local cerebral blood flow following transient cerebral...

534 Local Cerebral Blood Flow FollowingTransient Cerebral Ischemia

I. Onset of Impaired Reperfusion within the First Hour FollowingGlobal Ischemia

CALVIN L. MILLER, P H . D . , DOUGLAS G. LAMPARD, P H . D . ,

KARL ALEXANDER, M.B., AND WILLIAM A. BROWN, P H . D .

SUMMARY Using the hydrogen clearance technique, local cerebral blood flow (LCBF) in 22 dogs was es-timated at 6 parietal sites prior to and following 5 min of total global ischemia. Ischemia was immediatelyfollowed by an initial reactive hyperemia during which the electrocorticogram (ECoG) usually began torecover, and within the first 30 min, most of the LCBPs decreased to subnormal values. This onset of bypo-perfusion was accompanied by a concomitant decrease in ECoG activity. Two animals that maintained normallocal perfusion after the initial hyperemia recovered ECoG activity quickly. These results suggest that the sub-sequent poor reperfusion was caused by an increased microvascular resistance rather than by blood aggregates,increased blood viscosity, or a variety of other mechanisms which have been proposed. Increased vascular tonuswas, at least, partly responsible for the increased vascular resistance. This report supports the hypothesis thatimpaired reperfusion (which occurs some time after an initial hyperemia) may be responsible for ultimateneuronal death, rather than the period of global ischemic hypoxia per se.

Stroke, Vol 11, No 5, 1980

SEVERAL REPORTS have indicated that thevulnerability of the central nervous system to briefperiods of ischemic hypoxia may be due to impairedrecirculation, rather than to immediate cell death.Hossmann, Lechtape-Gruter, and Hossmann1

demonstrated the return of neuronal functions (EEG,pyramidal response, evoked potentials) in cats after 30or 60 minutes of complete cerebral ischemia.Hossmann and Kleihues2 concluded that normother-mic nerve cells can withstand 60 min of completeischemia and that revival of nerve cells is hindered(presumably) by deficits in recirculation. Additionalsupport to this hypothesis has been given by Yatsu,Lee, and Liao3 and others4'6 who have shown thatcerebral energy metabolism is not always irreversiblydamaged and is not primarily vulnerable to ischemia.

The cause of impaired cerebral microcirculationfollowing ischemia has been attributed to many fac-tors. Ames and co-workers8 concluded that the "no-reflow phenomenon" was due to increased bloodviscosity and endothelial and perivascular glial cellswelling. Fischer and Ames7'' noted that perfusiondeficits appeared in a scattered pattern and did notappear to be caused by platelet microemboli or pialarterial spasm. Chiang et al." implicated swollen endo-thelial "blebs" which they claimed obstructed thecapillary lumen. Little, Kerr and Sundt10 related im-paired microcirculation to the compression ofcapillaries by perivascular glial swelling, but alsonoted that severe neuronal injury preceded the micro-vascular obstruction. Conversely, Cuypers andMatakas11 concluded that no-reflow was produced byblood aggregates which obstructed larger vessels

From the Department of Electrical Engineering, Monash Univer-sity, Clayton, Victoria, Australia, 3168.

Dr. Alexander is from the Intensive Care Unit, Royal Children'sHospital, Parkville, Victoria, Australia, 3053.

rather than capillaries. Wade et al.12 proposed that theno-reflow state is due to an increase in potassium con-centration in brain extracellular fluid which causes apronounced contraction of vascular smooth muscle.Klatzo13 concluded that arterial spasm was responsi-ble for the no-reflow phenomenon. Hekmatpanah14

demonstrated capillary microthrombi and red cellaggregates during circulatory standstill. Micro-circulatory blockage has also been attributed to con-sumption coagulopathy18 and to an "ischemic tissue-blood interface" reaction that reduces blood flow byan unspecified mechanism."

