Brain Advance Access originally published online on April 27, 2007
Brain 2007 130(6):1631-1642; doi:10.1093/brain/awm071
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Normobaric hyperoxia improves cerebral blood flow and oxygenation, and inhibits peri-infarct depolarizations in experimental focal ischaemia
1Stroke and Neurovascular Regulation Laboratory, Department of Radiology, Massachusetts General Hospital, Harvard Medical School, Charlestown, MA 02129, 2Biomedical Engineering Department, University of Texas at Austin, Austin, TX 78712, 3Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Harvard Medical School, Charlestown, MA 02129, 4Neuroprotection Research Laboratory, Department of Radiology, Massachusetts General Hospital, Harvard Medical School, Charlestown, MA 02129 and 5Stroke Service and Neuroscience Intensive Care Unit, Department of Neurology, Massachusetts General Hospital, Harvard Medical School, Boston, MA 02114, USA
Correspondence to: Cenk Ayata, Stroke and Neurovascular Regulation Laboratory, Massachusetts General Hospital, 149 13th Street, Room 6403, Charlestown, MA 02129, USA E-mail: cayata{at}partners.org
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Normobaric hyperoxia is under investigation as a treatment for acute ischaemic stroke. In experimental models, normobaric hyperoxia reduces cerebral ischaemic injury and improves functional outcome. The mechanisms of neuroprotection are still debated because, (i) inhalation of 100% O2 does not significantly increase total blood O2 content; (ii) it is not known whether normobaric hyperoxia increases O2 delivery to the severely ischaemic cortex because of its short diffusion distance; and (iii) hyperoxia may reduce collateral cerebral blood flow (CBF) to ischaemic penumbra because it can cause vasoconstriction. We addressed these issues using real-time two-dimensional multispectral reflectance imaging and laser speckle flowmetry to simultaneously and non-invasively determine the impact of normobaric hyperoxia on CBF and oxygenation in ischaemic cortex. Ischaemia was induced by distal middle cerebral artery occlusion (dMCAO) in normoxic (30% inhaled O2, arterial pO2 134 ± 9 mmHg), or hyperoxic mice (100% inhaled O2 starting 15 min after dMCAO, arterial pO2 312 ± 10 mmHg). Post-ischaemic normobaric hyperoxia caused an immediate and progressive increase in oxyhaemoglobin (oxyHb) concentration, nearly doubling it in ischaemic core within 60 min. In addition, hyperoxia improved CBF so that the area of cortex with
20% residual CBF was decreased by 45% 60 min after dMCAO. Furthermore, hyperoxia reduced the frequency of peri-infarct depolarizations (PIDs) by more than 60%, and diminished their deleterious effects on CBF and metabolic load. Consistent with these findings, infarct size was reduced by 45% in the hyperoxia group 2 days after 75 min transient dMCAO. Our data show that normobaric hyperoxia increases tissue O2 delivery, and that novel mechanisms such as CBF augmentation, and suppression of PIDs may afford neuroprotection during hyperoxia.
Key Words: neuroprotection; laser speckle flowmetry; multispectral reflectance imaging; middle cerebral artery occlusion; acute stroke
Abbreviations: CBF, cerebral blood flow; dMCAO, distal middle cerebral artery occlusion; PID, peri-infarct depolarization
Received October 11, 2006. Revised January 23, 2007. Accepted March 12, 2007.
