Viable Ischemic Tissue Research Articles

Improvement of a Closed Chest Porcine Myocardial Infarction Model by Standardization of Tissue and Blood Sampling Procedures

Myocardial ischemia reperfusion (I/R) injury contributes to almost half of the necrotic area after myocardial infarction. To date there is no approved drug to prevent or reduce myocardial I/R injury. The study and understanding of the pathophysiological mechanisms of myocardial I/R injury is essential to develop successful treatments. Large animal experiments are an important step in translational methods. The porcine model of acute myocardial infarction has been established and described by ourselves and others. We aimed to further improve the value of the model by focusing in detail on the sampling techniques for use in future experiments. Furthermore, we emphasize small but important steps that can affect the quality of the final results. To mimic the clinical situation of myocardial I/R injury, a percutaneous coronary intervention (PCI) catheter was inserted into the left anterior descending coronary artery (LAD) of an anesthetized pig. °°°This model mimics acute myocardial infarction and PCI treatment in humans with the possibility of accurately determining the area at risk as well as the necrotic- and viable ischemic tissue. Here the model was used to investigate the effect of a bicyclic peptide inhibitor of FXIIa. The model can also be modified to allow longer reperfusion times to study later effects of myocardial infarction.

Improvement of a Closed Chest Porcine Myocardial Infarction Model by Standardization of Tissue and Blood Sampling Procedures.

Myocardial ischemia reperfusion (I/R) injury contributes to almost half of the necrotic area after myocardial infarction. To date there is no approved drug to prevent or reduce myocardial I/R injury. The study and understanding of the pathophysiological mechanisms of myocardial I/R injury is essential to develop successful treatments. Large animal experiments are an important step in translational methods. The porcine model of acute myocardial infarction has been established and described by ourselves and others. We aimed to further improve the value of the model by focusing in detail on the sampling techniques for use in future experiments. Furthermore, we emphasize small but important steps that can affect the quality of the final results. To mimic the clinical situation of myocardial I/R injury, a percutaneous coronary intervention (PCI) catheter was inserted into the left anterior descending coronary artery (LAD) of an anesthetized pig. °°°This model mimics acute myocardial infarction and PCI treatment in humans with the possibility of accurately determining the area at risk as well as the necrotic- and viable ischemic tissue. Here the model was used to investigate the effect of a bicyclic peptide inhibitor of FXIIa. The model can also be modified to allow longer reperfusion times to study later effects of myocardial infarction.

Hyperintense Vessels on FLAIR: Hemodynamic Correlates and Response to Thrombolysis

Hyperintense vessels on baseline FLAIR MR imaging of patients with ischemic stroke have been linked to leptomeningeal collateralization, yet the ability of these to maintain viable ischemic tissue remains unclear. We investigated whether hyperintense vessels on FLAIR are associated with the severity of hypoperfusion and response to thrombolysis in patients treated with intravenous tissue-plasminogen activator. Consecutive patients with ischemic stroke with an MR imaging before and within 24 hours of treatment, with proved vessel occlusion and available time-to-maximum maps were included (n = 62). The severity of hypoperfusion was characterized on the basis of the hypoperfusion intensity ratio (volume with severe/mild hypoperfusion [time-to-maximum ≥ 8 seconds / time-to-maximum ≥ 2 seconds]). The hypoperfusion intensity ratio was dichotomized at the median to differentiate moderate (hypoperfusion intensity ratio ≤ 0.447) and severe (hypoperfusion intensity ratio > 0.447) hypoperfusion. Good outcome was defined as a modified Rankin Scale score of ≤2. Hyperintense vessels on FLAIR were identified in 54 patients (87%). Patients with extensive hyperintense vessels on FLAIR (>4 sections) had higher NIHSS scores, larger baseline lesion volumes, higher rates of perfusion-diffusion mismatch, and more severe hypoperfusion (hypoperfusion intensity ratio). In stepwise backward multivariate regression analysis for the dichotomized hypoperfusion intensity ratio (including stroke etiology, age, perfusion deficit, baseline lesion volume, smoking, and extent of hyperintense vessels on FLAIR), extensive hyperintense vessels on FLAIR were independently associated with severe hypoperfusion (OR, 6.8; 95% CI, 1.1-42.7; P = .04). The hypoperfusion intensity ratio was an independent predictor of a worse functional outcome at 3 months poststroke (OR, 0.2; 95% CI, 0.5-0.6; P < .01). Hyperintense vessels on FLAIR are associated with larger perfusion deficits, larger infarct growth, and more severe hypoperfusion, suggesting that hyperintense vessels on FLAIR most likely indicate severe ischemia as a result of insufficient collateralization.

