Visualization of rat pial microcirculation using the novel orthogonal polarized spectral (OPS) imaging after brain injury

In situ assessment of the brain microcirculation in mechanically-ventilated rabbits using sidestream dark-field (SDF) imaging

Assessment of the cerebral microcirculation by on-line visualization has been impossible for a long time. Sidestream dark-field (SDF) imaging is a relatively new method allowing direct visualization of cerebral surface layer microcirculation using hand-held probe for direct contact with target tissue. The aim of this study was to elucidate the feasibility of studying the cerebral microcirculation in situ by SDF imaging and to assess the basic cerebral microcirculatory parameters in mechanically ventilated rabbits. Images were obtained using SDF imaging from the surface of the brain via craniotomy. Clear high contrast SDF images were successfully obtained. Total small-vessel density was 14.6+/-1.8 mm/mm(2), total all-vessel density was 17.9+/-1.7 mm/mm(2), DeBacker score was 12.0+/-1.6 mm(-1) and microvascular flow index was 3.0+/-0.0. This method seems to be applicable in animal studies with possibility to use SDF imaging also intraoperatively, providing unique opportunity to study cerebral microcirculation during various experimental and clinical settings.

In vivo imaging of cerebral hemodynamics using high-frequency micro-ultrasound

Assessment of cerebral vascular response is important in neuroscience research. Some imaging modalities that are commonly used to detect flow and/or vessel diameter changes in the brain include magnetic resonance imaging, positron emission tomography, and optical intrinsic signal imaging. Ultrasound has not typically been used to assess neurovascular response but recent advances in the technology have led to the development of micro-ultrasound systems with significant potential for this application. The state of the art in high frequency (15-50 MHz) micro-ultrasound is based on linear arrays specifically designed for small animal imaging. These systems can achieve axial resolution ranging from 30 to 200 microm. They are capable of quantifying brain hemodynamics in terms of red blood cell (RBC) velocity, flow, and vascular density in real time, up to 35 mm below the cortical surface, and can achieve temporal resolution of up to 1000 frames per second. This protocol describes imaging of the rat brain using various ultrasound imaging modes (power Doppler, color Doppler, pulsed-wave Doppler, and nonlinear contrast-enhanced imaging) to assess the state of cerebral microcirculation.

Epinephrine reduces cerebral perfusion during cardiopulmonary resuscitation

OBJECTIVE

Epinephrine has been the primary drug for cardiopulmonary resuscitation (CPR) for more than a century. The therapeutic rationale was to restore threshold levels of myocardial and cerebral blood flows by its alpha1 (alpha1) and alpha2 (alpha2)-adrenergic agonist vasopressor actions. On the basis of coincidental observations on changes in microvascular flow in the cerebral cortex, we hypothesized that epinephrine selectively decreases microvascular flow.

DESIGN

Randomized prospective animal study.

SETTING

University-affiliated research laboratory.

SUBJECTS

Domestic pigs.

INTERVENTIONS

Four groups of five male domestic pigs weighing 40 +/- 3 kg were investigated. After induction of anesthesia, endotracheal intubation was followed by mechanical ventilation. A frontoparietal bilateral craniotomy was created. Ventricular fibrillation was induced and untreated for 3 minutes before the start of precordial compression, mechanical ventilation, and attempted defibrillation. Animals were randomized to receive central venous injections during CPR of 1) placebo, 2) epinephrine, 3) epinephrine in which both alpha1- and beta (beta)-adrenergic effects were blocked by previous administration of prazosin and propranolol, and 4) epinephrine in which both alpha2- and beta-adrenergic effects were blocked by previous administration of yohimbine and propranolol.