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Introduction |
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Increasing the partial pressure of O2 in inspired air may be an effective therapeutic option in acute stroke. Earlier studies have focused on hyperbaric hyperoxia; however, its use has been hampered by limited access to a hyperbaric chamber in acute stroke, and potential side effects (Auer, 2001
). Normobaric hyperoxia is an alternative strategy that is generally well-tolerated with fewer potential side effects (e.g. loss of pulmonary surfactant) when administered for <24 h (Matalon et al., 1982; Royston et al., 1990; Carvalho et al., 1998; Brock and Di Giulio, 2006); it is readily available, inexpensive and can be initiated by emergency medical personnel within minutes after stroke symptom onset. Although normobaric hyperoxia reduced infarct size in all experimental stroke studies (Flynn and Auer, 2002; Singhal et al., 2002a, b; Kim et al., 2005), human experience has been limited. In one study, subjects with severe strokes did not benefit from hyperoxia, and those with mild or moderate stroke had a worse outcome (Ronning and Guldvog, 1999); however, low doses of oxygen were administered (3 l/min via nasal catheter) and the onset of treatment was relatively late (<24 h). The results of a subsequent preliminary human series were promising, where high-flow O2 therapy was associated with a transient improvement in neurological deficits and lesion volume on diffusion-weighted MRI in select patients with acute ischaemic stroke (Singhal et al., 2005). Reduced morbidity and improved survival was recently reported with 40% O2 in patients with large MCA strokes (Chiu et al., 2006). However, O2 therapy in stroke has been criticized citing the risk of enhanced free radical generation, blood–brain barrier breakdown, inflammation and haemorrhage into the infarct. Although some of these concerns have now been allayed (Singhal et al., 2002b; Kim et al., 2005; Liu et al., 2006), two issues remain outstanding. First, increasing the fraction of O2 in inspired air does not proportionally increase blood O2 content. This is because under physiological conditions haemoglobin is already saturated by more than 96% when breathing room air, and the direct solubility of O2 in blood as well as its diffusion distance in brain are limited. Hence, it is not known whether normobaric hyperoxia significantly impacts O2 delivery to the ischaemic cortex. Secondly, hyperoxia causes vasoconstriction in normal brain (Watson et al., 2000), and thus may impact collateral cerebral blood flow (CBF) supply to ischaemic cortex.
We addressed these questions by measuring changes in haemoglobin oxygenation and CBF in ischaemic cortex through intact skull, using real time simultaneous multispectral reflectance imaging and laser speckle flowmetry (Ruth, 1990; Dunn et al., 2003; Ayata et al., 2004). Our data suggest that normobaric hyperoxia improves both cerebral oxygenation and blood flow, in part by suppressing peri-infarct depolarizations (PIDs).
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Material and methods |
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General surgical preparation
Mice (C57BL/6J, 23–28 g) were housed under diurnal lighting conditions and allowed food and tap water ad libitum. Mice were anaesthetized with 2% isoflurane (in 70% N2O and 30% O2) and intubated via a tracheostomy. Femoral artery was catheterized for the measurement of blood pressure (BP; ETH 400 transducer amplifier). Anaesthesia was maintained by 1% isoflurane, the depth of anaesthesia was checked by the absence of cardiovascular changes in response to tail pinch. Rectal temperature was kept at 36.8–37.1°C using a thermostatically controlled heating mat (FHC, Brunswick, ME). Mice were paralysed (pancuronium bromide, 0.4 mg/kg/h, i.p.), mechanically ventilated (CWE, SAR-830, Ardmore, PA) and placed in a stereotaxic frame (David Kopf, Tujunga, CA). Arterial blood gases and pH were measured every 20 min in 25 µl samples (Corning 178 blood gas/pH analyzer, Ciba Corning Diagnostics, Medford, MA). The data were continuously recorded using a data acquisition and analysis system (PowerLab, AD Instruments, Medford, MA) and stored in a computer. Mice were allowed to stabilize for 30 min after surgical preparation.
Focal cerebral ischaemia
Focal cerebral ischaemia was induced by distal middle cerebral artery occlusion (dMCAO). After general surgical preparation, mice were placed in a stereotaxic frame, and skull surface was prepared for optical imaging as described earlier (Ayata et al., 2004; Shin et al., 2006). The temporalis muscle was separated from the temporal bone and removed. A burr hole (2 mm diameter) was drilled under saline cooling in the temporal bone overlying the MCA just above the zygomatic arch. The dura was kept intact and MCA was occluded using a microvascular clip.
Imaging
Multispectral reflectance imaging was performed as described previously in detail (Dunn et al., 2003). Briefly, light from a halogen fibre optic illuminator (Techniquip R150, Capra Optical, Natick, MA) was passed through six different 10-nm-wide bandpass filters raging from 560 to 610 nm, placed on a six-position filter wheel (Thorlabs, Newton, NJ), rotating continuously at 3–5 revolutions per second. The filtered light was then coupled into a 12-mm-diameter fibre optic bundle (Edmund Scientific, Tonawanda, NY) for illumination of the cortex. Images were acquired at each illumination band sequentially, captured using a variable magnification objective (x 0.75 to x 3, Edmund Optics, Barrington, NJ) and focused either through (infrared laser) or reflected off of (visible light) a dichroic mirror onto two CCD cameras (Coolsnap fx, Roper Scientific 1300 x 1030 pixels, for multispectral; Cohu 4600, San Diego, CA, 640 x 480 pixels, for speckle). The final image size for multispectral imaging was 433 x 343 pixels, after 3 x 3 binning. Raw multispectral data was collected in sequences of 30 frames at 10 Hz (5 frames/wavelength), and a 5s delay was added to acquire one sequence approximately every 7.5 s. The reflectance image from each wavelength was averaged over the sequence.
