(Stroke. 1997;28:1624-1630.)
© 1997 American Heart Association, Inc.
Articles |
Effects of the Allosteric Modification of Hemoglobin on Brain Oxygen and Infarct Size in a Feline Model of Stroke
From the Division of Neurosurgery and Department of Medicinal Chemistry (D.A.), Medical College of Virginia, West Hospital, Virginia Commonwealth University, Richmond.
Correspondence to M. Ross Bullock, MD, PhD, Division of Neurological Surgery, Medical College of Virginia, Virginia Commonwealth University, West Hospital, 8th Floor, Box 631, Richmond, VA 23298-0631. E-mail RBULLOCK{at}Gems.VCU.Edu
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Background and Purpose Cerebral ischemia and stroke are leading causes of morbidity and mortality. An approach to protecting the brain during ischemia is to try to increase the delivery of oxygen via the residual blood flow through and around ischemic tissue. To test this hypothesis, we used a novel oxygen delivery agent, RSR-13 (2-[4-[[(3,5-dimethylanilino)-carbonyl]-methyl]phenoxy]-2-methylpropionic acid). Intravenous administration of RSR-13 increases oxygen delivery through allosteric modification of the hemoglobin molecule, resulting in a shift in the hemoglobin/oxygen dissociation curve in favor of oxygen delivery.
Methods We studied RSR-13 in a feline model of permanent middle cerebral artery occlusion to assess its effects on cerebral oxygenation and infarct size. A randomized, blinded study of RSR-13 (n=6) versus 0.45% saline (n=12) was conducted, after an RSR-13 dose-escalation study (n=4). Drug was administered as a preocclusion bolus followed by a continuous infusion for the duration of the experiment (5 hours). Brain oxygen was measured continuously with the use of a Clark oxygen electrode. Infarct size was measured at 5 hours after occlusion with computer-assisted volumetric analysis.
Results The drug treatment group had consistently higher mean brain oxygen tension than controls (33±5 and 27±6 mm Hg, respectively) and significantly smaller infarcts (21±9% versus 33±9%, respectively; P<.008). We observed an inverse relationship between the dose response of RSR-13 (the shift in the hemoglobin/oxygen dissociation curve) and infarct size.
Conclusions These results are evidence that allosteric hemoglobin modification is protective to the brain after acute focal ischemia, providing a new opportunity for neuroprotection and raising the possibility of enhancing the protective effect of thrombolysis and ion channel blockade.
Key Words: cerebral ischemia • neuroprotection • hemoglobin • cats
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Introduction |
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The pharmacological agent RSR-13 (2-[4-[[(3,5-dimethylanilino)carbonyl]-methyl]phenoxy]-2-methylpropionic acid) is designed to increase tissue oxygen delivery by decreasing the affinity of hemoglobin for oxygen, effectively shifting the hemoglobin/oxygen dissociation curve to the right, similar to the effects of endogenous 2,3-DPG. RSR-13 was developed at the Medical College of Virginia from a search for a family of compounds that exploit this effect on hemoglobin for application in the treatment of sickle cell disease and as a radiosensitizer for cancer radiation therapy.1 2 3 4 5 Other potential applications of RSR-13 arose from its further experimental use. Administration of RSR-13 in vivo increased resting muscle tissue PO2 in mice.6 RSR-13 was found to have activity in the brain: hypoxia-induced vasodilatation was reversed with RSR-13 in a feline model.7 We subsequently hypothesized that RSR-13 might be clinically useful during brain ischemia.
Focal ischemia as seen in the MCA occlusion model in the cat creates a reproducible zone of infarction. The relative cerebroprotective effects of many agents have been tested in this model. Recently, we adapted a Clark electrode–based oxygen sensor for continuous measurement of brain oxygen tension in vivo. Using this technology, we have been able to measure brain oxygen during experimental ischemia. We have previously shown that the brain tissue PO2 was reduced in the area of infarction.8 Direct measurement of brain oxygen after administration of RSR-13 allows us to study the effects of RSR-13 on oxygen delivery in the ischemic brain. However, the Clark electrode measures only partial pressure that will indirectly reflect delivery and should not be confused with actual oxygen content (which is determined largely by blood flow and hemoglobin concentration as well as oxygen unloading). The true significance of brain tissue partial pressure of oxygen has yet to be defined in terms of the normal and pathological ranges since this is an evolving technology.