Oxygen Imaging by MRI

Mapping the ischemic penumbra (ie, the neurophysiologically silent but still viable ischemic tissue) is increasingly part of routine assessment in suspected acute stroke, although whether this approach is cost-effective is still unclear.1,2 The penumbra is characterized by both low cerebral blood flow (CBF; <20 mL · 100 g−1 · min−1) and elevated oxygen extraction fraction (OEF).3 The latter is considered a critical marker of tissue viability and is therefore a key imaging target. The penumbra was originally documented in humans using positron emission tomography (PET),2,3 a validated method to map CBF, OEF and cerebral metabolic rate of oxygen (CMRO2),4 but clinical access to PET is scarce. Consequently, MR-based diffusion and perfusion imaging (PWI) and CT-based perfusion imaging are widely used as substitutes.1 However, both methods have limitations for penumbra imaging2 and do not directly assess oxygen metabolism. A method combining the advantages of MR and the ability to map OEF would therefore be highly desirable. Blood oxygen level-dependent (BOLD) imaging has recently emerged as a possible candidate for this purpose. However, several BOLD techniques with different levels of validation, accuracy, and clinical applicability are in concurrent development, making the situation somewhat confusing. This review aims to clarify whether BOLD imaging might be of use to map oxygen in the clinical setting. We systematically review and critically discuss all studies published to date in English language, both experimental and clinical, that have applied BOLD in acute stroke. Because our focus is tissue oxygen metabolism, we do not address the T2*-weighted method to visualize leptomeningeal vessels,5 the mapping of ΔCMRO2 during physiological challenges,6 or the emerging 17O imaging method.7 ### Principles of BOLD MRI Oxyhemoglobin is diamagnetic, whereas deoxyhemoglobin (DHb) is paramagnetic. Transverse relaxation is sensitive to paramagnetic …

Clinical application of perfusion and diffusion MR imaging in acute ischemic stroke.

With the advances in new neuroimaging modalities, the role of imaging of acute ischemic stroke has broadened and progressed from making diagnoses to providing valuable information for patient management. The goal of thrombolytic therapy for acute ischemic stroke should be to salvage the ischemic tissue reversibility that can respond to recanalization and avoid reperfusion of the dead (nonviable) tissue. It is essential to have rapid diagnostic modalities that can distinguish viable ischemic tissue from irreversibly damaged tissue, because there is a risk of reperfusion injury such as hemorrhagic complications with early intervention. Although diffusion magnetic resonance (MR) imaging has been reported to have a high sensitivity and specificity for acute ischemia in acute stroke patients without early reperfusion therapy, the capability to differentiate reversible from irreversible ischemia by diffusion MR imaging has not been established. Perfusion MR imaging techniques provide direct information on parenchymal perfusion status (adequacy of the collateral circulation) and may have the potential for providing important information about tissue viability and/or reversibility for selecting appropriate patients for thrombolytic therapy.

Glucose delivery is a major determinant of glucose utilisation in the ischemic myocardium with a residual coronary flow.

Experimental data from isolated rat hearts suggest that glycolysis in severe myocardial ischemia is inhibited by accumulation of glycolytic metabolites. In contrast, positron emission tomography (PET) in patients with myocardial ischemia records a 'mismatch' between the decreased coronary flow in viable ischemic tissue and an increased fluorodeoxyglucose (18FDG) signal. To resolve this contradiction, we investigated glucose uptake at very low coronary flows in the isolated rat heart model. Rates of glucose uptake were measured in the isolated Langendorff-perfused Wistar rat heart, at control (12-15 ml/g wet wt/min) and low coronary flows (0.1, 0.2 and 0.5 ml/g wet wt/min) and at a range of glucose concentrations (2.75, 5.5, 11 and 22 mM). The steady-state rate of glucose uptake versus glucose concentration could be described by a double rectangular hyperbola at each coronary flow. Glucose uptake fell to levels significantly below control at low coronary flows. However, the extraction of glucose (glucose uptake as % of glucose delivered) rose sharply, from 1% at control coronary flows, to 25-30% at low coronary flows. Crossover analysis of glycolytic intermediates in freeze-clamped tissue indicated little inhibition at any specific site, although phosphofructokinase activity was restricted when glycolytic substrate availability was high. Insulin and preconditioning both increased glucose uptake with 11 mM glucose, possibly by increasing membrane transporter density and thus increasing glucose delivery to the cytosol. Despite the reduction in absolute glucose uptake at low coronary flow-rates, the extraction of glucose was greatly increased, possibly following GLUT4 translocation. Delivery of glucose to the glycolytic pathway appears to be a major controlling site of glycolysis in low-flow ischemia. Downstream regulation is then distributed along the pathway with no one site exerting greater inhibition than reduced glucose delivery.