Some investigations have shown that cerebralischemia may be initially followed by hyperemia, butwith a subsequent hypoperfusion within one hour.1' 17

During the initial post-ischemic hyperemia, thecerebral mean requirement of oxygen (CMROi) maybe temporarily reduced, but the CMRO2 may subse-quently increase beyond control values.18 Perfusiondeficits, accompanied by an elevated CMROi, resultin a prolonged cerebral hypoxia that may contributeto post-ischemic cerebral pathogenesis.19 The etiologyof perfusion deficits has still not been resolved whichemphasizes the need to investigate post-ischemiccerebral circulation so that methods of restoring ade-quate blood flow may be devised.

The purpose of this investigation was to closelyfollow the changes in local cerebral blood flow aftertotal ischemia and to determine whether there was anyevidence to support (or reject) the hypothesis thatcerebral recovery may be hindered by poor reperfu-sion rather than by immediate death of hypoxic nervecells.

Methods

Anesthesia was induced in adult, healthy mongreldogs, 15 to 25 kg, with intravenous thiopentonesodium (Intraval, May & Baker) and maintained with

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CBF AFTER TIA. I. IMPAIRED REPERFUSION/A/iV/er et al. 535

intravenous pentobarbitone sodium (Sagatal, May &Baker) on demand. Following endotracheal intubationand insertion of femoral arterial and venous cathetersfor pressure monitoring and blood sampling, artificialventilation was maintained with air and oxygen (40%Oj), 20 breaths per minute, at a tidal volume whichkept the arterial Pco, at about the normal canine levelof 38 mm Hg;10 this Paco, produced an end-expiredCO, (FEco2) typically between 4.5% and 5% asmeasured by a Godart Capnograph. A RadiometerBMS Mk2 blood microsystem was used for serialmeasurement of arterial pH and blood gases. Bodytemperature was maintained at 38°C and was con-tinuously monitored using a rectal thermistor. Theventilation system was computer controlled asdescribed by Lampard, Coles, and Brown."

A Cardiovascular Instrument cardiac output com-puter, which employs the thermodilution technique,was used for cardiac output estimates. A thermistorwas firmly fixed in the pulmonary artery with a pursestring suture; bleeding from the artery was slight andtransient.

Local Cerebral Blood Flow Measurement

The hydrogen clearance technique"' M was used forestimating local cerebral blood flow. Hydrogenclearance curves were essentially monoexponentialwith a small second order effect. Only the first ordertime constant was calculated. From the first order ratefunction, A0"

kt, k was calculated from the mono-exponential segment by dividing the half-time (time inseconds taken for the function to decay to half its ini-tial value, Ao) by 0.693; multiplication of k by 60seconds/minute and 100 gm tissue yielded the localcerebral blood flow in units of ml blood/minute/100gm tissue.

Platinum wire of 0.3 mm was silver soldered to 1mm Belling-Lee connectors and insulated with Epoxy-lite 6001 thermosetting epoxy. Insulation (0.5 mm)was removed from the electrode tip. This insulationtechnique yields an electrode which is impermeable tophysiological fluids or saline for at least 24 hours. Ab-solute insulation of the electrode is a prerequisite inobtaining inter-electrode independence and smooth,noiseless graphs. If the electrode resistance droppedbelow 20 megaohms, considerable "cross-talk" oc-curred among electrodes, which resulted in unstablebaselines, and it became difficult to properly bias theelectrodes to +300 millivolts. Adequate electrode in-sulation against ultimate leakage is a commonly en-countered problem, especially in chronic preparations,as more fully discussed by Donaldson."

Following reflection of the frontal and temporalmuscles along the external sagittal crest, the parietalbone was perforated lateral to the sagittal sinus with aNo. 4 dental burr, care being exercised to avoid abra-sion or laceration of the dura. In the event of bleedingfrom the bone the hole was stoppered with bone wax;otherwise, bleeding was slight or unnoticeable.Because of extreme differences among mongrel skulls,it was not possible to stereotaxically implant the elec-

trodes. However, electrodes were standardly placed inparietal lateral, ectolateral, and middle suprasylvianareas at depths ranging from 1 to 8 mm. A rigidmicromanipulator was used to prevent electrodemovement and minimize tissue trauma while the elec-trodes were fixed to the skull with Surgical Simplexbone cement. Consistent blood flow measurementsfrom acutely implanted platinum electrodes dependson the immobilization of the electrode after insertionand on the absence of subdural bleeding. An electrodedepth of at least 1 mm was necessary to avoid inac-curate flow estimates resulting from the rapid diffu-sion of hydrogen from the surface of the cortex."Local cerebral blood flow estimates were measuredfrom 5 or (usually) 6 electrode sites in each dog.