Each set of multispectral images was converted to changes in haemoglobin oxygenation and volume using a least squares fitting procedure based on a Monte Carlo model of light propagation in tissue. This approach was used rather than the traditional modified Beer Lambert relationship, which has been shown to be inaccurate for large haemoglobin concentration changes observed during cerebral ischaemia. Briefly, the difference between the intensity changes predicted by the Monte Carlo model for a given set of optical properties (absorption and scattering coefficients) and the measured intensity changes at each time point for all six wavelengths was minimized. The fitting parameters were the absorption and scattering coefficients of the tissue. OxyHb and deoxyHb were assumed to be the only chromophores in the tissue at these wavelengths such that the absorption coefficient was given by:
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is the molar extinction coefficient and C is the concentration of each chromophore. Pre-ischaemic baseline concentrations were assumed to be 
= 60 µM and 
= 40 µM.
Laser speckle flowmetry (LSF) was used to study the spatiotemporal characteristics of CBF changes during focal cerebral ischaemia. The technique for LSF has been described in detail elsewhere (Dunn et al., 2001; Ayata et al., 2004). Briefly, a CCD camera (Cohu, San Diego, CA) was positioned above the head, and a laser diode (780 nm) was used to illuminate the intact skull surface in a diffuse manner. The penetration depth of the laser is 
500 µm. Raw speckle images were used to compute speckle contrast, which is a measure of speckle visibility related to the velocity of the scattering particles, and therefore CBF. The speckle contrast is defined as the ratio of the standard deviation of pixel intensities to the mean pixel intensity in a small region of the image (Briers, 2001). Ten consecutive raw speckle images were acquired at 15 Hz (an image set), processed by computing the speckle contrast using a sliding grid of 7 x 7 pixels, and averaged to improve signal-to-noise ratio. Speckle contrast images were converted to images of correlation time values, which represent the decay time of the light intensity autocorrelation function. The correlation time is inversely and linearly proportional to the mean blood velocity (Briers, 2001). Relative CBF images (percentage of baseline) were calculated by computing the ratio of a baseline image of correlation time values to subsequent images. Laser speckle perfusion images were obtained every 7.5 s. Multispectral reflectance and laser speckle images were co-registered using visible surface landmarks, to ensure complete spatial overlap of regions of interest for analysis.
Image analysis
Images were analysed using three different methods (Ayata et al., 2004) to assess the impact of hyperoxia spatiotemporally:
- Based on the severity of CBF reduction during the first minute of dMCAO, three cortical regions of interest (ROIs, 250 x 250 µm) were manually selected corresponding to core (centre of severe CBF reduction), haemodynamic penumbra (steep portion of CBF gradient between core and non-ischaemic cortex) and non-ischaemic cortex. Haemoglobin oxygenation and CBF changes within these ROIs were recorded over time expressed as % of pre-ischaemic baseline.
- Ischaemic CBF deficit was analysed two-dimensionally over time by quantifying the area of cortex (mm2) with either severe (0–20% residual CBF, representing core) or moderate CBF reduction (21–30% representing penumbra) using a thresholding paradigm.
- In order to determine the gradient of CBF reduction between non-ischaemic cortex and core, we quantified the CBF (% of baseline) along a profile drawn between lambda (0 mm) and the centre of severely ischaemic core.
Peri-infarct depolarizations (PIDs) were identified by observing the attendant spreading CBF and oxygenation changes, which have been previously shown by us and others to reliably detect PIDs in focal ischaemia (Shin et al., 2006; Strong et al., 2006). The impact of PIDs on CBF deficit was two-dimensionally determined by calculating the change in area of severely hypoperfused cortex from 1–5 min before a PID (Pre1) to 5–10 min after that PID (Post1), and comparing this change to that from Post1 to 1–5 min before the next PID (Pre2). This was then repeated for each subsequent PID, and the average change from Pre to Post (i.e. change with an interval PID) was statistically compared to the average change from Post to Pre (i.e. change without an interval PID), as previously described in detail (Shin et al., 2006).
