Thresholds of focal cerebral ischemia in awake monkeys

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✓ An awake-primate model has been developed which permits reversible middle cerebral artery (MCA) occlusion during physiological monitoring. This method eliminates the ischemia-modifying effects of anesthesia, and permits correlation of neurological function with cerebral blood flow (CBF) and neuropathology. The model was used to assess the brain's tolerance to focal cerebral ischemia. The MCA was occluded for 15 or 30 minutes, 2 to 3 hours, or permanently. Serial monitoring evaluated neurological function, local CBF (hydrogen clearance), and other physiological parameters (blood pressure, blood gases, and intracranial pressure). After 2 weeks, neuropathological evaluation identified infarcts and their relation to blood flow recording sites.

Middle cerebral artery occlusion usually caused substantial decreases in local CBF. Variable reduction in flow correlated directly with the variable severity of deficit. Release of occlusion at up to 3 hours led to clinical improvement. Pathological examination showed microscopic foci of infarction after 15 to 30 minutes of ischemia, moderate to large infarcts after 2 to 3 hours of ischemia, and in most cases large infarcts after permanent MCA occlusion. Local CBF appeared to define thresholds for paralysis and infarction. When local flow dropped below about 23 cc/100 gm/min, reversible paralysis occurred. When local flow fell below 10 to 12 cc/100 gm/min for 2 to 3 hours or below 17 to 18 cc/100 gm/min during permanent occlusion, irreversible local damage was observed.

These studies imply that some cases of acute hemiplegia, with blood flow in the paralysis range, might be improved by surgical revascularization. Studies of local CBF might help identify suitable cases for emergency revascularization.

Lack of an ideal model has hampered investigation of ischemic stroke.19 We have recently developed a model of stroke involving awake primates; this model overcomes some important problems.16,20 A snare ligature permits reversible, minimally traumatic occlusion of the middle cerebral artery (MCA) in the unanesthetized state. Local cerebral blood flow (CBF) is determined serially at multiple brain loci. Ischemia-modifying variables are monitored sequentially. Neuropathological evaluation 2 weeks after the insult identifies infarction and local CBF recording sites. The method eliminates the ischemia-modifying effects of anesthesia; neurological function can be correlated with local CBF and neuropathology.

Preliminary experience with this model suggested a CBF threshold for infarction.20 The present study is an expanded assessment of the brain's tolerance to focal ischemia. Ischemic insults of varied severity and duration were produced. Neurological function, local CBF, and pathology were correlated. We found that local CBF defines ischemia thresholds for function and structure: when local flow falls below a paralysis threshold, function is reversibly lost; when flow drops further below an infarction threshold, anatomic damage becomes irreversible.

Materials and Methods
General Description

Thirty-one Macaca irus monkeys of both sexes, weighing 3 to 5 kg, were studied. Under anesthesia, we installed an aortic catheter, intracerebral CBF electrodes, an intracranial pressure (ICP) transducer, and a snare ligature around the right MCA. After recovery from surgery, the MCA ligature was tightened for 15 minutes, 30 minutes, 2 to 3 hours, or permanently (see Table 3). Two animals served as controls. We monitored arterial pressure and blood gases, neurological status, local CBF, and ICP. After 48 hours, animals were returned to their cages. After 2 weeks, neuropathological evaluation identified infarcts and CBF recording sites.

TABLE 3

Summary of clinical, pathological, and CBF (cc/100 gm/min) data*

Monkey No.Duration of IschemiaClinical ResponseElectrode
During OcclusionAfter Occlusion12345  
115 min hemiparesiscomplete recoverysiteTTPICWMCX
 CBF156164815
  % control75156593239
 infarct00000
 
217 min moderate deficitcomplete recoverysiteTICPCXWM
 CBF1924711
  % control1005583944
 infarct00000
 
317 min moderate deficitcomplete recoverysiteCICPCXWM
 CBF12300.25
  % control60210.10.636
 infarct00000
 
440 min moderate deficitpartial resolutionsiteCCICWM??
 CBF1216007
  % control601140.10.647
 infarct00000
 
530 min severe deficitresolution to mild monoparesissiteCPP-WMCXCX-WM
 CBF73506
  % control615240.0110
 infarct00000
 
630 min moderate to severe deficitresidual arm weaknesssiteCPWMWMCX
 CBF11671211
  % control6143263637
 infarct0± 1000
 
730 min severe deficitcomplete recoverysiteCCWMWMWM
 CBF1181082
  % control813610010017
 infarct00000
 
830 min mild to moderate deficitcomplete recoverysiteWMWMWMWMWM
 CBF197272015
  % control8227637145
 infarct00000
 
926 min mild deficitcomplete recoverysiteCICICPCX
 CBF13152525
  % control50562020
 infarct00000
 
1030 min moderate deficit of spastic monoparesis of lt upper limbmild residual arm weaknesssiteWM-VentCC-ICWMWM
 CBF14131113
  % control87816557
 infarct00000
 
11120 min mild deficitcomplete recoverysiteGPPWMWMCX
 CBF4123303318?
  % control11721344436
 infarct00000
 
12157 min severe deficitpartial recoverysiteAICPWMWMCX
 CBF119842
  % control26355088
 infarct+++00
 
13133 min moderate deficitpartial recoverysiteAICPWMWMCX
 CBF611251412
  % control720273332
 infarct++0?00

Abbreviations: CBF = cerebral blood flow (cc/100 gm/min); T = thalamus; PIC = posterior internal capsule; WM = white matter; CX = cortex; P = putamen; C = caudate; IC = internal capsule; AIC = anterior internal capsule; vent = ventricle; 0 = no infarct; + = definite infarct; ± I = possible infarct; ? = uncertain pathology.

