aI.,

1994).

In the highest-OEF region, the average CBF remained above the penumbral range and the average CMR02 was

not significantly reduced at any time during the occlu­

sion. This profile, which confirms with the definition of

"oligemia" rather than true "ischemia" (Lassen, 1990;

Baron, 1991), would be compatible with the consistently

good tissue outcome. This moderate reduction in perfu­

sion in the cortical area presumably reflects an efficient

collateral supply-although our "snapshot" measure­

ment may have missed initially more pronounced hy­

poperfusion. In contrast, the ultimately infarcted area ini­

tially exhibited significantly lower rCBF values at all times after occlusion than the highest-OEF area and the

rCMR02 was likewise significantly reduced (Tables 3

and 5). Nevertheless, at the individual level (Fig. 5b),

significant hypometabolism did develop in the highest­

OEF area (MCAO + 4 h) in two of our baboons. This

finding is consistent with the work of Pappata et aI.

( 1993), who also reported progressive metabolic deterio­

ration in two of five baboons in the highest-OEF area.

Thus, because oligemic tissue is hypoxic, it may dete­

riorate over time and reperfusion may have saved it fur­

ther deterioration and possibly even infarction.

The initial reduction in rCBF in the infarct-to-be re­

gion was on average only -40% relative to the preocclu­

sion measurement with an absolute value around 25 ml

100 g- l min- 1 (Table 3; Fig. 6a). This value is higher

than would be expected based on previously established

thresholds for infarction (Astrup et aI., 1981; Jones et aI.,

1981; Lassen, 1990), but it may have been overestimated

due to the small infarct size relative to the spatial reso­lution of the PET camera.

The coupled reduction in both rCBF and rCMR02 that

affects the highest-OEF area in the chronic stage might

be explained by two different, though possibly associ­

ated, phenomena: "incomplete infarction, " unnoticed on

chronic-stage MRI scans (We iller et aI. , 1993; Garcia et

aI., 1995; Nakagawara et aI. , 1995); and a diaschisis

phenomenon as a result of a deep-seated infarct (Feeney

and Baron, 1986; Baron, 1989; Sette et aI., 1993). There is some evidence that selective, partial neuronal loss may

TRANSIENT MeA 0 IN ANESTHETIZED BABOONS 1187

affect penumbral areas in the cat (Mies et aI., 1983;

Strong et aI., 1983). However, studies on human post­

mortem material indicate neuronal loss only exception­

ally extends more than a few millimeters from the bor­ders of the established infarct (Nedergaard et aI., 1986;

Torvik and Svindland, 1986; Nedergaard, 1988). Fur­

thermore, the lack of significant CMR02 reduction in the

highest-OEF area of the baboons in Group 2 (Fig. 7)

would speak strongly in favor of the "diaschisis" mechanism, as the selective neuronal loss mechanism, if

any, should have affected both groups in the same man­

ner.

In conclusion, following MCAO, the region that

shows an early maximal increase in the OEF is both topographically and physiologically distinct from the re­

gion with final consolidated infarction if reperfusion is allowed at 6 h. This high OEF, although indicative of a

threatened condition, is not an indicator of inescapable

consolidated infarction and is thus a situation in which

therapy could be envisaged. Whether or not it is at risk of

infarction and thus constitutes one target for therapy re­

mains to be seen.

Acknowledgment: We thank Mr. C. Le Poec and Mr. P. Lochon of the Cyclotron Unit at Cyceron and Ms. N. Ravenel and Mr. V. Beaudouin of the Computer Department. A special mention to Mr. G. Huguet for his assistance with the care of the animals and to Ms. A. Brocquehaye for her technical expertise. We are also most grateful to the radiographers of the University Hospital of Caen, in particular Mr. Dominique Luet, for assis­tance with the MRI scanning procedures.

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