Tissue damage during electrode insertion maypreclude reliable washout curves until healing has oc-curred,"- " but a series of experiments in thislaboratory showed LCBF estimates to be constant un-der controlled conditions."

Stainless steel Sherman bone screws were locatedparieto-occipitally as the reference electrodes for theplatinum electrodes. The epidural electrocorticogram(ECoG) was measured from bilateral Sherman bonescrews inserted into frontal bone. Bone screw depthwas carefully matched to skull thickness to avoid com-pression of the cortex with the screw tip. Epidural in-tracranial pressure (ICP) was measured using astainless steel adapter inserted into parietal bone.Care was taken to ensure that there was no bleedingfrom the bone that would clot the lumen of the ICPadapter.

Mean arterial pressure (MAP), systolic pressure,diastolic pressure, pulse rate, FEco2, I.C.P., rectaltemperature, and the mean rectified voltage (MRV) ofthe ECoG were recorded continuously on a Brush 8channel recorder.

Technique Tor Producing Total Cerebral Ischemia

Following a left thoracotomy and removal of thefourth rib, the brachiocephalic and left subclavianarteries were exposed and cotton thread was passedunder each artery near its origin. A length of Silastictubing (3 mm o.d.) was passed twice around thedescending aorta to form a loop. This loop allowedcomplete constriction of the aorta when the ends ofthe Silastic tubing were drawn. Dissection around theheart and ascending arteries was cautiously performedto avoid any trauma to the vagosympathetic trunk,cardiovagal branches and other nerves in the area.

Because of the collateral circulation to the dogbrain arising from the descending aorta, completecerebral ischemia cannot be achieved by clampingonly the brachiocephalic and left subclavian arteries.However, if the descending aorta is also occluded, it isnecessary to provide a pressure vent (or "shunt") forthe left ventricle to prevent blood congestion in theheart and lungs. To provide this shunt, a catheter waspassed from the femoral artery into the aortic arch.During the ischemia, this catheter allowed diversion of


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536 STROKE VOL 11, No 5, SEPTEMBER-OCTOBER 1980

arterial blood to a one liter Viaflex bag (TravenolLabs).

To produce the total cerebral ischemia, the brachio-cephalic and left subclavian arteries were clamped attheir origin. Assuming 86 ml blood per kg bodyweight," heparin was injected intravenously at a doseof one unit per ml blood. Immediately after theheparin injection, the intra-aortic catheter was con-nected to a one-liter plastic blood collection bag(Travenol Viaflex) and the descending aorta was con-stricted around the intra-aortic catheter by drawingthe ends of the Silastic loop. Blood returned from theblood collection reservoir via the femoral vein. It isimportant that the catheters have the widest possiblelumen so that blood can flow rapidly and freely intoand out of the blood bag.

The ECoG became flat (isoelectric) typically within30 seconds and this flattening served as the startingpoint of the 5 minute period of ischemia. At the end of5 minutes of isoelectric ECoG, the constricturearound the descending aorta was removed, the clampswere removed from the ascending arteries, and theintra-aortic catheter was withdrawn. Arterial Pco3quickly returned to pre-ischemia levels (36-40 mmHg), but arterial pH was predictably low. Arterial pHwas corrected by intravenous infusion of 4.2% sodiumbicarbonate at 1 ml/minute until pHa was within anormal range (7.36 to 7.44); usually, this requiredabout 60 to 80 ml of 4.2% bicarbonate. Bicarbonatewas not infused until the local blood flows weredemonstrated to be reduced below control values.Simultaneous LCBF estimates were recorded from 6electrode sites in each dog, except for 3 animals whichhad 5 electrode sites. This resulted in a total of 117electrode sites for the 20 dogs used in these ex-periments.

