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Stroke. 1995;26:1444-1452

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(Stroke. 1995;26:1444-1452.)
© 1995 American Heart Association, Inc.

Articles

Effects of Intermittent Reperfusion on Brain pHi, rCBF, and NADH During Rabbit Focal Cerebral Ischemia

Luca Regli, MD; Robert E. Anderson, BS Fredric B. Meyer, MD

From the Thoralf M. Sundt Jr Neurosurgical Research Laboratory, Mayo Clinic and Mayo Graduate School of Medicine, Rochester, Minn.


*    Abstract
up arrowTop
 *Abstract
down arrowIntroduction
 down arrowMaterials and Methods
 down arrowResults
 down arrowDiscussion
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Background and Purpose The use of intermittent reperfusion versus straight occlusion during neurovascular procedures is controversial. This experiment studied the effects of intermittent reperfusion and single occlusion on intracellular brain pH (pHi), regional cerebral or cortical blood flow, and nicotinamide adenine dinucleotide (NADH) fluorescence during temporary focal ischemia.

Methods Twenty fasted rabbits under 1.0% halothane anesthesia were divided into four groups: (1) nonischemic controls, (2) 60 minutes of uninterrupted focal ischemia, (3) 2x30-minute periods of focal ischemia separated by a 5-minute reperfusion, and (4) 4x15-minute periods of focal ischemia separated by three 5-minute reperfusion periods. Focal ischemia was produced by occlusion of both the middle cerebral and ipsilateral anterior cerebral arteries. After the final occlusion, there was a 3-hour reperfusion period in all groups. Regional cerebral and cortical blood flow, brain pHi, and NADH fluorescence were measured with in vivo panoramic fluorescence imaging.

Results During occlusion, regional cerebral and cortical blood flows and NADH fluorescence values were not different among the groups. Brain pHi was significantly lower in the 4x15-minute group compared with the 1x60-minute group (6.57±0.02 versus 6.73±0.06; P<.03) but not significant when compared with the 2x30-minute group. During the short reperfusion periods, all parameters returned to normal except for NADH fluorescence levels, which remained elevated. During the postischemic final reperfusion period, there was a mild brain alkalosis of approximately 7.1 in all groups. There were no significant differences in NADH fluorescence among groups during the final reperfusion. Regional cerebral and cortical blood flow returned to near normal values in all groups.

Conclusions This study demonstrates that intermittent reperfusion during temporary focal ischemia has different effects on the intracytoplasmic and the intramitochondrial compartments: worsening of brain cytoplasmic pHi but no significant differences in the oxidation/reduction level of mitochondrial NADH.


Key Words: brain intracellular pH • cerebral ischemia, focal • NADH fluorescence • reperfusion


*    Introduction
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 up arrowAbstract
 *Introduction
down arrowMaterials and Methods
 down arrowResults
 down arrowDiscussion
 down arrowReferences
 
During the neurosurgical treatment of complex vascular lesions, either planned or emergent temporary occlusion of parent vessels is occasionally required.1 2 3 Methods to provide cerebral protection during this temporary occlusion remain controversial.4 5 Within the research literature, the potential benefits or deleterious effects of either intermittent reperfusion or straight occlusion are debatable issues.6 7 8 Although the experimental literature is abundant with data suggesting that reperfusion increases blood-brain barrier breakdown, edema formation, and free radical generation, these reperfusion paradigms do not simulate the type of ischemia that occurs in the neurosurgical setting.9 10 11 12 13 14

Alternatively, at least one study suggests that brief periods of intermittent reperfusion during temporary focal ischemia designed to mimic the typical operative scenario decrease infarction size.15 The possible beneficial effects of intermittent reperfusion may include restoration of necessary metabolic substrates, waste product removal, and the mitigation of the development of an environment conducive to the production of free radicals.7 16 17 The purpose of this experiment was to examine the serial changes in pHi, regional cerebral (133Xe) or cortical (umbelliferone) blood flow, and the NADH oxidative/reduction state during intermittent reperfusion comparable with the operative setting. The hypothesis tested in this study was that brief intermittent reperfusion results in improved normal metabolic function as assessed by brain pHi and NADH fluorescence.