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Materials and Methods |
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All experiments were performed with approval of the institutional investigation review board and animal investigation review board. Twenty-two adult cats weighing 3 to 5 kg were studied. The animals were randomly assigned to receive RSR-13 or placebo. The team performing the experiments was kept blinded with respect to the drug versus placebo status of each animal during all parts of the study until the final data analysis. The animals were fasted overnight and then anesthetized with intravenous methohexital (Brevital) 10 mg/kg for endotracheal intubation by an animal care veterinarian. General anesthesia was then maintained with endotracheal halothane 1.0% to 2.0% for the duration of the experiment. Mechanical ventilation with a balanced oxygen and nitrogen mixture was used. A right femoral cutdown was performed, and the femoral artery and vein were each cannulated. Arterial blood pressure was monitored continuously with a standard oscilloscope and recorded continuously with the use of a computer using MacLab software (AD Instruments). Arterial blood gas samples were sent for pH, PO2, and PCO2 measurements. The ventilator settings were adjusted based on the arterial blood gas results to maintain normocapnia (PCO2 24 to 32 mm Hg), normoxia (PO2 100 to 200 mm Hg), and normal pH (7.32 to 7.42). The animals were then placed in a rigid Kopf head frame. Core body temperature was monitored with a rectal thermometer, and normothermia (36.5°C to 38°C) was maintained with an adjustable heating blanket.
Surgical Technique and MCA Occlusion
The left supraorbital skin and midline scalp were infiltrated
with 1 to 2 mL of 1% lidocaine and epinephrine (1:100 000
concentration). Permanent left MCA occlusion was performed as
previously described.8 After the 5-hour duration of the
experiment, the cat was killed with a bolus of potassium chloride. Its
brain was removed, chilled on dry ice (-20°C), and cut into
5-mm-thick coronal slices. The fresh brain slices were immersed in a
solution of 2,3,5-TTC at 37°C for 20 to 30 minutes. These slices were
then placed in 10% formalin for fixation and photographed.
Drug Dosing and Administration
Three dosing regimens were used: 2 cats received a bolus of
RSR-13 of 100 mg/kg with no continuous infusion; 2 received a
30-mg/kg bolus followed by a 40-mg/kg continuous infusion
given over 5 hours; and 2 animals received a 60-mg/kg bolus
followed by a 40-mg/kg infusion given over 5 hours. RSR-13 was
dissolved in sterile 0.45 NaCl. The bolus infusion was given over 15
minutes immediately before the occlusion, and the continuous infusion
was administered over 5 hours starting at occlusion. In the placebo
group, 0.45 NaCl was given for both the bolus and the continuous
infusion. The drug was prepared by an impartial assistant and was
administered in a blinded fashion in all cases. Four blood samples (2
mL each) were collected for hemoglobin/oxygen dissociation
analysis at the following intervals: before drug
administration; immediately after the bolus dose; at 2 hours after
bolus; and again at 5 hours after bolus.
Determination of p50 Values
The p50 value is the partial pressure of oxygen at which
hemoglobin is 50% saturated. This allows quantification of the
"shift" in the hemoglobin/oxygen saturation curve by measuring
the change in the p50 value (
p50) (Fig 1
). Four blood samples collected after
drug administration (see above) were analyzed in the Department
of Medicinal Chemistry.
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Brain Oxygen Measurement
Brain PO2 was measured with a
multiparameter sensor, as described previously by our
group, using the Paratrend 7 system (Pfizer Biochemical
Sensors).8 The sensor allows for simultaneous
measurement of tissue pH, PCO2, and temperature
along with PO2. Values for brain temperature,
pH, and PCO2 will be presented in this
report but are not the focus of this investigation.