Results

The mean LCBF was 43.0 ml/min/100 gm tissue,with a standard error of ±4.8 ml/min/100 gm and arange of 19 to 198 ml/min/100 gm. The wide rangeand relatively large standard error emphasize theheterogeneity of flow around cortical and subcorticalelectrode sites. For ease in comparative analysis eachpost-ischemic LCBF is expressed as a percentage of itspre-ischemic control value of 100%. Figure 1 showsthe local cerebral blood flow from 6 electrode sites inone representative animal following the 5 minuteperiod of ischemia; the changes indicated are typicallyseen after the ischemia. Most of the local flows wereinitially greatly increased, although occasionally someelectrode sites demonstrated an initial decrease belowcontrol flow. It is apparent that the initial flow in-crease is followed by a decrease which results in mostof the LCBF's falling substantially below the control.

All LCBF estimates for the first hour followingischemia are shown in figure 2. Except for 9 sites, allof the electrode sites recorded an increase in LCBFafter ischemia. For these 9 sites, it is not possible todetermine whether the flows were initially increasedand then began to diminish, or whether the flows were

6001

LOCAL CBF FOLLOWING

5 MINUTE S ISCHEMIA

21 JULY 78NORMAL PaC02

FIGURE I. Changes in local CBF at 6 electrode sites forone representative dog. Each symbol represents an electrodesite. PacOt was normal within 5 minutes after the ischemiaand the pH, was corrected to 7.4 after one hour. Note thatmost flows remain subnormal during subsequent hours.

initially decreased below control when cranial circula-tion was restored. After the ischemic episode, it tookat least 5 minutes to reposition the animal, rebias theelectrodes and resume estimates of LCBF's.

After the initial hyperemia, blood flows appeared todecrease relatively gradually (fig. 2), but occasionallya precipitous drop in blood flow was noted. This wasexemplified by a sudden decrease in the wash-out ofhydrogen and the introduction of a significant secondorder component in the decay curve (fig. 3). This acutedecrease in flow is a difficult event to capture using agas clearance method. In one animal, there was asimultaneous acute decrease in flow at 3 of 6 electrodesites. This was accompanied by a marked decrease inthe ECoG mean rectified voltage.

Intracranial pressure (ICP) was never elevated dur-ing the period of flow decrease. During the ischemia,ICP always decreased, indicating the partial emptyingof blood from the cerebral venules and a reduction incerebral blood volume. Following restoration of thecirculation, ICP usually increased sharply (not morethan 40 mm Hg) and then recovered to pre-ischemiclevels of about 3 mm Hg within 10 minutes (fig. 4). Insubsequent hours, the ICP never showed a tendency toincrease.


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CBF AFTER TIA. I. IMPAIRED REPERFUSI0N/A/;7/er et al. 537

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After restoration of circulation, end-expired CO2was elevated, but returned to normal within 5 to 10minutes. The post-ischemic arterial pH was between7.25 and 7.3, and was not corrected with sodium bicar-bonate during the first hour post-ischemia. It wasbelieved best to avoid the complication of introducinganother variable to the many physiological alterationsthat were happening during and immediately after theischemia. Bicarbonate may have varying effectsdepending on the rate of intravenous infusion and therate of change of the cerebrospinal fluid pH. Also, itwas important to initially observe the behavior of thepost-ischemic brain blood flow without interferencefrom other therapies or drugs. The only resuscitative

(go FIGURE 2. Line graph of all LCBF estimatesfrom all animals showing the progression from

| high initial flows to severely depressed flowsWO within the first hour.

action consisted of adequate ventilation and restora-tion of blood pressure and cardiac output. When pHawas corrected (typically 1 to 1.5 hours after ischemia),sodium bicarbonate (4.2%) was infused at a rate notexceeding 0.05 ml/min/kg. The correction of pHa didnot cause any significant increase or decrease inLCBF.