*    Materials and Methods
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 up arrowAbstract
 up arrowIntroduction
 *Materials and Methods
down arrowResults
 down arrowDiscussion
 down arrowReferences
 
Animal Preparation
On approval from the animal care and use committee, 20 overnight-fasted New Zealand White rabbits weighing between 3.5 and 4.5 kg were induced, operated, and studied under 4.0%, 2.5%, and 1.5% halothane anesthesia, respectively. A tracheostomy was performed, and the animals were placed on a Harvard respirator (Harvard Apparatus). The animals were given 0.15 mg/kg pancuronium bromide (Pavulon, Organon Inc) to abolish respiratory efforts. The animals were kept normoxic and normocarbic during the surgical procedure and through the experimental procedure with supplemental CO2 and O2.

Catheters were inserted into the right femoral artery and vein for monitoring blood pressure and sampling arterial blood gases and for administration of drugs. A PE-50 catheter was inserted into the right lingual artery so that its tip was located at the origin of the external carotid artery for the retrograde delivery of the indicator umbelliferone and 133Xe into the internal carotid artery.

The skin, subcutaneous tissue, and muscle were excised over the right supraorbital ridge and parietal area. A craniectomy was performed using a high-speed air drill (Hall Surgical, Division of Zimmer) with the aid of an Olympus operating microscope. The majority of the frontal and parietal cortex was exposed for imaging. The dura was removed and carefully cauterized at the margins of the craniectomy, then covered with plastic wrap to prevent surface oxygenation and to keep the brain moist. Blood loss for the surgical preparation did not exceed 5 mL.

After the surgical preparation, the animals were moved from the operating table and placed on an intravital-type microscope stand. The microscope was focused on an area centered about the suprasylvian gyrus with 1.5 cm2 of cortex imaged for pHi, rCtBF-UM, and NAD/NADH measurements. Arterial blood pressure was measured by a Statham strain gauge attached to the femoral artery catheter and recorded on a Grass model 78 polygraph. The animals were kept normothermic by the use of a heating blanket (K-Pad, Gorman-Rupp), and body temperature was monitored with a rectal digital thermometer. Arterial PaCO2, PaO2, and pH measurements were performed on a London Radiometer blood gas analyzer (PHM-73).

After control measurements were performed, focal cerebral ischemia was produced by occlusion of the proximal middle and anterior cerebral arteries using Mayfield miniature aneurysm clips. Twenty rabbits were divided into four groups of five each: (1) a nonischemic control observed for 4 hours; (2) a control of 60 minutes of straight ischemia followed by 3 hours of reperfusion; (3) two 30-minute periods of ischemia separated by 5 minutes of reperfusion followed by 3 hours of reperfusion; and (4) four 15-minute periods of ischemia each separated by 5 minutes of reperfusion followed by a final 3-hour period of reperfusion. All animals underwent a high PaCO2 reactivity test to ensure normal function before occlusion. A PaCO2 reactivity test was also performed at the end of 3 hours of reperfusion.

In Vivo Video Fluorescence Instrumentation
Instrumentation was designed to perform serial panoramic video imaging of cortical brain pHi and regional cortical blood flow with umbelliferone fluorescence.18 The optical characteristics were such that the majority of the entire hemisphere could be studied simultaneously through a large craniectomy. The use of umbelliferone as a noninvasive in vivo technique for measuring brain pHi and cortical blood flow has been previously described.18 19 20 21 Umbelliferone is nontoxic, fat-soluble, and freely diffuses across the blood-brain barrier, and it rapidly equilibrates across cell membranes and is distributed through the cytoplasm as an uncharged molecule.18 Umbelliferone was prepared for injection by dissolving 0.2 g of indicator in 200 mL 5% glucose-saline solution at 90°C for 30 minutes. The solution was then filtered through a 0.22-µm mesh filter before injection. The volume of injectate was 1.5 mL in this study.