MEASUREMENTS AND MAIN RESULTS

Cerebral cortical microcirculatory blood flow (MBF) was measured with orthogonal polarization spectral imaging. Cerebral cortical carbon dioxide and oxygen tensions (Pbco2 and Pbo2) were concurrently measured using miniature tissue optical sensors. Each animal was resuscitated. No differences in the number of electrical shocks for defibrillation or in the duration of CPR preceding return of spontaneous circulation were observed. Yet when epinephrine induced increases in arterial pressure, it significantly decreased Pbo2 tension and increased Pbco2 tension. Epinephrine therefore significantly decreased MBF and increased indicators of cerebral ischemia. Reduced MBF and magnified brain tissue ischemia during and after cardiopulmonary resuscitation were traced to the alpha1-adrenergic agonist action of epinephrine. When the alpha2 effects of epinephrine were blocked, reduced MBF and tissue ischemia persisted. No differences in cardiac output, end tidal Pco2, arterial Po2 and Pco2, and brain temperature were observed before inducing cardiac arrest and following return of spontaneous circulation.

CONCLUSIONS

In this model, epinephrine through its alpha1-agonist action had adverse effects on cerebral microvascular blood flow such as to increase the severity of cerebral ischemia during CPR.

Cerebral cortical microvascular flow during and following cardiopulmonary resuscitation after short duration of cardiac arrest

AIM

To examine changes in cerebral cortical macro- and microcirculation and their relationship to the severity of brain ischaemia during and following resuscitation from a short duration of cardiac arrest.

METHODS

Bilateral cranial windows were created in eight domestic pigs weighing 41+/-1 kg, exposing the frontoparietal cortex for orthogonal polarization spectral imaging together with estimation of cortical-tissue partial pressure of carbon dioxide, a quantitator of the severity of cerebral ischaemia. After 3 min of untreated ventricular fibrillation, cardiopulmonary resuscitation was begun and continued for 4 min before defibrillation. Aortic pressure, end-tidal and cortical-tissue partial pressure of carbon dioxide, and cortical microcirculatory blood flow in vessels of less and more than 20 microm in diameter were continuously measured.

RESULTS

Cerebral microcirculatory blood flow progressively decreased over the 3-min interval that followed onset of ventricular fibrillation. Chest compression restored cortical microvascular flow to approximately 40% of the pre-arrest value. Following return of spontaneous circulation, microvascular flow velocity was restored to baseline values over 3 min. Reversal of cerebral ischaemia with normalisation of cerebral cortical-tissue partial pressure of carbon dioxide occurred over 7 min after resuscitation. Cortical microcirculatory blood flow in microvessels less than 20 microm was highly correlated with flow in vessels more than 20 microm together with mean aortic pressure and end-tidal partial pressure of carbon dioxide.

CONCLUSION

Cerebral cortical microcirculatory flow ceased only 3 min after onset of cardiac arrest. Flow was promptly restored to 40% of its pre-arrest value after start of chest compression. After resuscitation, both macro- and microcirculatory flows were fully restored over 3 min, but cerebral ischaemia reversed more slowly.

Effects of epinephrine and vasopressin on cerebral microcirculatory flows during and after cardiopulmonary resuscitation

OBJECTIVES

Both epinephrine and vasopressin increase aortic and carotid arterial pressure when administered during cardiopulmonary resuscitation. However, we recently demonstrated that epinephrine reduces cerebral cortical microcirculatory blood flow. Accordingly, we compared the effects of nonadrenergic vasopressin with those of epinephrine on cerebral cortical microvascular flow together with cortical tissue Po2 and Pco2 as indicators of cortical tissue ischemia.

DESIGN

Randomized, prospective animal study.

SETTING

University-affiliated research laboratory.

SUBJECTS

Domestic pigs.

MEASUREMENTS AND MAIN RESULTS

The tracheae of ten domestic male pigs, weighing 40 +/- 2 kg, were noninvasively intubated, and the animals were mechanically ventilated. A frontoparietal bilateral craniotomy was created. Microcirculatory blood flow was quantitated with orthogonal polarization spectral imaging. Blood flow velocity in pial and cortical penetrating vessels measuring <20 microm was graded from 0 (no flow) to 3 (normal). Cerebral cortical tissue carbon dioxide and oxygen tensions (Pbco2 and Pbo2) were measured concurrently using miniature optical sensors. Ventricular fibrillation, induced with an alternating current delivered to the right ventricular endocardium, was untreated for 3 mins. Animals were then randomized to receive central venous injections of equipressor doses of epinephrine (30 microg/kg) or vasopressin (0.4 units/kg) at 1 min after the start of cardiopulmonary resuscitation. After 4 mins of cardiopulmonary resuscitation, defibrillation was attempted. Spontaneous circulation was restored in each animal. However, postresuscitation microvascular flows and Pbo2 were greater and Pbco2 less after vasopressin when compared with epinephrine. We observed that a significantly greater number of cortical microvessels were perfused after vasopressin.