Cardiac output (CO) was not measured in allanimals but measurements in 3 animals showed an im-mediate post-ischemic increase in CO that returned tonearly normal within the first hour. In most animals,the increase in cardiac output was accompanied by atransient hypertension (150 to 190 mm Hg systolicBP) that decreased to no less than 120 mm Hg systolic

ACUTE DECREASE OFPOST-ISCHEMIC LOCAL CBF

FIRST ORDER DECAY: 92-5 ml/min/lOOgm

SECOND ORDER DECAY: I6'5 ml/min/lOOgm

10 MINUTES /5 20

FIGURE 3. Sudden introduction of second-order component in the hydrogen wash-out curve,implying an acute decrease in blood flow at that electrode site.


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FIGURE 4. Slight recovery of the ECoG during hyperemia,and the subsequent decrease in ECoG following the decreasein LCBFs.

BP. The difference between systolic and diastolicpressures was at least 30 mm Hg. Heart rate was nor-mal or slightly increased. These cardiovascularmeasurements demonstrated that reduced LCBFswere not due to cardiovascular impairment. This alsoemphasizes that the technique for producing theischemia preserves heart function. It is important tohave a healthy heart after the ischemia so that changesin LCBF can be confidently ascribed to the cerebralvasculature rather than to a failing heart and a milieuof supportive drugs.

The electrocorticogram (ECoG) is a sensitive in-dicator of the decrease of LCBFs following the initialhyperemia. During the initial post-ischemic hyper-emia, the ECoG mean rectified voltage (MRV) beganto gradually recover, but then there was a subsequentdecrease concomitant with the reduction in LCBFs(fig. 4). This effect was noted in 14 of the 20 animals.In the remaining 6 dogs, the ECoG MRV remainednearly isoelectric during the hyperemia. Even thoughthe local flows remained depressed, most of the

animals gradually recovered control levels of ECoGMRV within 8 hours following ischemia. Recovery ofelectrocorticographic activity was defined as thereturn of ECoG MRV to approximately 100% of thecontrol (pre-ischemic) levels. In 2 animals, the ECoGMRV recovered quickly to control levels and theLCBFs did not decrease below control after the initialhyperemia. These 2 dogs were apparently not asvulnerable to ischemia as the other 20 dogs and,therefore, have not been included in figure 2.

During ischemia, the pupils were fully dilated.When circulation was re-established, the pupilsrecovered normal diameter (about 2 mm) within 10minutes. Mild and transient convulsions and tremorswere observed during and after the ischemia; theseevents typically subsided within the subsequent hour.Because of the ischemic insult, the animals did notrequire any further anesthetic. The animals hadtypically been anesthetized for 6 to 8 hours prior toischemia.

Discussion

Five minutes of ischemia was chosen as this periodis reported to be the threshold for irreversible braindamage in animals and man.10"" Other studies haveshown that animals may survive 12 to 15 minutes ofischemia"" and have an apparent clinical recoveryand normal EEG, but these periods are long in com-parison to the shorter periods compatible with perma-nent damage. Although several studies have shownthat energy metabolism returns quickly afterischemia,4'8 resumption of cerebral metabolism doesnot imply recovery of cerebral function." In animals itis difficult to assess clinical recovery in terms of subtlechanges in fine motor skills, personality, and intellect.

Definite histological evidence of brain damage maybe accompanied by a normal scalp EEG and apparentclinical recovery."'" In the present experiments, wewere interested in the initial changes in cerebral bloodflow that may contribute to neuronal death. Theseresults show that 5 minutes of total cerebral ischemiain dogs caused an initial reactive hyperemia that wassucceeded by a decrease in local cerebral blood flowsto subnormal values. Thirty minutes after ischemia,most of the flows had fallen to subnormal levels (figs.1, 2). Not all the electrode sites recorded an initialhyperemia. Within the first 10 minutes, 9 of 92 elec-trode sites recorded subnormal blood flows (fig. 2). Itis not possible to determine whether these electrodesites were initially hyperemic and then decreasedbefore resumption of recording, or whether flows weredepressed upon restoration of arterial flow.