The pH-sensitive indicator umbelliferone has two fluorophors, anionic and isobestic. The anionic and isobestic forms are excited at 370 nm and 340 nm, respectively, and have a common emission at 450 nm. The fluorescence of the anion varies directly with pH, whereas the fluorescence of the isobestic form varies directly only with the indicator concentration. Therefore, it is possible to create a nomogram from the ratio of 340-nm to 370-nm excitations to determine brain pHi. Acquired images were corrected for background NADH fluorescence before processing. NADH fluorescence images were stored for later analysis of mitochondrial function. The images from the 340-nm excitation were processed to compute rCtBF-UM using the 1-minute initial slope index. The partition coefficient for umbelliferone is one.21 The rCtBF-UM image was then displayed and stored on tape for final analysis. For processing of the pHi image, ratios of the paired images from the 340-nm and 370-nm excitations were made, and the resultant pHi image was then displayed and stored on tape for final analysis.

Umbelliferone has been shown to be a reliable indicator of regional cortical blood flow. Anderson et al21 have made a comparative analysis of the different techniques of measuring rCBF compared with that of umbelliferone. They found that a distinction can be made between rCBF as measured by radiolabeled compounds and that of regional cortical blood flow as measured by umbelliferone. rCBF by definition is defined as areas of flow that contain major vessels as well as capillaries and arterioles as measured by radiolabeled compounds. Regional cortical blood flow as measured by umbelliferone is defined as those areas that are relatively avascular and contain only capillaries. The imaging system allows the measurement of regional cortical blood flow by setting the number of pixels to cover specified areas.

133Xe Regional Cerebral Blood Flow Measurements
133Xe rCBF was measured using a CdTe (Cadmium Telluride) detector (RMD) system. The detector has a measurement volume of 0.50 mm3. The window discriminator was set at 76 keV and 200 keV to minimize Compton scatter.22 The resultant counts were recorded on a strip-chart recorder. The 1-minute initial slope index was used to calculate rCBF.23 The partition coefficient ({lambda}) used for 133Xe was 0.63.24

Statistical Analysis
Because of anatomic variation of the microvasculature from animal to animal, single points along an x-y coordinate cannot be averaged frame by frame from different animals at the same time period. Therefore, measurements of regional pHi, regional cortical blood flow–umbelliferone (rCtBF-UM), and NADH fluorescence were made in areas devoid of major vessels. Measurements were made over these relatively avascular areas by averaging 10 000 to 15 000 pixels (22 500 to 37 500 µm2), and the mean and standard error were tabulated. Student's t test was used for comparison between the nonischemic control group and each of the ischemic groups with correction for multiple comparisons. ANOVA followed by Tukey's test for multiple comparison was used to test the statistical significance of differences among the four groups studied. A value of P<.05 was considered significant.


*    Results
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 up arrowAbstract
 up arrowIntroduction
 up arrowMaterials and Methods
 *Results
down arrowDiscussion
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Systemic Parameters and Video Acquisition
There were no significant differences between animals studied in each of the three groups in the measurements of levels of PaCO2, PaO2, mean arterial blood pressure, core body temperature, glucose, lactate, and hematocrit (TableDown). The control nonischemic animals had stable pHi, rCBF, rCtBF-UM and NADH fluorescence measurements throughout the experiment, therefore confirming the stability of the preparation. These data are included on the graphs for comparison but are not discussed below.


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  		Table 1. Systemic Parameters

Group 1: 60-Minute Straight Occlusion Followed by 3-Hour Reperfusion
Intracellular Brain pH
Baseline brain pHi was uniform over the entire exposed cortex measuring 7.02±0.02. After 15 minutes of focal cerebral ischemia, pHi fell significantly (P<.005) to 6.69±0.02 and remained at this level for the next 45 minutes. Thirty minutes after restoration of flow, brain pHi showed a rise toward normalization with an initial value of 7.06±0.03, not significantly different from the baseline control values, and remained close to this level for the next 2.5 hours (Fig 1ADown).



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  		Figure 1. Time-course plots ({circ}, solid line) show brain pHi (A), regional cortical blood flow (rCBF)–umbelliferone (B), reduced NADH (C), and regional cerebral blood flow (rCBF)–133Xe (D) before, during 60 minutes of middle cerebral artery occlusion, and during 3 hours of reperfusion. A corresponding plot ({bullet}, dotted line) of nonischemic controls is also depicted. **P>.005, *P>.05 when compared with nonischemic controls.