Experimental protocol
Multimodal imaging of CBF and oxygenation was started 5 min prior to dMCAO and continued uninterrupted for 60 min after the onset or discontinuation of normobaric hyperoxia (75–105 min). The normoxia group was maintained on 30% oxygen, whereas in hyperoxia groups, the fraction of oxygen in inspired air was increased to 100% at 15 (Experiment I) or 45 min after dMCAO (Experiment II). In a third group, normobaric hyperoxia was instituted 15 min after dMCAO and discontinued at 45 min (Experiment III). Mean arterial blood pressure, pH and pCO2 were monitored and maintained within normal limits in all groups (Table 1).
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Infarct volume
The impact of hyperoxia on infarct volume was determined in a separate group of mice without intubation or mechanical ventilation, since these surgical procedures significantly increased morbidity and mortality during the recovery period. To determine infarct volume, microvascular clip was carefully removed after 75 min of dMCAO, and successful reperfusion confirmed using LSF; animals without reperfusion or with haemorrhage during clip removal were excluded from the study. As in the imaging protocol above, normoxic mice (n = 6) received 30% O2 throughout 75 min of ischaemia, whereas hyperoxic mice (n = 5) received 100% O2 starting 15 min after dMCAO. Following clip removal, both groups were maintained breathing room air. Mice were sacrificed 48 h after ischaemia and whole brains were incubated in TTC for 60 min. After photographing the hemispheric surface for measurements of the surface area of infarct, brains were cut into 1 mm thick coronal sections, and infarct area at each section was measured and integrated along the anteroposterior axis to calculate infarct volume. Because infarcts are relatively small in this dMCAO model compared to filament MCAO (fMCAO), only direct infarct volume was compared between groups.
Data analysis
The data were expressed as mean ± standard error of mean (SEM). Statistical comparisons were done using paired or unpaired Student's t-test, Mann–Whitney Rank Sum test and one-way ANOVA or two-way ANOVA for repeated measures followed by Fisher's protected least significant difference test, except when otherwise specified. P < 0.05 was considered statistically significant.
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Results |
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Cerebral oxygenation
OxyHb and total Hb concentrations abruptly decreased to 20–25 and 60% of baseline in ischaemic core, respectively, immediately after dMCAO, and remained unchanged in normoxic control group throughout the imaging period (Fig. 1). As expected, there was an increase in deoxyHb concentration in both core and penumbra (data not shown). In hyperoxia group, changing the inspired air to 100% O2 15 min after dMCAO (Experiment I) increased oxyHb within 10 min, reaching a plateau within 20 min. Sixty minutes after the onset of hyperoxia oxyHb was
45% of pre-ischaemic baseline, compared to 25% in normoxic group (P < 0.01, two-way ANOVA for repeated measures; Fig. 1A). Hyperoxia did not change total Hb concentration in ischaemic core (Fig. 1B), suggesting that the increase in oxyHb during hyperoxia reflects increased O2 saturation of Hb (Hbsat) rather than increased total Hb concentration. Delayed administration of 100% O2 45 min after dMCAO (Experiment II) also increased oxyHb concentration; this effect persisted for at least 60 min (Fig. 3B). Upon discontinuation of 100% O2 at 45 min (i.e. 30 min after its onset, Experiment III) oxyHb gradually decreased to pre-hyperoxia levels within 60 min (Fig. 3C).
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Cerebral blood flow
The area of cortex with severe CBF deficit (i.e.
20% residual CBF compared to pre-ischaemic baseline) expanded rapidly during the first 10 min of dMCAO, and did not differ between groups prior to the onset of hyperoxia (15 min; Figs 2 and 3). In normoxic mice, this area continued to expand over the next 60 min, as described previously (Shin et al., 2006). In contrast to normoxic mice, administration of 100% O2 at 15 min (Experiment I) prevented the expansion of severe CBF deficit over time (2.1 ± 0.4 versus 3.8 ± 0.5 mm2, hyperoxia and normoxia, respectively, at 75 min; P < 0.05, two-way ANOVA for repeated measures; Fig. 2). Furthermore, delayed administration of hyperoxia 45 min after dMCAO reversed the expansion of severe CBF deficit and restored it to its area at the onset of dMCAO (Experiment II, Fig. 3E). Unlike oxyHb levels, the CBF preserving effect of hyperoxia persisted for at least 60 min after discontinuation of 100% O2 at 45 min (Experiment III, Fig. 3F). The CBF profile through core, penumbra and non-ischaemic cortex showed that hyperoxia improved CBF in both core and penumbra without significantly impacting non-ischaemic cortex (Fig. 4).