CONCLUSIONS

Cortical microcirculatory blood flow was markedly reduced after epinephrine, resulting in a greater severity of brain ischemia after the restoration of spontaneous circulation in contrast to the more benign effects of vasopressin.

Liquid-crystal tunable filter spectral imaging for brain tumor demarcation

Past studies have demonstrated that combined fluorescence and diffuse reflectance spectroscopy can successfully discriminate between normal, tumor core, and tumor margin tissues in the brain. To achieve efficient, real-time surgical resection guidance with optical biopsy, probe-based spectroscopy must be extended to spectral imaging to spatially demarcate the tumor margins. We describe the design and characterization of a combined fluorescence and diffuse reflectance imaging system that uses liquid-crystal tunable filter technology. Experiments were conducted to quantitatively determine the linearity, field of view, spatial and spectral resolution, and wavelength sensitivity of the imaging system. Spectral images were acquired from tissue phantoms, mouse brain in vitro, and human cortex in vivo for functional testing of the system. The spectral imaging system produces measured intensities that are linear with sample emission intensity and integration time and possesses a 1 in. (2.54 cm) field of view for a 7 in. (18 cm) object distance. The spectral resolution is linear with wavelength, and the spatial resolution is pixel-limited. The sensitivity spectra for the imaging system provide a guide for the distribution of total image integration time between wavelengths. Functional tests in vitro demonstrate the capability to spectrally discriminate between brain tissues based on exogenous fluorescence contrast or endogenous tissue composition. In vivo imaging captures adequate fluorescence and diffuse reflectance intensities within a clinically viable 2 min imaging time frame and demonstrates the importance of hemostasis to acquired signal strengths and imaging speed.

Restoration of Cerebral Vasoreactivity by an L-Type Calcium Channel Blocker following Fluid Percussion Brain Injury

Traumatic brain injury (TBI) results in significant acute reductions in regional cerebral blood flow (rCBF). However, the mechanisms by which TBI impairs CBF and cerebral vascular reactivity have remained elusive. In the present study, the effect of verapamil, an L-type calcium (Ca(2+)) channel blocker, on post-traumatic vascular reactivity was evaluated following a lateral fluid percussion injury (FPI) in rats. rCBF was measured by [(14)C]-iodoantipyrine autoradiography 1 h after FPI. Following FPI, significant rCBF reductions were documented in all examined cortical areas. These reductions were the most prominent (72.0%) at the primary injury site. Intravenous infusion of verapamil (VE; 200 microg/kg/min), and norepinephrine (NE; 20 microg/mL/min) to maintain normal blood pressure, increased rCBF by 141.5% at the primary injury site when compared to untreated, FPinjured animals. Under stimulated conditions, both the ipsilateral and contralateral hemispheres failed to show any increases in rCBF at 1 h following FPI. In direct contrast, following VE+NE treatment all cortical areas measured showed near normal vascular reactivity to direct cortical stimulation (normal reactivity = 45% increase in rCBF vs. 47% increase in FPI+VE+NE cases). These findings suggest that the majority of post-traumatic hemodynamic depressions are closely related to mechanisms involving vasoconstriction. Furthermore, Ca(2+) may play a causative role in this vasoconstriction and the loss of vasoreactivity.