Regional Cortical Blood Flow–Umbelliferone
Baseline rCtBF-UM measured 48.1±3.9 mL · 100 g-1 · min-1. Fifteen minutes after occlusion, rCtBF-UM fell significantly (P<.005) to 18.0±2.3 mL · 100 g-1 · min-1 and remained at this level for the next 45 minutes. Thirty minutes after the restoration of blood flow, rCtBF-UM rose to 37.9±3.9 mL · 100 g-1 · min-1 and was relatively constant over the 2.5 hours, not significantly different from baseline control values (Fig 1BUp).

Regional Cerebral Blood Flow–133Xe
Baseline rCBF-133Xe measured 57.6±0.6 mL · 100 g-1 · min-1. Fifteen minutes after occlusion, rCBF-133Xe fell significantly (P<.05) to 32.9±2.7 mL · 100 g-1 · min-1 and remained at this level for the next 45 minutes. Thirty minutes after the restoration of blood flow, rCBF-133Xe rose to 47.6±5.0 mL · 100 g-1 · min-1 and was relatively constant over the 2.5 hours, not significantly different from baseline control values (Fig 1DUp).

NADH Fluorescence
The baseline NADH fluorescence gray scale level was 41.7±0.4. Fifteen minutes after occlusion, NADH fluorescence increased significantly by 136±11.0% (P<.05) when compared with baseline control values and remained at this level for the next 45 minutes. Thirty minutes after the restoration of blood flow, NADH fluorescence decreased to 126.0±3.0% of baseline control. After this time period, NADH fluorescence declined slightly for the remainder of the experiment but remained significantly greater than baseline control values (P<.005) (Fig 1CUp).

Group 2: Two 30-Minute Occlusions Followed by 3-Hour Reperfusion
Intracellular Brain pH
Baseline brain pHi was uniform over the entire exposed cortex measuring 7.06±0.02. After 30 minutes of focal ischemia, pHi fell significantly (P<.005) to 6.78±0.07. After 5 minutes of reperfusion, brain pHi was 7.00±0.03, not different from baseline values. Brain pHi at 30 minutes from the second ischemic insult was 6.73±0.03 (P<.005). Thirty minutes after restoration of blood flow, brain pHi rose to 7.05±0.04 and was slightly alkalotic for the next 2.5 hours, but it was not significantly different from baseline control values (Fig 2ADown).



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  		Figure 2. Time-course plots ({circ}, solid line) show brain pHi (A), regional cortical blood flow (rCBF)–umbelliferone (B), reduced NADH (C), and regional cerebral blood flow (rCBF)–133Xe (D) before, during two 30-minute periods of middle cerebral artery occlusion with 5 minutes of reperfusion after the first 30 minutes of ischemia, and 3 hours of reperfusion. A corresponding plot ({bullet}, dotted line) of nonischemic controls is also depicted. **P>.005, *P>.05 when compared with nonischemic controls; @5 minutes of reperfusion; {Phi}15 minutes of ischemia; {psi}30 minutes of ischemia.

Regional Cortical Blood Flow–Umbelliferone
Baseline rCtBF-UM measured 45.8±2.6 mL · 100 g-1 · min-1. Thirty minutes after occlusion, rCtBF-UM fell significantly (P<.005) to 16.9±1.7 mL · 100 g-1 · min-1. After 5 minutes of reperfusion, rCtBF-UM rose to 36.8±2.3 mL · 100 g-1 · min-1, not different from baseline control values. Thirty minutes from the second ischemic insult, rCtBF-UM fell to 18.7±4.3 (P<.005). Thirty minutes after the restoration of blood flow, rCtBF-UM rose to 24.0±1.11 mL · 100 g-1 · min-1, significantly different from baseline control values (P<.05). It continued to rise and at 1 hour after restoration of flow, it became not significantly different from baseline control values (Fig 2BUp).