Analysis of Infarct Size
Infarct volume was determined on the basis of TTC staining of
the fresh brain cut into 5-mm coronal sections (Fig 2). The slices were photographed, and the
pictures were scanned into a computer-based file. With computer
assistance, the infarcted region was outlined and then submitted to
volumetric analysis by a blinded technician using the MCID
image analyzer (Imaging Research Inc). Infarct volume was
expressed as total volume in cubic millimeters. To eliminate
variability in brain size between animals, the stroke volume was also
computed as a percentage of the left hemisphere volume. This percentage
was used for comparisons between groups.
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Statistical Analysis
Physiological parameters,
including mean blood pressure, core body temperature, and
arterial blood gas values, were compared between the two
groups with the use of a two-tailed t test with a
significance level of P<.05. Infarct size was also compared
with the two-tailed t test. Brain oxygen comparison was made
by means of nonparametric Mann-Whitney U
analysis.
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Results |
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Physiological Parameters
There were no significant differences between the saline control and RSR-13–treated groups with respect to blood pressure, core body temperature, brain temperature, or arterial blood gas parameters. Data are shown in the Table
, which compares values for blood
pressure, core and brain temperatures, and arterial blood
gas values between control and drug-treated animals at the intervals
before occlusion and at 1, 2, 3, 4, and 5 hours after occlusion. There
were no significant differences between the groups at any of these time
intervals except as indicated in the Table. The drop in brain
temperature relative to body temperature after MCA occlusion did not
differ significantly between control and RSR-13 animals at any time
point. Mild increase in the mean arterial pressure (5 to
10 mm Hg) occurred with the bolus injection. However, this was
transient and was seen in both control and RSR-13 animals. RSR-13 did
not appear to have any intrinsic effects on the measured
physiological parameters.
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Effect of RSR-13 on the Hemoglobin/Oxygen Dissociation
Curve
For 8 cats (7 drug treatment and 1 control), measurement of the
p50 values of hemoglobin was obtained. Actual data from the hemoglobin
analysis of a cat that received RSR-13 as a 60-mg/kg
bolus followed by a continuous infusion of 30 mg/kg per hour are
presented in Fig 3. This figure
demonstrates the method for calculating the right shift of the curve
(p50). Not only is the curve shifted to the right, but the slope is
slightly changed. The change in the p50 values was dose dependent (Fig 4). The largest shift was seen with the
100-mg/kg bolus, which gave a p50 value of 95 mm Hg. The
values seen at the 60-mg/kg dose were variable but were in
the desired range from 50 to 60 mm Hg.
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Limited data were obtained on the pharmacokinetics of RSR-13 in cats.
When a bolus-only dose was administered, we observed an apparent loss
of effect (in one animal given a 100-mg/kg bolus) (Fig 4
) by 2
hours after bolus. This drop in response was attenuated by the use of
the continuous infusion (Fig 4).
Brain Oxygen
Brain oxygen, carbon dioxide, and pH data were available in 6 of
the 10 RSR-13–treated animals and in 7 control animals. In the 9
animals in which these data were not available, technical problems were
responsible in 5, sensors were not available for 2, and probe
malposition, discovered after brain removal, occurred in 2. Results of
the multiparameter sensor measurements are
presented in Fig 5. Predrug brain
oxygen tension was the same for the two groups. After administration of
RSR-13, the mean±SD oxygen measurements were higher in the drug
treatment group versus control, at 33±5 versus 27±6 mm Hg,
respectively, although this did not reach statistical significance.
This trend was unrelated to arterial oxygen tension. As is
demonstrated in Fig 5, there were no consistent differences
with regard to the pH or carbon dioxide data.
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Effects of RSR-13 on Infarct Size
The control MCA occlusion animals (n=12) developed infarcts of
approximately one third of the left hemisphere (33±9% [±SD]). The
RSR-13–treated animals (n=10) had mean infarcts of 21±9%. This
difference was statistically significant (P<.008,
two-tailed t test). This is shown graphically in Fig 6.