These monkeys had Grade 1 to 2 infarcts distant from the electrode tips.

‡No electrode sites are given for Monkey 18, since electrodes were not localized in these sections.

TABLE 3 (continued)*
Monkey No.Duration of IschemiaClinical ResponseElectrode
During OcclusionAfter Occlusion12345  
14 permanent no lasting deficitsiteAICPPWMCX
  CBF2340274945
   % control74337571102
  infarct00000
  
15† permanent persistent, distal upper limb paresissiteCC-ICPWMWM-CX
  CBF204333105?
   % control59537710031
  infarct00000
  
16 permanent persistent, severe lt upper limb weaknesssiteCCICPWM
  CBF1551023
   % control94243356
  infarct++++0
  
17 permanent moderate deficit which became severesiteCICICPWM
  CBF171381826
   % control714269108
  infarct0+++0
  
18‡ permanent severe, persistent deficitsite
  CBF4375715
   % control84116835468
  infarct?+++0

Abbreviations: CBF = cerebral blood flow (cc/100 gm/min); T = thalamus; PIC = posterior internal capsule; WM = white matter; CX = cortex; P = putamen; C = caudate; IC = internal capsule; AIC = anterior internal capsule; vent = ventricle; 0 = no infarct; + = definite infarct; ± I = possible infarct; ? = uncertain pathology.

These monkeys had Grade 1 to 2 infarcts distant from the electrode tips.

No electrode sites are given for Monkey 18, since electrodes were not localized in these sections.

Investigations were patterned after a previously described approach.16,20 There were three significant modifications: 1) electrode insertion, orbital surgery, and MCA occlusion were staggered over several days to permit study of the effects of operative manipulations on CBF; 2) in most animals, we performed limited studies of autoregulation and CO2 reactivity; and 3) local CBF was determined by hydrogen clearance, with on-line data analysis by a programmable calculator.

Preparation Procedures

For preparation, animals were anesthetized with phencyclidine (4 mg intramuscularly,) and pentobarbital (18 mg/kg intravenously). Transfemoral arterial and venous catheters were placed. For CBF determinations, an array of platinum-iridium electrodes was implanted stereotaxically in the cortical and subcortical territory of the MCA. Five electrodes were placed in the right hemisphere and one in the left hemisphere. An epidural transducer for ICP recording was fixed with dental acrylic in the right parietal area.10 Animals awakened in a primate-restraining chair. On Day 2, baseline CBF values were determined by hydrogen clearance.4,8,9,22 Then animals were anesthetized with phencyclidine, and a snare ligature was placed around the right MCA.11 In this transorbital procedure, we exposed the MCA microsurgically, passed a 10–0 nylon suture around the MCA, and brought the suture out of the wound through a fine polyethylene catheter. If there was any subarachnoid bleeding during surgery or a postoperative deficit, the animal was eliminated from further study. On Day 3, the MCA was occluded under extensive monitoring.

Local Cerebral Blood Flow

Local CBF was measured using a hydrogen clearance technique.4,8,9,22,26 Details of the instrumentation and methodology are described elsewhere.20 On-line least-squares regression5 analysis of local CBF data was performed by a programmable calculator.* To check the reliability of the programmable calculator and for multiexponential desaturation curves, curve stripping was performed to calculate mean weighted flow.

Serial CBF flow studies were obtained at 30- to 60-minute intervals during occlusion in monkeys undergoing temporary occlusion. The CBF studies were repeated at 24 hours, 48 hours, and approximately 7 and 14 days thereafter. Fourteen animals underwent limited studies of autoregulation and CO2 reactivity.

Clinical Evaluation

In each phase of the experiment, monkeys were examined for evidence of neurological deficit. Deficits were graded using a simple five-point scale, as follows:

Grade 0 = no clinical deficit: normal examination, including normal climbing in cage

Grade 1 = minimal clinical deficit: decreased dexterity of the contralateral hand

Grade 2 = mild clinical deficit: posturing of the contralateral arm at rest, with some difficulty with contralateral leg during climbing; equivocal facial paresis or hemianopsia

Grade 3 = moderate clinical deficit: pronounced contralateral hemiparesis preventing climbing; facial paresis, ipsilateral circling and hemianopsia

Grade 4 = severe clinical deficit: hemiplegia; inability to stand; marked ipsiversive turning of eyes and head.