Regional Cerebral Blood Flow–133Xe
Baseline rCBF-133Xe measured 53.4±1.9 mL · 100 g-1 · min-1. Thirty minutes after occlusion, rCBF-133Xe fell significantly (P<.05) to 33.4±4.7 mL · 100 g-1 · min-1. After 5 minutes of reperfusion, rCBF-133Xe rose to 44.3±3.3 mL · 100 g-1 · min-1, not different from baseline control values. Thirty minutes from the second ischemic insult, rCBF-133Xe fell to 34.8±3.3 (P<.05). Thirty minutes after the restoration of blood flow, rCBF-133Xe rose to 38.5±2.2 mL · 100 g-1 · min-1, not different from baseline control values. It remained at this level for the duration of the experiment (Fig 2DUp).

NADH Fluorescence
Baseline NADH fluorescence gray scale level was 41.3±0.5. Thirty minutes after occlusion, NADH fluorescence increased significantly to 132.0±5.0% (P<.005) when compared with baseline control values. After 5 minutes of reperfusion, NADH fluorescence decreased to 124.0±4.0% of baseline control, still significantly different (P<.005). Thirty minutes from the second ischemic insult, NADH fluorescence rose to 135.0±7.0% of baseline controls (P<.005). Thirty minutes after the restoration of blood flow, NADH fluorescence decreased to 129.0±5.0% of baseline control but was still significantly different (P<.005). After this time period, NADH fluorescence declined for the remainder of the experiment and became not significantly different 1.5 hours after restoration of flow (Fig 2CUp).

Group 3: Four 15-Minute Occlusions Followed by 3-Hour Reperfusion
Intracellular Brain pH
Baseline brain pHi was uniform over the exposed cortex measuring 7.08±0.01. After 15 minutes of focal ischemia, pHi fell significantly (P<.005) to 6.65±0.03. After 5 minutes of reperfusion, brain pHi was 6.94±0.03, not different from baseline values. After three 15-minute ischemic insults and three 5-minute reperfusions, brain pHi at 15 minutes from the fourth ischemic insult was 6.57±0.02 (P<.005). Thirty minutes after restoration of blood flow, brain pHi rose to 7.10±0.02 and became significantly alkalotic (P<.05) for the next 2.5 hours (Fig 3ADown).



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  		Figure 3. Time-course plots ({circ}, solid line) show brain pHi (A), regional cortical blood flow (rCBF)–umbelliferone (B), reduced NADH (C), and regional cerebral blood flow (rCBF)–133Xe (D) before, during four 15-minute periods of middle cerebral artery occlusion with 5 minutes of reperfusion after each of first three periods of ischemia, and 3 hours of reperfusion. A corresponding plot ({bullet}, dotted line) of nonischemic controls is also depicted. **P>.005, *P>.05 when compared with nonischemic controls; @ 5 minutes of reperfusion; {Phi}15 minutes of ischemia.

Regional Cortical Blood Flow–Umbelliferone
Baseline rCtBF-UM measured 43.5±3.3 mL · 100 g-1 · min-1. After 15 minutes of focal ischemia, rCtBF-UM fell significantly (P<.005) to 20.7±3.1 mL · 100 g-1 · min-1. After 5 minutes of reperfusion, rCtBF-UM rose to 48.1±9.2 mL · 100 g-1 · min-1, not different from control baseline values. After three 15-minute ischemic insults and three 5-minute reperfusions, rCtBF-UM at 15 minutes from the fourth ischemic insult was 21.2±2.2 mL · 100 g-1 · min-1 (P<.005). Thirty minutes after restoration of blood flow, rCtBF-UM rose to 36.2±4.83 mL · 100 g-1 · min-1, not different from baseline control values. It remained at this level for the duration of the experiment (Fig 3BUp).