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Peri-infarct depolarizations
We have previously shown that PIDs negatively impact CBF in core and penumbra, and their suppression improves perfusion in focal ischaemia (Shin et al., 2006
). Therefore, we determined the frequency of PIDs and their impact on CBF and oxygenation, and compared these between normoxia and hyperoxia groups. In normoxic mice, PIDs occurred throughout the recording period (Fig. 5A and C). The frequency of PIDs in hyperoxia groups prior to the onset of 100% O2 did not differ from normoxic mice (Table 2). Normobaric hyperoxia administered 15 min after dMCAO (Experiment I) abolished PIDs within 15–30 min, and the effect persisted for 60 min (Fig. 5B and D). Delayed administration of 100% O2 at 45 min (Experiment II) also tended to suppress PIDs (Fig. 5E, Table 2). Upon discontinuing 100% O2 at 45 min (Experiment III), PIDs recurred within 15–30 min (Fig. 5F, Table 2).
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Each PID was associated with a transient hypoperfusion (Shin et al., 2006
) accompanied by a reduction in oxyHb in ischaemic core and penumbra (Fig. 6). In normoxic mice, both oxyHb and CBF failed to recover to pre-PID baseline; therefore, each PID caused a stepwise worsening of CBF and oxygenation thus contributing to the expansion of ischaemic cortex over time (13 ± 8% increase in the area of severe CBF deficit by each PID, P < 0.05). In contrast to normoxia, hyperoxia ameliorated both the magnitude and the duration of transient hypoperfusion and oxyHb reduction during PIDs (Fig. 6). In addition, CBF and oxyHb completely recovered to baseline after a PID in hyperoxic mice. Hence, PIDs did not significantly expand the area of hypoperfused cortex in hyperoxic group (1 ± 3% increase in the area of severe CBF deficit by each PID).
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Infarct size
Hyperoxia reduced infarct volume by 45% compared to normoxic mice when measured 48 h after 75 min transient dMCAO (Fig. 7). This neuroprotective effect was evident at all anteroposterior slice levels.
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Normobaric hyperoxia in normal cortex
In a separate group of non-ischaemic mice (n = 4), 15 min of normobaric hyperoxia did not significantly alter oxyHb (13 ± 6% increase, P > 0.05), and induced a small but statistically significant decrease in CBF (9 ± 6%, P < 0.05, one-way ANOVA for repeated measures).
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Discussion |
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Improving brain tissue oxygenation via 100% O2 inhalation is a potential therapeutic strategy to save salvageable tissue in acute ischaemic stroke. Here, we provide evidence that normobaric hyperoxia administered 15 min after the onset of focal ischaemia increases oxygen delivery, improves CBF and inhibits PIDs and their deleterious effects on CBF and oxygenation, thereby reducing infarct size. Delayed administration of hyperoxia at 45 min still improved CBF and oxygenation and suppressed PIDs, suggesting that the therapeutic window for these physiological effects is favourable for clinical use. Furthermore, early discontinuation of hyperoxia was associated with return of oxyHb to pre-hyperoxia levels and recurrence of PIDs, whereas CBF preservation was longer lasting. Our data are consistent with previous studies showing reduction in infarct size after normobaric hyperoxia, and reveal novel mechanisms of hyperoxic neuroprotection in focal ischaemia related to cerebrovascular function and PIDs.