Observations on the microcirculation of the human burn wound using orthogonal polarization spectral imaging

Orthogonal polarization spectral imaging (OPS) utilizes the illumination of the tissue with polarized light within the haemoglobin spectrum. We report here on OPS for the assessment of the skin microcirculation non-invasively through the surface of the human burn wound. This allows inspection of individual capillaries of the cutaneous microcirculation and flow through these vessels in real time. Two distinct microcirculatory patterns were seen. Superficial burns had small visible dermal capillaries studied throughout the field of view. The flow of individual erythrocytes through these capillaries was clearly visible in real-time. Conversely, deep burns showed large thrombosed vessel coursing in a criss-cross fashion. There was marked difference between the mean optical densities for normal skin and superficial burns (65.8+/-15.6 and 64+/-14.6, respectively) and deep burns (131.2+/-31.1). These findings indicate that OPS may have utility in the assessment of cutaneous microcirculation in burns.

Small volume resuscitation with HyperHaes improves pericontusional perfusion and reduces lesion volume following controlled cortical impact injury in rats

The hyperosmolar and hyperoncotic properties of HyperHaes (HHES) might improve impaired posttraumatic cerebral perfusion. Possible beneficial effects on pericontusional perfusion, brain edema, and contusion volume were investigated in rats subjected to controlled cortical impact (CCI). Male Sprague-Dawley rats (n = 60) anesthetized with isoflurane were subjected to a left temporoparietal CCI. Thereafter, rats were randomized to receive HHES (10% hydroxyethylstarch, 7.5% NaCl) or physiological saline solution (4 mL/kg body weight) intravenously. Mean arterial blood pressure (MABP) and intracranial pressure (ICP) were determined before and following CCI, after drug administration and 24 h later. Regional pericontusional cortical perfusion was determined by scanning laser Doppler flowmetry before CCI, and 30 min, 4 and 24 h after injury. At 24 h brain swelling and water content were measured gravimetrically. At 7 days, cortical contusion volume was determined planimetrically. MABP was not influenced by HHES. ICP was significantly decreased immediately after HHES infusion (5.7 +/- 0.4 vs. 7.1 +/- 1.0 mm Hg; p < 0.05). Pericontusional cortical perfusion was significantly decreased by 44% compared to pre-injury levels (p < 0.05). HHES significantly improved cortical perfusion at 4 h after CCI, approaching baseline values (85 +/- 12%). While increased posttraumatic brain edema was not reduced by HHES at 24 h, cortical contusion volume was significantly decreased in the HHES-treated rats at 7 days after CCI (23.4 +/- 3.5 vs. 39.6 +/- 6.2 mm3; p < 0.05). Intravaneous administration of HHES within 15 min after CCI has a neuroprotective potential, as it significantly attenuated impaired pericontusional perfusion and markedly reduced the extent of induced structural damage.

Effects of early and late intravenous norepinephrine infusion on cerebral perfusion, microcirculation, brain-tissue oxygenation, and edema formation in brain-injured rats

OBJECTIVES

Reduction of cerebral perfusion during the early phase after traumatic brain injury is followed by a later phase of normal to increased perfusion. Thus, pharmacologically elevating mean arterial blood pressure with the aim of improving cerebral perfusion may exert different time-dependent effects on cortical perfusion, microcirculation, tissue oxygenation and brain edema formation after traumatic brain injury.

DESIGN

Randomized, placebo-controlled trial.

SETTING

Experimental laboratory at a university hospital.

SUBJECTS

A total of 37 male Sprague-Dawley rats subjected to a focal cortical contusion.

INTERVENTIONS

At 4 or 24 hrs after focal traumatic brain injury, mean arterial blood pressure was increased to 120 mm Hg for 90 mins by infusing norepinephrine. In rats receiving physiologic saline, mean arterial blood pressure remained unchanged. In the first series, pericontusional cortical perfusion was measured using the laser Doppler flowmetry scanning technique before injury and before, during, and after the infusion period. In a second series, intracranial and cerebral perfusion pressure and intraparenchymal perfusion and tissue oxygen measured within the contused and pericontusional cortex were recorded continuously before, during, and after norepinephrine infusion. Changes in cortical microcirculation were investigated by orthogonal polarization spectral imaging. At the end of each experiment, hemispheric swelling and water content were determined gravimetrically.