Neuropathology

Monkeys that did not die early on were sacrificed 2 to 3 weeks after the onset of the ischemic insult. Monopolar cauterization was applied to each electrode to help in identification of electrode tip sites. Brains were fixed in situ by transcardiac perfusion with 10% phosphate-buffered formalin. The right internal carotid artery was injected with Microfil to investigate patency of the right MCA at the point of ligation. In every case of temporary occlusion, the vessel was patent. In every case of permanent occlusion, the vessel was totally blocked by the ligature. Brains were removed and immersed in fixative for an additional 3 weeks. Except for the midbrain, brain stem, and cerebellum, the entire brain was embedded in celloidin. The whole brain was then serially sectioned in the horizontal plane at gapless 15-µ intervals. Sections were examined sequentially to locate the electrode tracts and tips (cauterized tissue) and to identify size, location, and character of infarction in relation to the electrode tips.20

Infarcts were graded on a scale of 0 to 4; 0 = no infarct; 1 = microscopic infarct; 2 = infarct up to 1 cm in deep structures; 3 = deep infarct 1 to 2 cm in diameter; and 4 = infarct larger than 2 cm in diameter extending to the surface.

Results
Unanesthetized Primate Model

Because of its complexity, the model is difficult to manage. Of 33 animals prepared, only 20 (including two controls) were suitable for detailed study. Others were eliminated because of unwanted subarachnoid hemorrhage, malfunction of the snare ligature, or other technical problems.

Systemic Variables

As in our previous report,20 systemic factors deviated from normal values in a predictable fashion (Table 1). Mild hypertension, chronic hyperventilation, and a low-grade anemia were characteristic.

TABLE 1

Systemic factors in unanesthetized monkeys before, during, and after MCA occlusion*

StudyControl ValuesDuring OcclusionAfter Occlusion
MABP (mm Hg)112.2 ± 15.3111.6 ± 11.0114.0 ± 17.0
no. of monkeys101010
pO2 (mm Hg)96.0 ± 8.895.2 ± 13.4100.7 ± 19.0
no. of monkeys101010
pCO2 (mm Hg)29.2 ± 6.028.5 ± 5.230.6 ± 3.6
no. of monkeys101010
pH7.4 ± 0.057.4 ± 0.077.4 ± 0.06
no. of monkeys101010
hematocrit (%)34.0 ± 6.029.2 ± 1.626.3 ± 3.5
no. of monkeys653

MABP = mean arterial blood pressure; MCA = middle cerebral artery. Numbers are mean values with standard deviations.

Local Cerebral Blood Flow

Electrode tips were well placed in 18 of the 20 monkeys. Good localization was not possible in the other two because of acute infarction with edema or inadequate cauterization of electrode tips. Electrodes lay astride the central territory of the MCA, extending from the caudate nucleus medially to the cortex laterally (Fig. 1).

Fig. 1.
Fig. 1.

Sites of cerebral blood flow recording. Drawing of horizontal whole brain section shows five electrode sites in the right (ischemic) hemisphere and one control electrode (R) on the left. Typical infarct is shown in stippled area.

Electrodes and instrumentation used in these studies registered errors of ± 5% when checked against directly measured flow in an in vitro system (as described by Willis, et al.).26 In anesthetized monkeys, immediately repeated local CBF determinations varied by ± 10%. In most awake monkeys, local CBF at individual electrodes fluctuated ± 15% over the 2-week follow-up period. Before MCA occlusion, about 30% of washout curves were biexponential. After MCA occlusion, virtually all washout curves in the ischemic zone were monoexponential.

Table 2 presents local CBF values obtained from various anatomic structures. Large standard deviations are evident. Blood flow in the cortex and basal ganglia exceeded flow in white matter.

TABLE 2

Local CBF and temporary MCA occlusion*

Testing SiteBefore OcclusionDuring Occlusion
Value% of Control  
caudate28 ± 2712 ± 444
no. of monkeys1314
putamen57 ± 4418 ± 331
no. of monkeys1011
internal capsule28 ± 3120 ± 2271
no. of monkeys98
insular cortex45 ± 3017 ± 1638
no. of monkeys108
thalamus26 ± 1132 ± 26123
no. of monkeys33
centrum semiovale33 ± 2318 ± 1148
no. of monkeys2524

CBF = cerebral blood flow; MCA = middle cerebral artery. Values are mean local CBF in cc/100 gm/min ± standard deviation.

Response to MCA Occlusion

In all monkeys, MCA occlusion caused a neurological deficit. Deficits appeared within seconds of occlusion, usually became maximal within 10 minutes, and occasionally increased up to 30 minutes. No animal in this study suffered delayed deterioration. Most animals sustained a moderate deficit (Grade 3), while some deficits ranged from mild (Grade 1) to severe (Grade 4).

In all the monkeys, MCA occlusion caused immediate decreases in local CBF, the largest in the putamen (Figs. 2 and 3, and Table 2). Moderate decrease in CBF was widespread, including in the centrum semiovale (but see Monkey 8, Table 3) and insular cortex. The percentage decrement of CBF in the internal capsule was less than for other areas. Significant decreases in contralateral local CBF (diaschisis) were not observed. The ICP and cerebral perfusion pressure did not change.

Fig. 2.
Fig. 2.

Reversible ischemia. Neurological deficit is graded as follows: 1 = minimal, 2 = mild, 3 = moderate, 4 = severe. Each symbol on the chart represents flow from one electrode (see location on the horizontal section). Occlusion of the middle cerebral artery caused a moderate to severe deficit with profound ischemia at all electrode sites. After 17 minutes, reversal of occlusion led to complete recovery and a brief hyperemia. Pathological evaluation showed no infarction.

Fig. 3.
Fig. 3.