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When the biological effect of the drug was known, ie, the p50, a
relationship between drug response and infarct size was seen. The
infarct size varied inversely with the amount of right shift of the
hemoglobin/oxygen dissociation curve (p50). This relationship is
demonstrated in Fig 7. There were two
large infarcts in the RSR-13 60/40 group, whose sizes were on the order
of control infarcts. Of these two animals, the change in p50 is known
for one, and there was very little change in the hemoglobin/oxygen
dissociation (p50=10). Unfortunately, the p50 of the other animal
in the RSR-13 group with a large infarct was not measured.
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Discussion |
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Since a major consequence of focal ischemia is decreased oxygen delivery, we hypothesized that increasing oxygen delivery to the brain before and during ischemia might protect the brain from infarction. Our data demonstrate that pretreatment with RSR-13 decreases the infarct volume seen with MCA occlusion in the cat. This protection appears to be related to the amount of right shift of the hemoglobin/oxygen dissociation curve, implying that oxygen unloading, ie, delivery, is the mechanism of its protection.
We believe that reporting the infarct size as a percentage of the left hemisphere is a valid method for comparing groups with respect to ischemic damage. Other authors have used infarct volume in cubic millimeters, but this fails to correct for differences in the brain sizes between animals which, in our experience with the cats, may be as much as 1 cm in length. Others have believed it necessary to correct for the contribution of cerebral edema. However, this is only necessary when gross morphological change in cerebral volume from edema is present. After only 5 hours of ischemia, significant edema was not present. Furthermore, correction for edema is not necessary for comparison between groups, but we acknowledge that the contribution of edema may play a part in the difference in infarct size observed between the control and RSR-13 groups.
Other investigators have studied the effects of oxygen delivery on cerebral protection. An attempt at increasing oxygen delivery to patients with severe head injury with hyperbaric oxygen was examined in a prospective randomized study.9 The authors found a decrease in mortality in the treatment group but no improvement in outcome of the survivors. Another strategy has been the use of PFCs. PFCs are oxygen-carrying compounds that can substitute in that capacity for hemoglobin. Kontos et al7 10 showed that cerebrospinal fluid perfusion of PFCs, much like RSR-13, could reverse cerebral vasodilatation from hypoxia with the cat pial window technique. Subsequently, Peerless et al11 demonstrated reduction in cerebral infarct using an intravenous PFC, Fluosol-DA, in a cat MCA occlusion model. Sakas et al12 have recently shown reduction in cerebral infarct size in animal models of ischemia using "new generation" intravenous PFCs. These results, along with our findings using RSR-13, suggest that increasing cerebral oxygen during ischemia protects the brain, presumably by providing more oxygen to the region of the ischemic penumbra.
Although we did observe higher average brain oxygen tension in the RSR-13–treated animals in the region of ischemia, this difference failed to reach statistical significance. Perhaps this is a result of the small sample of animals in which the data were available (n=13), which decreased our statistical power. A plausible explanation relates to the placement of the oxygen sensor. The probes were inserted in the core of the infarct. Temperature data as well as pH and PCO2 data are consistent with values recorded from the site of infarction, as previously shown by Zauner et al.8 Since RSR-13 can only exert its effects in regions of some preserved cerebral blood flow, one would not expect to see a profound effect of RSR-13 on brain oxygen in the center of the ischemic territory. The ideal region in which to monitor the tissue oxygen is the border zone of ischemia, which represents the ischemic penumbra. Because of the length of the Clark electrode (5 mm) and the variability of infarct distribution, this was not technically feasible. Measurements were limited by lack of direct visualization of electrode placement, and in two animals the sensor appeared to be outside of the ischemic zone by TTC staining (these measurements were excluded from our analysis). Optimization of oxygen sensor design (ie, reduction in length of the sensor tip from 25 to 10 mm) and placement (eg, stereotaxic guidance) are goals for future studies with this new technique to reduce these technical limitations.