MEASUREMENTS AND MAIN RESULTS

At 4 and 24 hrs after traumatic brain injury, intravenous norepinephrine significantly increased pericontusional cortical perfusion, which was also reflected by an increase in diameters and flow velocities of pericontusional arterioles and venules. Cerebral perfusion pressure and intraparenchymal perfusion and tissue oxygen were significantly increased during norepinephrine infusion at 4 and 24 hrs. Hemispheric swelling and water content showed no difference between the groups.

CONCLUSIONS

After cortical impact injury, early and late intravenous norepinephrine infusion pressure-dependently increased cerebral perfusion and tissue oxygenation without aggravating or reducing brain edema formation. Future studies are warranted to determine long-term changes of short and prolonged norepinephrine-induced increases in mean arterial blood pressure and cerebral perfusion pressure.

Effects of LY379268, a selective group II metabotropic glutamate receptor agonist on EEG activity, cortical perfusion, tissue damage, and cortical glutamate, glucose, and lactate levels in brain-injured rats

Activating presynaptic group II metabotropic glutamate (mGlu II) receptors reduces synaptic glutamate release. Attenuating glutamatergic transmission without blocking ionotropic glutamate receptors, thus avoiding unfavorable psychomimetic side effects, makes mGlu II receptor agonists a promising target in treating brain-injured patients. Neuroprotective effects of LY379268 were investigated in rats following controlled cortical impact injury (CCI). At 30 min after CCI, rats received a single intraperitoneal injection of LY379268 (10 mg/kg/body weight) or NaCl. Changes in EEG activity and pericontusional cortical perfusion were determined before trauma, at 4, 24, and 48 h, and 7 days after CCI. Brain edema and contusion volume were determined at 24 h and 7 days after CCI, respectively. Before brain removal pericontusional cortical glutamate, glucose, and lactate were measured via microdialysis. During the early period following CCI, EEG activity and cortical perfusion were significantly reduced in rats receiving LY379268. At 7 days, cortical perfusion was significantly increased in rats treated with LY379268, while EEG activity was depressed as in control rats. While brain edema remained unchanged at 24 h, cortical contusion was significantly decreased by 56% at 7 days after CCI. Cortical glutamate, glucose, and lactate were not influenced. Significant reductions in EEG activity and contusion volume by LY379268 do not appear mediated by attenuated excitotoxicity and energetic impairment. Overall, an additional decrease in cortical perfusion seems to interfere with the anti-edematous potential of LY379268 during the early period following CCI, while an increase in perfusion in LY379268-treated rats at 7 days might contribute to tissue protection.

Segmental vascular resistance after mild controlled cortical impact injury in the rat

In an effort to localize the site at which increased resistance occurs after brain trauma, pial arteriole diameter and pressure were assessed after mild controlled cortical impact (CCI) injury in rats using an open cranial window technique. The authors tested the hypothesis that an increase in resistance accompanied by vasoconstriction occurs at the level of the pial arterioles within the injured cortex of the brain. At 1 hour after mild CCI injury, ipsilateral cerebral blood flow was significantly reduced by 42% compared with sham injury (n = 4; < 0.05). Pial arteriole diameter and pressure remained unchanged. Resistance in the larger arteries (proximal resistance), however, was significantly greater after CCI injury (1.87 +/- 0.26 mm Hg/[mL. 100 g. min]) compared with sham injury (0.91 +/- 0.21 mm Hg/[mL. 100 g. min]; < 0.0001). Resistance in small vessels, arterioles, and venules (distal resistance) was also significantly greater after CCI injury (1.13 +/- 0.05 mm Hg/[mL. 100 g. min]) compared with sham injury (0.74 +/- 0.13 mm Hg/[mL. 100 g. min]; = 0.0001). The authors conclude that, at 1 hour after mild CCI injury, changes in vascular resistance comprise a 53% increase in the resistance distal to the area of injury and, surprisingly, a 105% increase in resistance in the arteries proximal to the injury site.