Irreversible infarction. Same symbols as in Fig. 2. Permanent middle cerebral artery occlusion caused a marked deficit and marked ischemia. Gradual mild improvement in deficit was noted over the 16-day survival period. Pathology showed a Grade 3 infarct encompassing the zones with lowest cerebral blood flow values.

Response to Release of MCA Occlusion

The release of the MCA occlusion always led to clinical improvement (Table 3). The speed and completeness of recovery were related to the duration of MCA occlusion. After 15 minutes of occlusion, all animals made a prompt, full recovery (Fig. 2). After occlusion for 30 minutes, four animals recovered over several hours and three improved partially. After 2 to 3 hours, three monkeys improved, but only one achieved a full recovery. No animals in this series seemed injured by release of the occlusion.

Reversal of the MCA occlusion invariably led to increased local CBF (“reactive hyperemia”). The magnitude and duration of reactive hyperemia were related to the magnitude and duration of CBF reduction. After 15 minutes of occlusion, hyperemia was modest and brief. After occlusion for 2 to 3 hours, hyperemia was marked and long-lasting. The magnitude and duration of hyperemia were roughly proportional to eventual infarct size. The ICP did not change significantly with reversal of MCA occlusion.

Neuropathological Observations

Careful gross inspection of the 20 brains failed to show subarachnoid blood, MCA occlusion, or significant anomalies of the cerebral arteries. In general, the longer the occlusion, the bigger the infarct (Table 3). After 15 minutes of MCA occlusion, three animals showed no evidence of infarction. After 30 minutes of occlusion, three animals had no infarct, and four animals showed infarct sizes ranging from Grade 1 (two) to Grade 2 (two). After 120 to 188 minutes of occlusion, one animal had no infarct and two animals showed infarct sizes of Grade 1 to 3. After permanent occlusion, one animal had no infarct, and four animals showed infarcts of Grade 2 to 4 (Fig. 3).

As to localization, the bigger the infarct, the more lateral its extension. Smaller infarcts involved the head and body of the caudate nucleus and putamen, sparing the centrum semiovale, internal capsule, and cerebral cortex. Such low-grade lesions were often multiple. Larger infarcts extended across the internal capsule to the insular and opercular cortex, sometimes involving the entire territory of the MCA.

The neurohistology of the lesions was relatively constant within each infarct grade. Large infarcts generally showed a central region of coagulation necrosis, with a relatively discrete peripheral rim of phagocytic cells, reactive astrocytes, and occasional chronic inflammatory cells. Smaller lesions showed sharply outlined multiple foci of neuronal destruction with variable preservation of the neuropil. Some of these small lesions exhibited poorly outlined borders of mixed reactive astrocytosis and incomplete neuronal dropout, with progressively greater injury toward the topographic center of lesion.

Correlations: Deficits, Local CBF, and Infarcts

Decrease in local CBF during MCA occlusion was directly related to neurological deficit (Fig. 4). For individual animals, local CBF values from the electrodes in the basal ganglia and internal capsule were averaged and correlated to simultaneous neurological deficit. A close linear relationship is seen: the less the occlusion flow, the worse the paralysis. When average local CBF was greater than 21 to 23 cc/100 gm/min, little or no deficit could be detected. When average local CBF dropped below this paralysis threshold, mild deficits were noted. When average local CBF was less than 8 to 9 cc/100 gm/min, hemiplegia was complete.

Fig. 4.
Fig. 4.

Local cerebral blood flow (1CBF) and neurological deficit. Each point represents one monkey's average CBF determined from basal ganglia and capsule electrodes (usually 1 to 3). Note that decreasing average CBF correlates with increasing deficit. No deficit was detected if average CBF remained above about 23 cc/100 gm/min, an apparent paralysis threshold.

The severity of neurological deficit at sacrifice was directly related to the size of the ceredral infarction (Table 3). The worse the deficit, the bigger the infarct. Five of six animals without deficit had no infarct; one animal without deficit had a small infarct in the putamen.

As gleaned from Table 3, absolute local CBF correlated in an interesting way with local tissue damage. Even intense ischemia for brief periods (15 to 30 minutes) was borne without local infarction (Fig. 5, open circles). Severe ischemia (10 to 12 cc/100 gm/min) for 2 to 3 hours caused local infarction (Fig. 5, filled squares). Moderate ischemia (18 cc/100 gm/min) with permanent occlusion caused irreversible necrosis. Thus, severity and duration of local flow reduction appear to define an infarction threshold.

Fig. 5.
Fig. 5.

Local cerebral blood flow (1CBF) and infarction. Pooled data from animals undergoing middle cerebral artery (MCA) occlusion. Open circles represent electrodes in normal tissue. Triangles represent electrodes on the periphery of the infarct. Filled boxes represent electrodes located in infarcted tissue. Dashed line approximates an infarction threshold: when 1CBF falls below this critical value, infarction occurs.

When electrodes were replotted for percentage of control CBF (occlusion local CBF/control local CBF × 100) versus time, the infarction threshold was less clear; at 120 to 180 minutes, CBF less than 50% of control led to infarction about half the time. For permanent occlusion, a CBF of 50% to 116% of control was sometimes associated with infarction.

White matter was sometimes especially resistant to ischemia. In three instances, areas of white matter survived marked, sustained ischemia (permanent occlusion with local CBF about 10 cc/100 gm/min).