Normobaric hyperoxia is neuroprotective in experimental cerebral ischaemia across a wide range of species including rats, rabbits (Kaminogo, 1989) and baboons (Branston et al., 1976). For example, in rats normobaric hyperoxia reduced diffusion weighted magnetic resonance imaging abnormalities and infarct volumes, improved functional outcome and extended the therapeutic window for thrombolysis, without increasing markers of oxidative stress or blood–brain barrier breakdown (Miyamoto and Auer, 2000; Flynn and Auer, 2002; Singhal et al., 2002a, 2002b; Kim et al., 2005; Liu et al., 2006). The magnitude of reduction in infarct volume by hyperoxia was somewhat smaller in our study; however, differences in ischaemia model (dMCAO versus fMCAO) and duration, duration of hyperoxia, and species preclude a comparison of efficacy of normobaric hyperoxia in this and previous studies. The importance of ischaemia model is highlighted in a recent study where normobaric hyperoxia was beneficial in permanent dMCAO but not fMCAO in mice (Veltkamp et al., 2006). Furthermore, normobaric hyperoxia was less efficacious in permanent ischaemia (Veltkamp et al., 2006) underscoring the importance of achieving reperfusion. Published data also suggest that normobaric hyperoxia has a therapeutic window of <45 min, at least in rats (Singhal et al., 2002a), although in mouse dMCAO model normobaric hyperoxia was still effective when administered 120 min after ischaemia onset (Veltkamp et al., 2006). We administered hyperoxia 15 or 45 min after ischaemia onset in order to maximize its potential benefit, and because this is an achievable period of time within which normobaric hyperoxia can be instituted by emergency medical technicians in human hyperacute stroke. Our data suggest a benefit from hyperoxia when administered early after stroke onset (45 min or less), and more work is needed to determine the therapeutic window for normobaric hyperoxia to improve CBF and oxygenation, inhibit PIDs and reduce infarct size.
Tissue pO2 in normal brain increases in a linear fashion with normobaric hyperoxia (Duong et al., 2001). The presumed mechanism of neuroprotection by normobaric hyperoxia has been increased O2 delivery to ischaemic neurons and glia thus improving oxidative metabolism. Although our imaging technique does not measure tissue pO2 directly, we confirmed increased O2 delivery indirectly by showing a rapid and sustained increase in oxyHb concentration in the severely ischaemic core. Increased tissue pO2 by normobaric hyperoxia was previously demonstrated by Liu et al. (2004, 2006) using electron paramagnetic resonance (EPR) oximetry. However, these investigators observed pO2 increases only in ischaemic penumbra but not core when measured during 70% O2 inhalation (Liu et al., 2004); in their subsequent study, where 100% O2 was tested, core pO2 was not measured (Liu et al., 2006). In addition to a lower concentration of inhaled O2 (70 versus 100% in our study), the apparent lack of increase in core pO2 in their study may be due to: (i) severe ischaemia model yielding greater infarction volumes (filament MCAO) compared to dMCAO used here, (ii) striatal localization of the EPR probe and (iii) local tissue factors due to the invasive nature of EPR oximetry requiring implantation of a lithium phthalocyanine crystal in the region where pO2 is measured. In this respect, using multimodal imaging we measured oxygenation in cerebral cortex, where the impact of hyperoxia is much stronger compared to striatum (Miyamoto and Auer, 2000; Flynn and Auer, 2002). Furthermore, multimodal imaging through intact skull is non-invasive, better preserving the integrity of the neurovascular unit and cerebrovascular physiology. In support of our data, Kaminogo. (1989) showed that mild hyperoxia (40–50% O2) increases tissue O2 availability in focal ischaemic core in rabbits measured using polarography. Although the relatively small size of mouse brain may facilitate the entry of O2 into ischaemic core, indirect evidence from baboons as well as humans suggest that normobaric hyperoxia augments oxygenation in ischaemic cortex in gyrencephalic brains as well (Branston et al., 1976; Hoffman et al., 1997).
Although hyperoxia causes vasoconstriction in normal brain, this effect is mostly observed during hyperbaric hyperoxia. Studies in different species including humans suggest that normobaric hyperoxia causes only a mild reduction in CBF in normal brain (15% of baseline CBF) (Busija et al., 1980; Bergo and Tyssebotn, 1995; Sjoberg et al., 1999; Matsuura et al., 2000; Watson et al., 2000; Duong et al., 2001; Johnston et al., 2003; Bulte et al., 2007). Hence, it has been difficult to predict whether the vasoconstrictive effect of normobaric hyperoxia would impact CBF deficit in focal ischaemia. It has been proposed that hyperoxic vasoconstriction in non-ischaemic cortex may redirect the blood flow to ischaemic territories, thus providing a net benefit. An alternative view is that vasoconstriction in non-ischaemic cortex may increase the vascular resistance in collateral vessels, thereby causing a net reduction in CBF. We directly addressed this question using two-dimensional high spatiotemporal resolution CBF mapping, and showed that normobaric hyperoxia augments CBF in ischaemic penumbra and core without altering CBF in mildly ischaemic or non-ischaemic territories. As a result, the area of severe CBF deficit (i.e. 20% residual CBF) was reduced by 45% compared to normoxic mice. Our data is in agreement with previous studies in both animals and humans, showing improvement in both CBV and CBF within ischaemic regions during normobaric hyperoxia, in the absence of arterial recanalization (Singhal et al., 2002a, 2005; Liu et al., 2006). Hyperoxic cerebral vasoconstriction is absent or reversed in acutely ischaemic hemisphere (Nakajima et al., 1983).