The ECoG was a sensitive monitor of the hyperemiaand the subsequent decrease in local CBFs. The post-ischemic rise in the ECoG activity (during hyperemia)was followed by a diminution of ECoG activity thatwas concomitant with the LCBF reduction to subnor-mal levels (fig. 4). This decrease in LCBFs was seen in20 of 22 dogs. The remaining 2 animals displayed aninitial hyperemia that was followed by LCBFs return-ing to approximately normal values; the ECoG mean


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CBF AFTER TIA. I. IMPAIRED REPERFUSION/A////er et al. 539

rectified voltage (MRV) recovered quickly to controllevels within 2 hours in these animals.

The decrease in LCBF could be a normal responseto reduced oxygen demand by the brain; a reducedCMROj would be expected to result in lower LCBFs.Hossmann et al.40 correlated CMRO, with post-ischemic recovery in cats. Post-ischemic CMRO2 wasalways reduced within the first hour after ischemia,but then increased to normal levels. If the O2 uptakeremained low, the animals had no electro-physiological recovery but if Oa uptake returned tonormal, animals showed good recovery. Yatsu et al.8

concluded that the energy metabolism was notprimarily vulnerable to ischemia and that themachinery for energy synthesis remained intactfollowing 5 minutes of ischemia. In fact, rather thanmetabolism being depressed after 5 minutes ofischemia, mitochondrial ATP synthesis was greaterthan control. Schutz et al.5 also reported brain mito-chondrial function to be intact after ischemia.Nemoto18 reported that after 16 minutes of ischemiain cats, the CMRO, was not different from controlCMRO, within the first 30 minutes after ischemia;over the subsequent 2 hours, CMRO2 increased by 2-fold.

Catecholamine release during ischemia has beenshown to increase CMRO2, a condition that may leadto a secondary hypoxia if oxygen availability is notproportionately increased.18

Post-ischemic brain hypermetabolism may inciteneuron damage." After global ischemia in dogs,Snyder et al.17 noted a reduced CMRO2 and CBFcoincident with an abnormally low cerebral venousoxygen content, which indicated that a hypoxic stateexisted. They negate "the idea that post-ischemicreduction in CBF is simply a matching of CBF toreduced cerebral metabolic demands (reducedCMRO,)". Lang et al.41 also concluded that post-ischemic reduced CBF is not a normal response toreduced CMRO2; decreased oxygen consumption wasthe result of metabolic derangement and reduced oxy-gen availability.

The present experiments did not measure CMRO2,but we believe that it is unlikely that diminished CBFafter ischemia is a normal consequence of a non-injurious reduced CMROj. Because of the possibilityof normal or elevated O2 demand, reduced CBF mayrepresent a post-ischemic hypoxia that may initiatenerve cell death. The reduced local blood flows (within30 minutes post-ischemia) seen in these experimentsare most likely causing an oxygen debt that is beingreflected by the concomitant decrease in ECoG ac-tivity (fig. 4).

These results indicate that a general "no-reflow"phenomenon was not present following the ischemicinsult, but that gross impairment of local reperfusionoccurred some time after the resumption of arterialflow. This is in contrast to other investigations whichhave reported that vessels develop blockage during (orimmediately subsequent to) the ischemia."-7> '• "• "Hence it appears that the impairment in reperfusionoccurs well after the end of ischemia. The initial rise in

ECoG activity was followed by a decrease that coin-cided with the decrease in local CBF. It is tempting tospeculate that the ECoG would have recoveredquickly if the post-ischemic LCBFs could have beenmaintained at normal or supranormal levels.

The hyperemia may be explained by vasodilationdue to tissue acidosis from lactic acid and carbondioxide, or by a slight increase in extracellularpotassium." Some of the increases in local blood flowwere remarkable. For example, one electrode sitewent from a control flow of 35 ml/min/100 gm to apostischemic flow of 230 ml/min/100 gm. Figure 2shows that 27 flows were initially greater than 400% ofcontrol.