Discussion
Unanesthetized Monkey Model

This model presents several advantages.19,20 It eliminates the confusing effects of anesthesia, which may be protective against ischemia.13,18,27 In addition, the model minimizes vascular trauma which might be substantial with larger pneumatic24 or mechanical17 occlusion devices. The snare ligature provides the reversible occlusion needed for studies of tolerance to ischemia. Hydrogen clearance monitors ischemia and reperfusion. The model also provides surveillance of ischemia-modifying variables. Since infarcts are allowed lowed to develop fully in histological reaction, indisputable evidence of tissue damage or viability is obtained.

The model has limitations, including expense and difficulty. Variability in local CBF in the awake state and uncontrollable changes in systemic factors may contribute to variability in infarct size.

Local Cerebral Blood Flow

The hydrogen clearance method seems well suited to these experiments. The method maximizes spatialtemporal resolution of CBF by monitoring multiple sites over multiple points in time. Careful histological studies allow correlation of electrode recording sites with evidence of infarction. Such correlation of local CBF with local histology provides the best evidence of a CBF threshold for tissue damage.

Despite these advantages, the hydrogen clearance method has distinct limitations, as discussed by Halsey, et al.9 Multiexponential desaturation curves were found at about one-third of our electrodes, and the origin of these components remains uncertain. Local CBF values reported here are somewhat lower than those reported elsewhere, perhaps related to the awake state with chronic respiratory hypocapnea. Because of the small size of brain structures relative to the 2-mm electrodes, we have doubtless sampled from several local structures simultaneously in these experiments. Diffusion from neighboring areas likely explains the unexpectedly low values of local CBF obtained from “insular cortex,” which is adjacent to the white matter and Sylvian cistern.

Effects of Ischemia

As in humans, MCA occlusion in awake monkeys causes serious effects. All animals suffered immediate neurological deficits, maximal within 30 minutes. Decreases in local CBF occurred in all monkeys, and were most pronounced in the basal ganglia, where infarction was most common. Washout curves from ischemic zones were almost always monoexponential, perhaps related to a predominant effect of ischemia on one compartment.

Occlusion of the MCA caused variable change in local CBF. In a few animals with verified MCA occlusion, only mild reduction in flow occurred (local CBF about 25 cc/100 gm/min). In other animals, ischemia was profound (local CBF 10 cc/100 gm/min). Most animals showed an intermediate response. Blood flow in the basal ganglia decreased most, and medial and lateral areas (Electrodes 1 and 5) were less affected. Variability in CBF during MCA occlusion seems best explained as variability in available leptomeningeal collateral circulation.

Local CBF was rather stable during MCA occlusion. In most cases, collateral circulation neither rallied nor failed after acute MCA occlusion in primates, but rather depended upon existing collateral circulation at the moment of occlusion. In a single unique animal with verified MCA occlusion, gradual resolution of a mild deficit corresponded to gradual recovery of CBF over 24 hours.

Effects of Reperfusion

Ligature release led to prompt elevation of local CBF in all cases studied. This confirms the contention that such release actually produces antigrade reperfusion. Evidence of “no reflow”2 was not obtained, although the sampling method could overlook small or patchy zones of impaired reperfusion.

Reperfusion commonly brought local CBF to levels exceeding preocclusion values (“reactive hyperemia”). The magnitude and duration of hyperemia corresponded roughly to both 1) the intensity and duration of ischemia, and 2) the size of the infarct. The data are insufficient to demonstrate a causal relationship.

Thresholds for Paralysis and Infarction

In all monkeys studied, clinical deficit was directly related to the magnitude of local CBF reduction (Fig. 4). When average deep hemispheral CBF fell below 20 to 25 cc/100 gm/min, a mild neurological deficit could be detected. When average deep hemispheral CBF fell below about 8 cc/100 gm/min, the deficit became complete (flaccid hemiplegia with ipsiversive turning). If the CBF was restored promptly, perfect neurological recovery followed, even after profound ischemia and deficit. Moderate ischemia (15 cc/100 gm/min) was tolerated for 2 to 3 hours without substantial permanent damage. The data suggest a paralysis threshold, independent of time, at about 23 cc/100 gm/min, below which normal tissue function is reversibly altered (Figs. 2 and 6).

Fig. 6.
Fig. 6.

Ischemia thresholds. When local cerebral blood flow (1CBF) falls below about 23 cc/100 gm/min, reversible paralysis occurs. Even profound ischemia is reversible for a brief time. When 1CBF falls below 10 cc/100 gm/min for 2 hours, or below 18 cc/100 gm/min during permanent occlusion, irreversible infarction occurs. MCA = middle cerebral artery.

The development of infarction appears to be a function of intensity and duration of ischemia. The aggregate data support this concept (Fig. 5). Monkeys with brief occlusions tolerated very marked ischemia without evidence of infarction. After 2 to 3 hours of occlusion, local CBF below 10 to 12 cc/100 gm/min led to infarction. With permanent MCA occlusion, local CBF below 17 to 18 cc/100 gm/min was associated with infarction. The data strongly suggest an infarction threshold, rising over some hours to a plateau at about 17 to 18 cc/100 gm/min, below which normal tissue structure is irreversibly damaged.