Technetium 99m—HL-91: A potential new marker of myocardial viability assessed by nuclear imaging early after reperfusion

Objective. 99mTc-HL-91 (Prognox) is a potential new hypoxia-avid myocardial imaging agent. The purpose of this study was to determine whether this tracer would demonstrate increased activity in nonviable myocardium in vivo. Methods and Results. A 3-hour left circumflex artery (LCx) occlusion was followed by 1 hour of reperfusion, injection of 99mTc-HL-91 (185 MBq), and 2 hours of gamma camera imaging in 6 open-chest canine experiments. Microspheres were injected during baseline, at occlusion, at the time of tracer injection, and at the end of the experiment. After the animals were killed, heart slices were imaged. Blood flow and tracer activity were determined by well counting. Mean infarct size was 19.2% ±2.2% (SEM) . All six dogs demonstrated no increased 99mTc-HL-91 myocardial activity other than small foci on in vivo and ex vivo gamma camera images. The mean large region of interest (ROI)-determined LCx/LAD (left anterior descending) ratio was 1.10 ±0.03 in vivo, and 1.0 ±0.02 ex vivo. Mean clearance curves from LCx and LAD ROI were not significantly different, and 2-hour retention was 15.2% ± 2.1% for the LCx and 18.6% ± 2.7% for the LAD ( p = NS) . ROI clearance curves demonstrated biexponential clearance over the first hour and linear clearance over the second hour. Myocardial blood flow (microspheres) versus well-counted tracer uptake curves were linear with near-zero slopes for viable tissue, nonviable tissue, and mosaic tissue. Blood clearance was triexponential with a 2-hour retention of 7.8% ±1.1%. Conclusions. In contrast to viable ischemic tissue, normal and nonviable myocardium demonstrate similar 99mTc-HL-91 uptake and retention kinetics. This agent warrants further clinical studies in situations where there is a need to differentiate ischemic viable from nonviable myocardium.

Prevention and Management of Medical Complications of the Hospitalized Elderly Stroke Patient

Medical complications may account for 30% or more of the deaths resulting from acute ischemic stroke in the elderly. In descending order of frequency, the most deadly complications are bacterial pneumonia, pulmonary embolism, myocardial infarction, and sepsis without pneumonia (often in the setting of a urinary tract infection or a necrotic decubitus). Normal aging is associated with declining pulmonary and cardiovascular functions as well as declining immunocompetence and physical barriers to infection. The neurological effects of acute ischemic brain injury compound these susceptibilities. Accordingly, a high degree of vigilance is emphasized in the diagnostic and therapeutic guidelines provided for care of the lungs, the heart, the urinary tract, and the skin. Guidelines are also provided for management of blood pressure during the first hours and days following stroke onset. Treatment should be withheld unless specific medical indications are identified. When antihypertensive agents are administered, the appropriate dose may be lower than usually recommended (e.g. labetalol) in order to minimize abrupt drops in blood pressure that may result in further injury to potentially viable ischemic brain tissue.

Ultrastructural damage and Ca2+-shifts in the canine myocardium subjected to regional incomplete ischemia

The role of Ca2+ in the pathogenesis leading to ischemic myocardial cell death is still controversial. To gain insight into this phenomenon a cytochemical procedure, the phosphate pyroantimonate method, was used to localize different subcellular Ca2(+)-pools at the ultrastructural level. After 45 min of left anterior descending coronary artery (LAD) occlusion, the coronary arteries were perfused with triphenyltetrazoliumchloride staining (TTC) to identify viable ischemic and infarcted tissue. In non-ischemic tissue, Ca2(+)-deposits were confined to the sarcolemma, sarcolemma-derived vesicles, transverse tubules, and intercalated disks. In infarcted tissue (TTC-negative), the sarcolemma lost its Ca2(+)-binding capacity and mitochondria were either overloaded with Ca2(+)-precipitate or they contained amorphous densities. In viable ischemic areas (determined with the TTC-technique) the sarcolemma was virtually devoid of Ca2(+)-deposits. Mitochondria in this area frequently showed clumping of the cristae, associated with an accumulation of Ca2(+)-precipitate in between the clustered cristae. The results of this study indicate that Ca2(+)-shifts occur in ischemic myocardial cells before the occurrence of other ultrastructural signs of irreversible injury which, therefore, narrows the possibility that Ca2(+)-overload is only a consequence of ischemic cell death.