It is possible that RSR-13 affected infarct size through a mechanism
other than oxygen unloading. There were no consistent
differences in blood pressure between the RSR-13– and placebo-treated
animals. A difference in mean arterial pressure observed at
the 4-hour interval between RSR-13 animals and controls did reach
statistical significance (Table). However, the actual difference was
small (10 mm Hg), and the blood pressure was lower in the drug
treatment group, which does not explain protection during
ischemia. Effects on the microcirculation of the brain are
unclear. RSR-13 did not affect brain temperature compared with the
control group. Fluid status was similar in the two groups. The
experimental protocol controlled for differences in volume per kilogram
of crystalloid administered to each group. We did not anticipate any
differential effects on the hematocrit with RSR-13, but this was not
measured. Other factors, such as systemic acidosis or pH and
PCO2 of the brain, were not different between
the groups (Fig 5).
The study was not designed to evaluate the biochemistry of RSR-13 in
the cat. However, we were able to make several observations concerning
the pharmacology of RSR-13 in felines. We observed a dose-related
increase in the right shift of the oxygen dissociation curve of
hemoglobin reported as the change in p50. Furthermore, just as is seen
with 2,3-DPG, not only is the curve shifted to the right but the slope
is changed (depressed), as shown in Fig 3. This property, referred to
as cooperativity, is also a result of the conformational change that
decreases the affinity for oxygen. It is important to note that with
RSR-13, the p50 alone does not explain the change in oxygen
affinity. However, the p50 allows us to roughly quantify this change
for comparison between animals. Although the dose-response relationship
of RSR-13 and infarct size suggests that the mechanism of brain
protection is through a change in oxygen affinity from the allosteric
modification of hemoglobin, statistical validation of this relationship
is lacking (r=.7, P=.18). p50 may provide a
surrogate marker of adequate dosing and biological effect. Such
measurements are obtainable in humans and may facilitate optimal drug
dosing. In our experiments, it is doubtful that any suggested
dose-response relationship would have been observed with this small
number of animals if the hemoglobin/oxygen assay had not been
available.
A finite limit exists for the amount of p50 that is desirable. If
the curve is right-shifted too much, deoxyhemoglobin predominates, and
it becomes impossible to saturate the hemoglobin in the lungs. This
would result in decreased oxygen delivery. For example, the highest p50
value, seen after the 100-mg/kg dose of RSR-13, was 95
mm Hg. Since the normal range of human arterial
PO2 is 95±5 mm Hg at sea level, the
hemoglobin would only be 50% saturated at room air. With a high
inspired concentration of oxygen, as is readily available in the
hospital setting, this is less of an issue. This was the case with our
experiments in which the inspired oxygen was high. However, in a
patient with compromised gas exchange and low resting oxygen saturation
due to lung disease or chest trauma, significant right shifts of the
curve would not be well tolerated. Interestingly, the body's natural
response to cope with the lower PO2 of altitude
is to increase 2,3-DPG and right-shift the oxygen dissociation curve of
hemoglobin to increase oxygen delivery. In effect, RSR-13 allows us to
use this natural response to attempt to ameliorate "unnatural"
states of hypoxia (ischemia).
A theoretical drawback to increasing oxygen delivery to ischemic brain may be the generation of oxygen free radicals. As a result of their evanescent nature, there is no way to directly measure free radicals in tissue. Indirect measurement of free radical production is possible, and this is the focus of current experiments with RSR-13 in our laboratory. Agardh et al13 used an aminotriazole/catalase method to indirectly measure hydrogen peroxide production in postischemic rat brain in hypoxic, normoxic, and hyperoxic conditions. Their findings did not show any increased ischemic damage from the increased oxygen. It is unclear whether increasing oxygen will have an adverse effect on the brain, but there was no evidence of this based on our data.