Regional Cerebral Blood Flow–133Xe
Baseline rCBF-133Xe measured 49.1±1.9 mL · 100 g-1 · min-1. After 15 minutes of focal ischemia, rCBF-133Xe fell significantly (P<.005) to 27.2±1.4 mL · 100 g-1 · min-1. After 5 minutes of reperfusion, rCBF-133Xe rose to 44.8±3.4 mL · 100 g-1 · min-1 but was not different from baseline control values. After three 15-minute ischemic insults and three 5-minute reperfusions, rCBF-133Xe at 15 minutes from the fourth ischemic insult was 27.2±1.1 mL · 100 g-1 · min-1 (P<.005). Thirty minutes after restoration of blood flow, rCBF-133Xe rose to 38.1±3.7 mL · 100 g-1 · min-1, not different from baseline control values. It remained at this level for the duration of the experiment (Fig 3DUp).

NADH Fluorescence
The baseline NADH fluorescence gray scale level was 42.4±0.9. After 15 minutes of focal ischemia, NADH fluorescence increased significantly (P<.005) to 127.0±4.0% of baseline control values. After 5 minutes of reperfusion, NADH fluorescence decreased to 122.0±4.0% of baseline control, significantly different from preocclusion values (P<.05). After three 15-minute ischemic insults and three 5-minute reperfusions, 15 minutes from the fourth ischemic insult NADH fluorescence increased to 127.0±6.0% of baseline control values (P<.05). Thirty minutes after restoration of blood flow, NADH fluorescence decreased to 124.0±7.0% of baseline control, still significantly different (P<.05) from preocclusion. After this time period, NADH fluorescence declined for the remainder of the experiment and became not significantly different 1 hour after restoration of flow (Fig 3CUp). An intergroup comparison of NADH fluorescence during the final reperfusion period was made among all three groups. This intergroup analysis demonstrated no significant difference in the NAD/NADH redox state during the final postischemic reperfusion period.

PaCO2 Reactivity
The PaCO2 reactivity test performed at the beginning and at the end of the experiment demonstrated little or no change in pHi in all groups studied. The reactivity with rCtBF-UM showed no significant difference in the 60-minute straight-occlusion group. In the 2x30-minute reperfusion group, there was a 6% decline in 133Xe-rCBF reactivity, which was not significantly different. In the 4x15-minute reperfusion group, there was a 33.5% decline in 133Xe-rCBF reactivity (P<.01) compared with baseline preischemic values. However, an intergroup analysis of postischemic CO2 reactivity showed no significant differences.


*    Discussion
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 up arrowAbstract
 up arrowIntroduction
 up arrowMaterials and Methods
 up arrowResults
 *Discussion
down arrowReferences
 
The purpose of this study was to examine the effects of brief periods of intermittent reperfusion during transient focal cerebral ischemia mimicking to some degree the surgical setting. The most consistent observations from this study are the following. First, multiple periods of intermittent reperfusion led to a worsening of intracellular acidosis during the ischemic period followed by a postischemic alkalosis. Second, although intermittent reperfusion was associated with a more rapid normalization of the NAD/NADH mitochondrial redox state, an intergroup comparison showed that there were no significant differences between intermittent reperfusion and straight occlusion during the final reperfusion. Third, multiple periods of intermittent reperfusion may worsen postischemic CO2 reactivity, with the 4x15-minute occlusion group having the greatest decline.

The regulation of brain pHi is primarily dependent on the Na+/H+ pump located along the cytoplasmic membrane and intracellular organic bases.25 26 27 Conceptually, the measurement of pHi can be considered a cytoplasmic marker of cellular function. The presence of a severe brain acidosis and the accumulation of lactic acid may determine in part the differences between selective neuronal injury and pannecrosis.28 29 Intracellular acidosis has the following postulated detrimental effects: (1) increasing glial edema with subsequent extravascular capillary-bed compression leading to reductions in potential collateral blood flow,27 30 31 (2) the inhibition of mitochondrial respiration,32 33 (3) the retardation of NADH regeneration,34 and (4) the promotion of free radical generation by facilitating the Haber-Weiss reaction.35 36 It should also be recognized, however, that there is some experimental evidence that suggests that acidosis might be neuroprotective by blocking the N-methyl-D-aspartate–gated calcium channel.37 38 These controversial data regarding the effects of brain acidosis during ischemia are reflected in the inconclusive results on the effects of hyperglycemia during focal ischemia. It has long been thought that hyperglycemia increases brain acidosis and injury through increased lactic acid production.39 40 However, the effects of hyperglycemia during focal cerebral ischemia are inconclusive with both an increase in, a decrease in, and no effect on infarction sizes being reported with hyperglycemia.41 42 43 Therefore, the significance of the observation that brief periods of intermittent reperfusion in this present experiment led to a worsening in acidosis during the ischemic period remains indeterminate.