The mechanism of improved CBF in ischaemic cortex is unclear. Our data do not support shunting of blood from non-ischaemic to ischaemic cortex, because, normobaric hyperoxia did not cause vasoconstriction in non-ischaemic cortex. Other potential mechanisms include improved endothelial nitric oxide synthase (eNOS) function, and attenuation of vasoconstrictive neurovascular coupling (Shin et al., 2006). It is well-known that NO synthesized by eNOS plays a pivotal role in maintaining CBF in ischaemic cortex. Oxygen is one of the substrates for NO production by eNOS, and may become a rate limiting factor under hypoxic and ischaemic conditions. Therefore, it is possible that hyperoxia augments eNOS activity in ischaemic cortex, thereby increasing CBF; this hypothesis remains to be tested. Alternatively, hyperoxia may attenuate vasoconstrictive coupling, an abnormal neurovascular coupling phenomenon operational under pathological circumstances including artificially elevated extracellular [K+], inhibition of nitric oxide synthase, subarachnoid haemorrhage and cerebral ischaemia (Dreier et al., 1998, 2000; Strong et al., 2005; Shin et al., 2006). We have recently demonstrated that intense ischaemic depolarizing events, such as anoxic depolarization and PIDs, are associated with vasoconstriction and worsening in CBF effectively expanding the CBF deficit over time in a stepwise manner (Shin et al., 2006). Here, we show that the stepwise CBF worsening during a PID and the expansion of CBF deficit are abolished by normobaric hyperoxia (Figs 2, 3 and 6). Therefore, our data suggest that the improvement in CBF during hyperoxia is, at least in part, due to attenuation of vasoconstrictive neurovascular coupling during PIDs (Shin et al., 2006), although normobaric hyperoxia administered after reperfusion was still neuroprotective in one study (Flynn and Auer, 2002).
Normobaric hyperoxia also significantly reduced the frequency of PIDs (Fig. 5). The mechanism may involve improved oxygenation and stabilization within ischaemic penumbra. In separate experiments, we determined that normobaric hyperoxia does not inhibit cortical spreading depression in normal cortex (data not shown), suggesting that hyperoxia does not decrease cortical susceptibility to spreading depolarizations such as PIDs. Regardless of its mechanism, inhibition of PIDs is expected to reduce the hemodynamic and metabolic burden on penumbra and thus reduce infarct size. The relevance of suppression of PIDs to human stroke is underscored by recent demonstration of frequent PIDs in human brain after traumatic cerebral injury and subarachnoid haemorrhage (Strong et al., 2005; Fabricius et al., 2006).
In summary, our results show that normobaric hyperoxia increases tissue O2 availability in focal cerebral ischaemia, and CBF augmentation and suppression of PIDs are two other implicated mechanisms. Our data also suggest that to fully observe hyperoxic neuroprotection, hyperoxia may need to be administered early after ischaemia onset and preferably continued until reperfusion is achieved. In order to minimize pulmonary toxicity, however, normobaric hyperoxia should be considered a transitional tool (24 h or less) to synergize with more conventional therapies aimed to achieve reperfusion. As such, stroke subtypes and the therapeutic window within which normobaric hyperoxia is efficacious, and the optimal duration of administration beyond which it may potentially become harmful remain to be determined in acute stroke.
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Acknowledgements |
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This work was supported by the American Heart Association (0335519N, CA) and National Institutes of Health (P50 NS10828 and PO1 NS35611, MAM K25NS041291, AKD; R01EB00790-01A2, DAB; R01-NS37074 and R01-NS56458, EHL). Funding to pay the Open Access publication charges for this article was provided by American Heart Association SDG0335519N.
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