Pretreatment with medications that protect the brain has clinical applications, such as temporary carotid occlusion during endarterectomy, temporary parent vessel occlusion during aneurysm surgery, and high-risk cardiac and aortic root surgeries. However, the ability to protect the brain after an insult has occurred greatly expands the usefulness of a pharmacological neuroprotectant. Recent studies have shown that a significant time window is present after stroke during which brain injury may be reduced with pharmacological intervention. This has been most clearly shown with thrombolysis with the use of recombinant tissue plasminogen activator, which reduces the magnitude of acute occlusive stroke when given within 3 hours of the ictus.14 This 3-hour window appears to be significant for another thrombolytic agent, streptokinase.15 16 We hypothesize that the major benefit of enhanced oxygen delivery may be seen when agents such as RSR-13 are used in combination with other techniques, such as thrombolysis and excitatory amino acid antagonists. Another potential application of RSR-13 is the treatment of ongoing ischemia, such as after severe head injury and during cerebral vasospasm after subarachnoid hemorrhage. Experience with brain oxygen measurements after head injury, in both experimental settings and preliminary clinical human data, has shown a derangement in brain oxygen tension.17 18
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Selected Abbreviations and Acronyms |
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Acknowledgments |
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This study was supported by National Institutes of Health grant 12587. We are grateful to Mary Lee Giebel for performing the volumetric image analysis and to Dr Xiao Di for RSR-13/placebo preparation. We also acknowledge Allos Therapeutics, Inc, Denver, Colo, the manufacturers of RSR-13, and Michael Gerber, MD, for providing RSR-13 and technical assistance with hemoglobin analysis.
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Footnotes |
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Reviews of this manuscript were directed by Richard J. Traystman, PhD.
Received December 17, 1996; revision received May 22, 1997; accepted May 22, 1997.
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References |
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- Randad RS, Mahran MA, Mehanna AS, Abraham DJ. Allosteric modifiers of hemoglobin, I: design, synthesis, testing, and structure-allosteric activity relationship of novel hemoglobin oxygen affinity decreasing agents. J Med Chem. 1991;34:752-757.[Medline] [Order article via Infotrieve]
- Abraham DJ, Wireko FC, Randad RS, Poyart C, Kister J, Bohn B, Liard JF, Kunert MP. Allosteric modifiers of hemoglobin: 2-[4-[[(3,5-disubstituted anilino)carbonyl]methyl]phenoxy]-2-methylpropionic acid derivatives that lower the oxygen affinity of hemoglobin in red cell suspensions, in whole blood, and in vivo in rats. Biochemistry. 1992;31:9141-9149.[Medline] [Order article via Infotrieve]
- Abraham DJ, Mehana AS, Williams FL. Design, synthesis, and testing of potential antisickling reagents, I: halogenated benzyloxy and phenoxy acids. J Med Chem. 1982;25:1015-1017.[Medline] [Order article via Infotrieve]
- Khandelwal SR, Lin PS, Hall CE, Truong QT, Lu J, Laurent JJ, Joshi GS, Abraham DJ, Schmidt-Ullrich RK. Increased radiation response of FsaII fibrosarcomas in C3H mice following administration of an allosteric effector of hemoglobin-oxygen affinity. Radiat Oncol Invest. 1996;4:51-59.
- Teicher BA, Ara G, Emi Y, Kakeji Y, Ikebe M, Maehara Y, Buxton B. RSR-13: effects of tumor oxygenation and response to therapy. Drug Dev Res.. 1996;38:1-11.
-
Khandelwal SR, Randad RS, Lin P, Meng H, Pittman RN,
Kontos HA, Choi SC, Abraham DJ, Schmidt-Ullrich R. Enhanced
oxygenation in vivo by allosteric
inhibitors of hemoglobin saturation. Am
J Physiol. 1993;265:H1450-H1453.
[Abstract/Free Full Text] -
Wei EP, Randad RS, Levasseur JE, Abraham DJ, Kontos
HA. Effect of local change in O2 saturation of
hemoglobin on cerebral vasodilation from hypoxia and
hypotension. Am J Physiol. 1993;265:H1439-H1443.