It is unlikely that transient brain swelling associatedwith hyperemia was responsible for a subsequent ar-tifactual decrease of LCBF around each electrode site.In previous experiments from this laboratory," hyper-capnea was never followed by low LCBF when theanimals were returned to normocapnea; similarly,when animals were hyperventilated,28 a return to nor-mocapnea was never associated with LCBF changesfrom previous control values.

Other investigators have reported a reactivehyperemia following either total or focal ischemia.Cuypers and Matakas11 found that sustainedhyperemia (either spontaneous or sympatho-mimetically induced) was beneficial in preventingpost-ischemic no-reflow and intracranial hyperten-sion. Osburne and Halsey44 reported that post-ischemic hyperemia in gerbils was associated with ul-timate clinical and EEG recovery; regional bloodflows demonstrated a reactive hyperemia that laterdeclined to subnormal levels. Hossmann et al.1

reported that post-ischemic hyperemia was a prereq-uisite for the recovery of brain electrical activity; theirresults also demonstrated a transient phase ofhyperemia that was followed by generally subnormalcerebral blood flow." Safar et al." showed thattherapeutic measures aimed at improving the cerebralcirculation resulted in eventual clinical recoveryfollowing 12 minutes of cardiac arrest in dogs. Ourresults suggest, that initial hyperemia may bebeneficial in restoring the ECoG, and only when thehyperemic phase ended, did the ECoG demonstrate anabrupt decrease in activity (fig. 4).

Conversely, Mchedlishvili et al.48 concluded thatpost-ischemic hyperemia is a contributory factor inthe development of brain edema. Heiss et al.47 alsoconcluded that reactive hyperemia in focal (middlecerebral artery occlusion) ischemia appeared to poten-tiate cerebral edema. Other investigations have alsoimplicated hyperemia as a harmful consequence ofischemia4860 but some of these involved prolongedfocal ischemia and/or induced arterial hypertension;therefore, comparisons with transient ischemia andreactive hyperemia (without arterial hypertension)may not be valid and should be interpreted cautiously.When comparing these results against those fromother laboratories, it is important to recognize thedifferences in experimental design and duration ofischemia. While the conclusions drawn by other in-


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vestigators are usually convincing, the methodologiesare often so different as to make comparisons difficult.This is an important consideration and is furtheremphasized by Molinari and Laurent.61

The results of these experiments do not supportsome hypotheses presented by other investigators con-cerning the etiology of "no-reflow." Obstruction ofcapillaries by swollen astroglia,*' " • " bloodaggregates and thrombi,11' M microthrombi," swollenerythrocytes" and increased blood viscosity8 havebeen implicated as major antecedents of "no-reflow."Wade et al.ia reported that high concentration of ex-tracellular potassium and consequent vascular con-striction may account for "no-reflow." However, ifany of these effects were significant contributors to thederangement of local CBF, then it would be expectedthat these would be manifest by initial low flows. Itmay be possible to mount an argument that would ex-plain how one of the above hypotheses may have adelayed effect after a period of hyperemia. In all ourexperiments, ischemia was immediately followed byhyperemia. Even at those sites which displayeddepressed initial flows, LCBFs were between 52% and89% of control (fig. 2). This is an "impaired reflow"rather than a "no-reflow" and implies vascularnarrowing rather than occlusion. Only at later times(30 to 60 minutes) did the LCBFs become so seriouslydecreased as to constitute in some instances a nearly"zero" flow.

It is uncertain whether the hyperemia per se wasresponsible for the subsequent decrease in LCBFs.While some reports have concluded that excess reper-fusion may contribute to brain edema,47 none of theanimals in the present experiments demonstratedraised ICP after the hyperemic phase. Local edema(e.g. perivascular glial swelling) may have developed,but if this were a significant effect, then the ICP wouldbe expected to become elevated because most of theLCBFs were subnormal, and widespread local edema(affecting nearly all the local flows) would be expectedto show itself by raised ICP. Hyperemia may havecaused extravasation, rupture of weak endothelia, orsloughing of intraluminal "blebs." Chiang et al.9 dis-covered diffuse intraluminal "blebs" which were morefrequent 30 minutes after ischemia. However, Fischeret al.56 have reinvestigated the "bleb" hypothesis andhave concluded that "blebs" may have been artifactscaused by an improper histological perfusion-fixationtechnique. Also, Little et al.10 found only minorcapillary changes even after hours of ischemia andconcluded that these changes did not appear to beproducing significant obstruction.