These thresholds for function and structure, represented schematically in Fig. 6, raise several questions. The exact locations of break-points in the infarction threshold curve should be better defined by further experiments with varied occlusion times. The existence of long-lasting reversible paralysis without infarction,14 a very interesting possibility for the clinician, should likewise be tested by further studies. The possibility of separate thresholds for gray and white matter also needs investigation.

The thresholds we have found seem in general agreement with other threshold phenomena reported in the literature. With regard to reversible changes, narcotized baboons lose somatosensory evoked potentials when regional CBF falls below 15 cc/100 gm/min.20 According to Heiss, et al.,12 neuronal firing in anesthetized cats ceases when regional CBF falls below 18 cc/100 gm/min. In anesthetized patients undergoing carotid clamping for endarterectomy, electroencephalographic changes occur if regional CBF falls below about 18 cc/100 gm/min.23 With regard to irreversible changes, narcotized baboon brain demonstrates a massive outpouring of potassium into the extracellular space when regional CBF falls below about 6 to 10 cc/100 gm/min.3

Tolerance to focal cerebral ischemia in these experiments was clearly related to residual local CBF, which must be provided by collateral circulation.6,7,25 Unexpected resilience of central nervous tissue to global ischemia, as documented by Ames and Gurian1 and Hossmann and Olsson,15 probably helps brain tissue survive a focal ischemic insult. This tolerance is likely enhanced by barbiturate anesthesia.13,18,27 Barbiturate effect may underlie the difference between the present study, with substantial infarcts after 3 hours, and our prior work in anesthetized monkeys which showed substantial infarcts after 4 to 8 hours.6 However, differences in technique, such as placement of monkeys supine as opposed to seated, and pCO2, could play a role, and further studies of barbiturate effect, with unanesthetized animals as controls, should be performed.

Clinical Implications

The threshold concept is a hopeful one for the management of acute ischemic stroke. The concept implies that some cases of fresh hemiplegia, with CBF in the paralysis range, might be improved by surgical revascularization. Recovery of animals after MCA occlusion of ½ to 2½ hours and the results of limited clinical experience support this suggestion.21 On the other hand, cases of acute hemiplegia with CBF in the infarction range will not be helped by restoration of flow. Rapid studies of local CBF and metabolism, possibly by positron emission tomography, might help identify suitable cases for emergency cerebral revascularization.

References

  • 1.

    Ames A IIIGurian BS: Effects of glucose and oxygen deprivation on function of isolated mammalian retina. J Neurophysiol 26:6176341963J Neurophysiol 26:

  • 2.

    Ames A IIIWright RLKowada Met al: Cerebral ischemia. II. The no-reflow phenomenon. Am J Pathol 52:4374531968Am J Pathol 52:

  • 3.

    Astrup JSymon LBranston NMet al: Cortical evoked potential and extracellular K+ and H+ at critical levels of brain ischemia. Stroke 8:51571977+ and H+ at critical levels of brain ischemia. Stroke 8:

  • 4.

    Aukland KBower BFBerliner RW: Measurement of local blood flow with hydrogen gas. Circ Res 14:1641871964Circ Res 14:

  • 5.

    Colton T: Statistics in Medicine. Boston: Little, Brown and Co1974372 ppColton T: Statistics in Medicine.

  • 6.

    Crowell RMOlsson YKlatzo Iet al: Temporary occlusion of the middle cerebral artery in the monkey: clinical and pathological observations. Stroke 1:4394481970Stroke 1:

  • 7.

    Denny-Brown DMeyer JS: The cerebral collateral circulation. 2. Production of cerebral infarction by ischemic anoxia and its reversibility in early stages. Neurology 7:5675791957Neurology 7:

  • 8.

    Fein JMWillis JHamilton Jet al: Polarographical measurement of local cerebral blood flow in the conscious and anesthetized primate. Stroke 6:42511975Stroke 6:

  • 9.

    Halsey JH JrCapra NFMcFarland RS: Use of hydrogen for measurement of regional cerebral blood flow. Problem of intercompartmental diffusion. Stroke 8:3513571977Stroke 8:

  • 10.

    Hayakawa TWaltz AG: Changes of epidural pressures after experimental occlusion of one middle cerebral artery in cats. J Neurol Sci 26:3193331975J Neurol Sci 26:

  • 11.

    Hayakawa TWaltz AG: Immediate effects of cerebral ischemia: evolution and resolution of neurological deficits after experimental occlusion of one middle cerebral artery in conscious cats. Stroke 6:3213271975Stroke 6:

  • 12.

    Heiss W-DHayakawa TWaltz AG: Cortical neuronal function during ischemia. Effects of occlusion of one middle cerebral artery on single-unit activity in cats. Arch Neurol 33:8138201976Arch Neurol 33:

  • 13.

    Hoff JTSmith ALHankinson HLet al: Barbiturate protection from cerebral infarction in primates. Stroke 6:28331975Stroke 6:

  • 14.

    Holbach K-HWassmann HHohelüchter KL: Reversibility of the chronic post-stroke state. Stroke 7:2963001976Stroke 7:

  • 15.

    Hossmann K-AOlsson Y: Suppression and recovery of neuronal function in transient cerebral ischemia. Brain Res 22:3133251970Brain Res 22:

  • 16.