[Abstract/Free Full Text] - Zauner A, Bullock R, Di X, Young HF. Brain oxygen, CO2, pH, and temperature monitoring: evaluation in the feline brain. Neurosurgery. 1995;37:1168-1177.[Medline] [Order article via Infotrieve]
- Rockswold GL, Ford SE, Anderson DC, Bergman TA, Sherman RE. Results of a prospective randomized trial for treatment of severely brain-injured patients with hyperbaric oxygen. J Neurosurg. 1992;76:929-934.[Medline] [Order article via Infotrieve]
- Kontos HA, Wei EP, Raper AJ, Rosenblum WI, Navari RM, Patterson JL Jr. Role of tissue hypoxia in local regulation of cerebral microcirculation. Am J Physiol. 1978;234:H582-H591.
-
Peerless SJ, Ishikawa R, Hunter IG, Peerless
MJ. Protective effect of Fluosol-DA in acute cerebral
ischemia. Stroke. 1981;12:558-563.
[Abstract/Free Full Text] - Sakas DE, Whittaker KW, Crowell RM, Zervas NT. Perfluorocarbons: recent developments and implications for neurosurgery. J Neurosurg. 1996;85:248-254.[Medline] [Order article via Infotrieve]
- Agardh CD, Zhang H, Smith ML, Siesjo BK. Free radical production and ischemic brain damage: influence of postischemic oxygen tension. Int J Dev Neurosci. 1991;9:127-138.[Medline] [Order article via Infotrieve]
-
The National Institute of Neurological Disorders and
Stroke r-TPA Stroke Study Group. Tissue plasminogen
activator for acute ischemic stroke. N
Engl J Med. 1995;333:1581-1587.
[Abstract/Free Full Text] - Donnan GA, Davis SM, Chambers BR, Gates PC, Hankey GJ, McNeil JJ, Rosen D, Stewart-Wynne EG, Tuck RR. Streptokinase for acute ischemic stroke with relationship to time of administration. JAMA. 1996;276:961-966.[Abstract]
-
The Multicenter Acute Stroke Trial–Europe Study Group.
Thrombolytic therapy with streptokinase in acute
ischemic stroke. N Engl J Med. 1996;335:145-150.
[Abstract/Free Full Text] - Maas AIR, Fleckenstein W, de Jong DA, van Santbrink H. Monitoring cerebral oxygenation: experimental studies and preliminary clinical results of continuous monitoring of cerebrospinal fluid and brain tissue oxygen tension. Acta Neurochir (Wien).. 1993;59:50-57.
- Schmoker JD, Shackford SR, Zhuang J. Hemorrhagic hypotension after brain injury causes an early and sustained reduction in cerebral oxygen delivery despite normalization of systemic arterial oxygen delivery. J Trauma. 1992;32:714-720.[Medline] [Order article via Infotrieve]
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Department of Anesthesiology, Johns Hopkins Hospital, Baltimore, Md
In the foregoing article the authors report the effect of a compound injected intravenously that allosterically alters the oxygen affinity of hemoglobin in blood in a focal ischemia model in cats. Compounds like RSR-13 shift the oxygen affinity curve to the right as measured in vitro. This is commonly reported as a change in P50, the partial pressure in which the hemoglobin is 50% saturated. A shift to the right would make the hemoglobin molecule more likely to release oxygen at a higher partial pressure. This has the theoretical advantage of unloading more oxygen at the tissue level. Another effect of RSR-13 as seen in this study is a change in the slope of the oxygen affinity curve. The slope of this curve is an indication of cooperativity of binding of hemoglobin for oxygen. As one or more oxygen molecules is either loaded or unloaded from hemoglobin, cooperativity between the protein subunits allows easier binding or unbinding of subsequent oxygen molecules. A flatter slope of the oxygen affinity curve could hamper the release of oxygen at the tissue level and make it more difficult (require higher partial pressure of oxygen) to saturate hemoglobin in the lung.