As discussed in Part II, the local vasculature wasstill highly reactive to increased arterial Pco, after theischemia. Of 70 electrode sites, 14 sites did not showan increase in blood flow during hypercarbia; theremaining 56 sites indicated a significant vasodilata-tion during hypercarbia. These increases in LCBF in-dicate that the post-ischemic vasculature was notgenerally paralytic and was not blocked by variouscellular aggregates or fragments. The most likely ex-planation for the reduced blood flows is an increase incerebral vascular resistance, i.e. a reduction in the

caliber of the microvessels. It is not certain whetherthe resistance is increased by delayed perivascularglial swelling" or by a sudden increased vessel tonus."The precipitous reduction in flow at some electrodesites (fig. 3) suggests that increased tonus must be acontributing factor. This sudden flow decrease wouldbe more likely due to smooth muscle contractionrather than to a sudden swelling of many endothelialor perivascular glial cells.

An important consideration in these results is thebarbiturate anesthesia, which has been shown to haveprotective effects in cerebral hypoxia and on ICP."Anesthesia was kept at a relatively light level by givingsmall doses of pentobarbitone only when the eyelidreflex reappeared. Even though pentobarbitone mayreduce the vulnerability of brain cells to hypoxia, 4 to5 minutes of complete global ischemia is sufficient tocause motor and behavioral disabilities or death.80- "• M

Five minutes of total ischemia and isoelectric ECoGwas enough to seriously derange the local CBF's in thepresent experiments.

It is highly unlikely that any heparin would havebeen in the cerebral vasculature during ischemia. Ifthrombi form during ischemia11- "• " heparin may in-hibit thrombogenesis. Heparin was added after thebrachiocephalic and left subclavian arteries wereclamped and immediately prior to bypass from theaorta to the external reservoir. After the ischemia,heparinized blood may be beneficial by preventingsecondary clotting in areas of low flow or by dissolvingthrombi, but others have shown7 that heparin did notappear to improve post-ischemic recirculation.

These results show that relatively short periods ofglobal ischemia (but sufficient to incur permanentdamage) are followed by generally increased LCBFs.This initial hyperemia may be accompanied by in-cipient ECoG activity, but when the LCBFs begin todecrease to subnormal levels, ECoG activity becomesmarkedly depressed. Two animals that maintainednormal LCBFs after the initial hyperemia recoveredECoG activity quickly. Results suggest that subse-quent hypoperfusion was due to microvascularnarrowing and that increased vascular tonus was atleast partly responsible for decrease vessel caliber.This report supports the hypothesis that the brain maynot suffer severe neuronal death during ischemia, butthat recovery may be hindered by microcirculatorydeficits.

Acknowledgment

This investigation was funded by a grant from the NationalHealth and Medical Research Council of Australia.

References1. Hossmann K-A, Lechtape-Gruter H, Hossmann V: The role of

the cerebral blood flow for the recovery of the brain afterprolonged ischemia. Z Neurol 204: 281-299, 1973

2. Hossmann K-A, Kleihues P: Reversibility of ischemic braindamage. Arch Neurol 29: 375-384, 1973

3. Yatsu FM, Lee LW, Liao CL: Energy metabolism during brainischemia: stability during reversible and irreversible damage.Stroke 6: 678-683, 1975

4. Siesjo BK, Ljunggren B: Cerebral energy reserves afterprolonged hypoxia and ischemia. Arch Neurol 29: 400-407,


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C L Miller, D G Lampard, K Alexander and W A Brownreperfusion within the first hour following global ischemia.

Local cerebral blood flow following transient cerebral ischemia. I. Onset of impaired

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