    Jones THMorawetz RBOjemann RGet al: Thresholds of focal cerebral ischemia in unanesthetized monkeys. Stroke 9:1011978 (Proceedings)Stroke 9:

  • 17.

    Little JR: Implanted device for middle cerebral artery occlusion in conscious cats. Stroke 8:2582601977Little JR: Implanted device for middle cerebral artery occlusion in conscious cats. Stroke 8:

  • 18.

    Michenfelder JDMilde JHSundt TM Jr: Cerebral protection by barbiturate anesthesia. Use after middle cerebral artery occlusion in Java monkeys. Arch Neurol 33:3453501976Arch Neurol 33:

  • 19.

    Molinari GFLaurent JP: A classification of experimental models of brain ischemia. Stroke 7:14171976Stroke 7:

  • 20.

    Morawetz RBDeGirolami UOjemann RGet al: Cerebral blood flow determined by hydrogen clearance during middle cerebral artery occlusion in unanesthetized monkeys. Stroke 9:1431491978Stroke 9:

  • 21.

    Ojemann RGCrowell RMRoberson GHet al: Surgical treatment of extracranial carotid occlusive disease. Clin Neurosurg 22:2142631974Clin Neurosurg 22:

  • 22.

    Pasztor ESymon LDorsch NWCet al: The hydrogen clearance method in assessment of blood flow in cortex, white matter and deep nuclei of baboons. Stroke 4:5565671973Stroke 4:

  • 23.

    Sharbrough FWMessick JM JrSundt TM Jr: Correlation of continuous electrocephalograms with cerebral blood flow measurements during carotid endarterectomy. Stroke 4:6746831973Stroke 4:

  • 24.

    Spetzler RFWeinstein PMehdorn M: New model for chronic reversible cerebral ischemia. Presented at the 46th Annual Meeting of the American Association of Neurological SurgeonsNew OrleansApril, 1978 (Paper No. 6)

  • 25.

    Vander Eecken HMAdams RD: The anatomy and functional significance of the meningeal arterial anastomoses of the human brain. J Neuropathol Exp Neurol 12:1321571953J Neuropathol Exp Neurol 12:

  • 26.

    Willis JADoyle TFRamirez Aet al: A practical circuit for hydrogen clearance in blood flow measurement. TN 74-2. Washington, DC: Armed Forces Radiation Research Institute1974

  • 27.

    Yatsu FMDiamond IGraziano Cet al: Experimental brain ischemia: protection from irreversible damage with a rapid-acting barbiturate (methohexital). Stroke 3:7267321972Stroke 3:

Hewlett-Packard Model 9815A programmable calculator manufactured by Hewlett-Packard Corp., San Diego, California.

This work was supported in part by the National Institute of Neurological Diseases and Stroke Grants NS13165 and NS10828, and in part by Teacher-Investigator Award NS11001 to Dr. Crowell.

Article Information

Address for Dr. DeGirolami: Department of Pathology (Neuropathology), University of Massachusetts Medical Center, 55 Lake Avenue North, Worcester, Massachusetts 01605.

Address reprint requests to: Robert M. Crowell, M.D., Barrow Neurological Institute, 350 West Thomas Road, Phoenix, Arizona 85013.

© AANS, except where prohibited by US copyright law."

Headings

Figures

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    Sites of cerebral blood flow recording. Drawing of horizontal whole brain section shows five electrode sites in the right (ischemic) hemisphere and one control electrode (R) on the left. Typical infarct is shown in stippled area.

  • View in gallery

    Reversible ischemia. Neurological deficit is graded as follows: 1 = minimal, 2 = mild, 3 = moderate, 4 = severe. Each symbol on the chart represents flow from one electrode (see location on the horizontal section). Occlusion of the middle cerebral artery caused a moderate to severe deficit with profound ischemia at all electrode sites. After 17 minutes, reversal of occlusion led to complete recovery and a brief hyperemia. Pathological evaluation showed no infarction.

  • View in gallery

    Irreversible infarction. Same symbols as in Fig. 2. Permanent middle cerebral artery occlusion caused a marked deficit and marked ischemia. Gradual mild improvement in deficit was noted over the 16-day survival period. Pathology showed a Grade 3 infarct encompassing the zones with lowest cerebral blood flow values.

  • View in gallery

    Local cerebral blood flow (1CBF) and neurological deficit. Each point represents one monkey's average CBF determined from basal ganglia and capsule electrodes (usually 1 to 3). Note that decreasing average CBF correlates with increasing deficit. No deficit was detected if average CBF remained above about 23 cc/100 gm/min, an apparent paralysis threshold.

  • View in gallery

    Local cerebral blood flow (1CBF) and infarction. Pooled data from animals undergoing middle cerebral artery (MCA) occlusion. Open circles represent electrodes in normal tissue. Triangles represent electrodes on the periphery of the infarct. Filled boxes represent electrodes located in infarcted tissue. Dashed line approximates an infarction threshold: when 1CBF falls below this critical value, infarction occurs.

  • View in gallery

    Ischemia thresholds. When local cerebral blood flow (1CBF) falls below about 23 cc/100 gm/min, reversible paralysis occurs. Even profound ischemia is reversible for a brief time. When 1CBF falls below 10 cc/100 gm/min for 2 hours, or below 18 cc/100 gm/min during permanent occlusion, irreversible infarction occurs. MCA = middle cerebral artery.