The major finding of the present work is reduction in infarction volume when RSR-13 is given to animals that have had occlusion of the middle cerebral artery. In an attempt to determine whether the more favorable oxygen dissociation characteristics of altered hemoglobin are contributing to the reduction in infarction volume, the authors measure PO2, PCO2, and pH at the site of injury with a multiparameter probe. Despite a tendency for higher oxygen tension in the RSR-13–treated group, the difference fails to reach significance. This may be the result of too few animals to demonstrate what is a small effect or, as indicated by the large standard deviation, a matter of probe placement in the infarcted region and too large a probe to sample tissue PO2 per se. However, it appears that the probe was reasonably placed, judging from the expected changes in PCO2 and pH during brain ischemia. More importantly, the failure to raise tissue PO2 may be the result of incomplete saturation of arterial hemoglobin in the RSR-13 group. It takes approximately four times the P50 in partial presssure of oxygen to achieve 96% to 100% saturation of hemoglobin. For P50 in the 50–mm Hg range, an arterial PO2 of 200 mm Hg would be expected to be necessary for full saturation of hemoglobin, more than reported for the RSR-13 group. And finally, any beneficial effect of a right-shifted P50 on oxygen unloading may be counteracted by the change in cooperativity that results in a finite ability of compounds like RSR-13 to impact on oxygen unloading.
Oxygen delivery to brain is highly regulated and has several determinants other than oxygen affinity of hemoglobin. Blood-borne factors, such as viscosity (affected mostly by the red cell mass) and oxygen content, and differences in tissue metabolism are important in control of blood flow and hence oxygen delivery. Subsequent studies measuring hematocrit, arterial oxygen content, blood flow, and an estimate of the effects of RSR-13 on these and cerebral metabolism will be necessary to fully determine whether increased oxygen delivery is the cause of reduced infarct volume.
Revascularization with thrombolytic agents will likely benefit only a subset of patients suffering from stroke. Chemical modification of hemoglobin in vivo may provide an exciting alternative in the treatment of stroke in humans by enhancing oxygen delivery and reducing penumbral injury. There appears to be little systemic effect of short-term infusion of an allosteric modifier of hemoglobin on blood pressure and arterial blood gases, an attractive characteristic for treating acutely ill patients. Furthermore, if only modest increases in P50 are needed clinically, it is likely that patients can be safely treated despite the increased inspired oxygen concentration needed to maintain arterial hemoglobin oxygen saturation. However, the impact of potential oxygen radical formation in tissue and pulmonary oxygen toxicity must be addressed with outcome studies prior to use of these compounds in humans.
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M. S. Wainwright, H. Sheng, Y. Sato, G. Burkhard Mackensen, R. P. Steffen, R. D. Pearlstein, and D. S. Warner Pharmacological correction of hypothermic P50 shift does not alter outcome from focal cerebral ischemia in rats Am J Physiol Heart Circ Physiol, May 1, 2002; 282(5): H1863 - H1870. [Abstract] [Full Text] [PDF]
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 J. A. Wahr, M. Gerber, J. Venitz, and N. Baliga Allosteric Modification of Oxygen Delivery by Hemoglobin Anesth. Analg., March 1, 2001; 92(3): 615 - 620. [Abstract] [Full Text] [PDF]
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 K. S. Kilgore, C. F. Shwartz, M. A. Gallagher, R. P. Steffen, R. S. Mosca, and S. F. Bolling RSR13, a Synthetic Allosteric Modifier of Hemoglobin, Improves Myocardial Recovery Following Hypothermic Cardiopulmonary Bypass Circulation, November 9, 1999; 100(90002): II-351 - 356. [Abstract] [Full Text] [PDF]
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 H. P. Grocott, R. D. Bart, H. Sheng, Y. Miura, R. Steffen, R. D. Pearlstein, D. S. Warner, and E. P. Wei Effects of a Synthetic Allosteric Modifier of Hemoglobin Oxygen Affinity on Outcome From Global Cerebral Ischemia in the Rat • Editorial Comment Stroke, August 1, 1998; 29(8): 1650 - 1655. [Abstract] [Full Text] [PDF]
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