The redox state of NAD/NADH is an assessment of mitochondrial function.44 45 The intergroup comparison in this experiment demonstrated no significant differences in NADH fluorescence during the final reperfusion. This would support the findings of Selman et al,46 who measured ATP levels in both Wistar and spontaneously hypertensive rats subjected to intermittent or single middle cerebral artery occlusion. Their metabolic data in the spontaneously hypertensive rats demonstrated no significant differences in ATP levels between the single and multiple occlusion groups. This would suggest that intermittent reperfusion did not prevent critical ATP depletion. It should be noted that, in their study, the intermittent reperfusion group had relatively preserved ATP levels in the ischemic core region, which would agree with the NAD/NADH data provided in the present experiment. Surprisingly, in their study this preservation of ATP did not attenuate the degree of histological injury.

A review of the current literature examining the effects of intermittent reperfusion in focal cerebral ischemia remains indeterminate in providing a definitive answer regarding the potential benefits or adverse effects of brief periods of reperfusion. For example, Sakaki et al47 studied the effects of three 20-minute episodes of middle cerebral artery occlusion compared with a straight 1-hour period of ischemia in cats. They noted that intermittent occlusion led to a reduction in the degree of pathological injury. Unfortunately, the time of the reperfusion periods was not specified. Steinberg and colleagues48 provided evidence that intermittent reperfusion led to a reduction in the degree of cortical neuronal injury in a rabbit model of focal cerebral ischemia. However, there was no difference in the degree of striatal ischemic injury. In a prior study from this laboratory,15 10-minute periods of ischemia separated by 5 minutes of reperfusion led to a significant reduction in infarct size when compared with single occlusion times of 60, 90, or 120 minutes. As indicated above, the more recent study by Selman and colleagues46 found no difference in histopathology and ATP, lactate, and glucose levels between single or intermittent ischemia. Ohtaki et al49 showed that there was no significant difference between intermittent and single ischemia. However, their model did not mimic the operative setting, since the ischemic times were three 1-hour episodes or a single 3-hour insult. Spetzler and colleagues,50 using a baboon model, demonstrated that short 10-minute periods of repeated occlusions were no different from single occlusions. Finally, the present report provides contradictory data on brain pHi and NADH fluorescence. An interesting observation in these experiments was that the PaCO2 response declined as the number of reperfusions increased. This could be explained in part by a weakening of vessel wall tone due to abrupt surges in perfusion pressure.

Accordingly, it may become evident that, at least in focal cerebral ischemia, there are no significant differences between intermittent or straight occlusion with total ischemic times that approximate the neurosurgical setting. It therefore may prove more valuable to examine the effects of potential neuroprotective agents within the experimental paradigms of single or intermittent reperfusion to better design techniques that provide intraoperative cerebral protection.


*    Selected Abbreviations and Acronyms
 
NADH = nicotinamide adenine dinucleotide
pHi = intracellular brain pH
rCBF = regional cerebral blood flow
rCtBF-UM = regional cortical blood flow–umbelliferone


*    Acknowledgments
 
This project was funded by National Institutes of Health grant RO1-25374. The authors thank Patricia Friedrich and Robert Carlson for their technical assistance and Mary Soper for preparation of the manuscript. Dr Regli was supported by Foundation Sicpa and Decker.


*    Footnotes
 
Reprint requests to Fredric B. Meyer, MD, Department of Neurosurgery, Mayo Clinic, 200 First St SW, Rochester, MN 55905.

Received September 22, 1994; revision received April 17, 1995; accepted May 10, 1995.


*    References
up arrowTop
 up arrowAbstract
 up arrowIntroduction
 up arrowMaterials and Methods
 up arrowResults
 up arrowDiscussion
 *References
 

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