References

1.

Ames A IIIGurian BS: Effects of glucose and oxygen deprivation on function of isolated mammalian retina. J Neurophysiol 26:6176341963J Neurophysiol 26:

2.

Ames A IIIWright RLKowada Met al: Cerebral ischemia. II. The no-reflow phenomenon. Am J Pathol 52:4374531968Am J Pathol 52:

3.

Astrup JSymon LBranston NMet al: Cortical evoked potential and extracellular K+ and H+ at critical levels of brain ischemia. Stroke 8:51571977+ and H+ at critical levels of brain ischemia. Stroke 8:

4.

Aukland KBower BFBerliner RW: Measurement of local blood flow with hydrogen gas. Circ Res 14:1641871964Circ Res 14:

5.

Colton T: Statistics in Medicine. Boston: Little, Brown and Co1974372 ppColton T: Statistics in Medicine.

6.

Crowell RMOlsson YKlatzo Iet al: Temporary occlusion of the middle cerebral artery in the monkey: clinical and pathological observations. Stroke 1:4394481970Stroke 1:

7.

Denny-Brown DMeyer JS: The cerebral collateral circulation. 2. Production of cerebral infarction by ischemic anoxia and its reversibility in early stages. Neurology 7:5675791957Neurology 7:

8.

Fein JMWillis JHamilton Jet al: Polarographical measurement of local cerebral blood flow in the conscious and anesthetized primate. Stroke 6:42511975Stroke 6:

9.

Halsey JH JrCapra NFMcFarland RS: Use of hydrogen for measurement of regional cerebral blood flow. Problem of intercompartmental diffusion. Stroke 8:3513571977Stroke 8:

10.

Hayakawa TWaltz AG: Changes of epidural pressures after experimental occlusion of one middle cerebral artery in cats. J Neurol Sci 26:3193331975J Neurol Sci 26:

11.

Hayakawa TWaltz AG: Immediate effects of cerebral ischemia: evolution and resolution of neurological deficits after experimental occlusion of one middle cerebral artery in conscious cats. Stroke 6:3213271975Stroke 6:

12.

Heiss W-DHayakawa TWaltz AG: Cortical neuronal function during ischemia. Effects of occlusion of one middle cerebral artery on single-unit activity in cats. Arch Neurol 33:8138201976Arch Neurol 33:

13.

Hoff JTSmith ALHankinson HLet al: Barbiturate protection from cerebral infarction in primates. Stroke 6:28331975Stroke 6:

14.

Holbach K-HWassmann HHohelüchter KL: Reversibility of the chronic post-stroke state. Stroke 7:2963001976Stroke 7:

15.

Hossmann K-AOlsson Y: Suppression and recovery of neuronal function in transient cerebral ischemia. Brain Res 22:3133251970Brain Res 22:

16.

Jones THMorawetz RBOjemann RGet al: Thresholds of focal cerebral ischemia in unanesthetized monkeys. Stroke 9:1011978 (Proceedings)Stroke 9:

17.

Little JR: Implanted device for middle cerebral artery occlusion in conscious cats. Stroke 8:2582601977Little JR: Implanted device for middle cerebral artery occlusion in conscious cats. Stroke 8:

18.

Michenfelder JDMilde JHSundt TM Jr: Cerebral protection by barbiturate anesthesia. Use after middle cerebral artery occlusion in Java monkeys. Arch Neurol 33:3453501976Arch Neurol 33:

19.

Molinari GFLaurent JP: A classification of experimental models of brain ischemia. Stroke 7:14171976Stroke 7:

20.

Morawetz RBDeGirolami UOjemann RGet al: Cerebral blood flow determined by hydrogen clearance during middle cerebral artery occlusion in unanesthetized monkeys. Stroke 9:1431491978Stroke 9:

21.

Ojemann RGCrowell RMRoberson GHet al: Surgical treatment of extracranial carotid occlusive disease. Clin Neurosurg 22:2142631974Clin Neurosurg 22:

22.

Pasztor ESymon LDorsch NWCet al: The hydrogen clearance method in assessment of blood flow in cortex, white matter and deep nuclei of baboons. Stroke 4:5565671973Stroke 4:

23.

Sharbrough FWMessick JM JrSundt TM Jr: Correlation of continuous electrocephalograms with cerebral blood flow measurements during carotid endarterectomy. Stroke 4:6746831973Stroke 4:

24.

Spetzler RFWeinstein PMehdorn M: New model for chronic reversible cerebral ischemia. Presented at the 46th Annual Meeting of the American Association of Neurological SurgeonsNew OrleansApril, 1978 (Paper No. 6)

25.

Vander Eecken HMAdams RD: The anatomy and functional significance of the meningeal arterial anastomoses of the human brain. J Neuropathol Exp Neurol 12:1321571953J Neuropathol Exp Neurol 12:

26.

Willis JADoyle TFRamirez Aet al: A practical circuit for hydrogen clearance in blood flow measurement. TN 74-2. Washington, DC: Armed Forces Radiation Research Institute1974

27.

Yatsu FMDiamond IGraziano Cet al: Experimental brain ischemia: protection from irreversible damage with a rapid-acting barbiturate (methohexital). Stroke 3:7267321972Stroke 3:

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