Kinetics of rubidium-82 after coronary occlusion and reperfusion. Assessment of patency and viability in open-chested dogs.

Currently available noninvasive techniques are unable to rapidly assess artery patency and tissue viability during acute myocardial infarction. In prior studies, rubidium-82 (Rb-82), a short-lived positron emitter obtained from a generator, was validated as an indicator of flow with a model that included the rate constants for transfer into and out of the cell. Accordingly, in the current study, 20 open-chested dogs with experimental infarction were studied serially at base line, after coronary occlusion, and at reperfusion. Time-activity curves acquired with beta probes on the epicardial surface were used to measure flow and net transfer of rubidium. Flow decreased to 0.41 +/- 0.08 ml/min per gram during occlusion and increased to 2.73 +/- 0.56 ml/min per gram in potentially viable ischemic tissue, whereas flows were 0.32 +/- 0.08 during occlusion (P less than 0.05 vs. viable) and 1.58 ml/min per gram (P less than 0.002 vs. viable) in irreversibly injured tissue. The transfer rate constant for Rb-82, kT, at base line was +1.22 +/- 0.60 X 10(-3) s-1 and did not change significantly during occlusion in viable vs. nonviable samples (+1.41 +/- 1.27 vs. +0.93 +/- 1.51 X 10(-3) s-1, respectively), except that 4 out of 11 nonviable tissue samples had negative kTs. At reperfusion, viable myocardial samples were all positive (+1.26 +/- 1.58 X 10(-3) s-1), whereas all irreversibly injured tissues had a negative kT, indicating leakage of tracer (-1.50 +/- 1.10 X 10(-3) s-1, P less than 0.001). This study suggests that Rb-82 time-activity curves can be useful to determine patency of an infarct related artery and potential viability after reperfusion during myocardial infarction.

Use of Thermography to Predict Intestinal Viability and Survival After Ischemic Injury: A Blind Experimental Study

Reactive hyperemia is a characteristic of viable ischemic tissue that has been revascularized. In 14 normal dogs, segments of bowel were made ischemic for periods of 2--8 hours. Thermograms and surface temperature measurements of exposed ischemic and nonischemic segments were obtained before and every 5 minutes for 30 minutes after revascularization. Thermograms demonstrated uniform reactive hyperemia throughout the segments ischemic for 2--3 hours; hyperemia with some nonuniform, patchy areas in segments ischemic for 3--7 hours; and no hyperemic response in segments ischemic for 8 hours. All five dogs demonstrating uniform hyperemia survived and the bowels were found to be histologically normal. Four of the six dogs showing hyperemia with some nonuniform areas survived, but all six showed areas of transmural damage with fibrosis and scarring histopathologically. The three dogs with no hyperemic response died of pathologically proved bowel infarction. Our results indicate that thermograms can document the degree and extent of reactive hyperemia. Thermograms, therefore, may be an important aid in assessing bowel viability during surgery.

Thermographic assessment of intestinal viability following ischemic damage.

Reactive hyperemia has been shown to be a characteristic of viable ischemic tissue that has been revascularized. A segment of small bowel was made ischemic by arterial occlusion and the ischemia maintained for 2 1/2 hrs (ischemic, viable bowel) or 8 hrs (ischemic, nonviable bowel) before circulation was restored. Thermograms and surface temperature measurements of exposed ischemic and non-ischemic bowel segments were obtained before and every five minutes after revascularization. Thermograms of segments revascularized after 2 1/2 hrs of ischemia demonstrated reactive hyperemia and a one to four degree increase in surface temperature compared to normal surrounding bowel. No hyperemic response was seen in bowel segments that were ischemic for 8 hrs. Our results indicate that thermograms can document reactive hyperemia of an exposed ischemic bowel that has been successfully revascularized and may be an important aid in assessing bowel viability during surgery.

Myocardial imaging with 123I-hexadecenoic acid.

123I-hexadecenoic acid is a terminally iodinated, 17-carbon fatty acid analog which is rapidly degraded in the myocardium. By determining regional myocardial distribution patterns and clearance rates, it may become useful as a single agent for estimating regional myocardial perfusion and for distinguished viable ischemic tissue from infarcted tissue. The high count rates obtainable with the iodine label permit acquisition of qualitative multiprojection images in only 3 min. per view, or quantifiable single projection high count images in 10 min. Ischemic defects may be observed in anginal patients without subjecting them to stress.