Mechanisms of the “No-Reflow” Phenomenon After Acute Myocardial Infarction

Figures

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  Figure 1. Pathognomonic Findings Related to the No-Reflow Phenomenon (A) An example from the first description of no reflow in the cat heart by Krug et al,9 where they injected acridine orange in the beating heart to define the risk area and light green dye to define the perfused bed after reperfusion. Then risk area and no reflow was defined post mortem. The dark area with clearly demarcated borders not staining with either acridine orange or green dye denotes the no-reflow zone. This animal had undergone 2 hours of coronary occlusion followed by 3 hours of reperfusion.9 Bar = 5 mm. Reproduced with permission of the American Heart Association. (B) An example of histopathologic findings in another animal in the same study by Krug et al,9 where the heart was examined 6 hours after reperfusion following 2 hours of coronary occlusion. Myocardial capillaries are dilated, partially ruptured, and packed with erythrocytes.9 Bar = 40 μm. Reproduced with permission of the American Heart Association. (C) The first ultrastructural findings in coronary reflow reported by Kloner et al11 from a dog heart undergoing 20 minutes of coronary occlusion and 90 minutes of reperfusion. The capillary walls are…

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  Figure 2. In vivo Short-Axis MCE Images From Dogs During Reperfusion Following Coronary Occlusion and Infarct Size After Reperfusion In vivo short-axis myocardial contrast echocardiographic (MCE) images from dogs during 3 hours of reperfusion following 3 hours of coronary occlusion as well as infarct size measured at 3 hours after reperfusion. (A) Examples from a dog with a nearly transmural anterior wall infarction involving the left anterior descending coronary artery. (A1) After 45 minutes of reperfusion, a small defect localized to the endocardium in the anterior wall is noted as shown by arrows. (A2) After 3 hours of reperfusion, this defect was slightly larger, as depicted by arrows. The addition of dipyridamole (A3) resulted in a much larger relative contrast defect, shown by arrows, which closely delineated the true size and shape of the infarct (A4), indicated by arrows.34(B) Examples from a dog with a near transmural lateral wall infarction localized involving the left circumflex coronary artery. After 15 minutes of reperfusion, a moderate sized contrast defect is seen (B1) that underestimates the infarct size (B3). However, when adenosine is given, the perfusion defect…

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  Figure 3. Radiolabeled Microsphere–Derived Transmural MBF Data in Dogs Undergoing Coronary Occlusion Followed by Reperfusion (A) Temporal changes in myocardial blood flow (MBF) from individual animals over 3 hours of coronary reperfusion.34 MBF within the risk area is expressed as a percentage of transmural blood flow in the normal myocardium on the y-axis, and the interval after reperfusion is depicted on the x-axis. DP indicates measurements made during the infusion of dipyridamole after 3 hours of reflow. Marked temporal heterogeneity in resting MBF is noted in individual animals, with (B) temporal changes in the mean transmural MBF from all dogs. On average, the resting MBF in the risk area is similar to that of the normal bed and starts declining only at 3 hours after reperfusion. When DP is administered, infarcted regions show markedly reduced MBF reserve compared with the MBF reserve of the normal bed. It is during DP infusion that the spatial extent of perfusion defect on MCE reflects infarct size. (C) Relation between MCE defect size as a percentage of the short-axis slice (y-axis) and resting normalized endocardial blood flow within the infarct bed (x-axis) from another set of…

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  Figure 4. Schematic to Depict the Potential Role of Pericytes in Tissue Fluid Hemostasis When the mean coronary perfusion pressure (CPP) ranges within the autoregulatory range, between approximately 45 mm Hg and 120 mm Hg, the vascular smooth muscle cells (VSMCs) on the resistance arterioles contract or relax to maintain a constant capillary hydrostatic pressure (CHP). When CPP falls below 45 mm Hg, the arterioles are fully dilated and cannot dilate anymore. When CPP is above 120 mm Hg, they are fully constricted and cannot constrict anymore. At CPP <45 mm Hg, pericyte contraction causes capillary constriction in order to maintain a constant CHP. At CPP >120, precapillary sphincters contract to preserve CHP and this diverts flow through arteriovenous shunts that are not operative under normal conditions. Contraction of precapillary sphincters shuts of capillary beds, thus causing capillary rarefaction, which could lead to myocardial ischemia and heart failure.37,42,66,82 R = resistance.

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  Figure 5. Anatomy and Topography of Pericytes (A) Schematic of the vasculature showing arteriole (left) ensheathed by arteriolar smooth muscle cells (blue), the capillary network (center) enwrapped by pericytes (green) of different morphologies,74 and the draining venule covered by a thin, sparse layer of smooth muscle cells (right). (B) Images of the microvascular network in a skeletal muscle (left)42 and cardiac tissue (right). These images were obtained from neural glial antigen-2 (NG2)-dsRed transgenic mice in which pericytes express the red fluorescent protein dsRed under the NG2 promoter and vessels were labeled by intravascular administration of isolectin B4 (green) to label the vascular basement membrane. Bars = 50 μm. (C) Electron microscope image of capillary sectioned obliquely showing pericytes (P) at polar ends. Bar = 2 μm. (D) Scanning electron micrographs of capillary pericytes from a pit viper.70(Left) A pericyte extends many primary processes (black arrows) tightly enwrapping the endothelial tube. The asterisk shows a flattened secondary process. In addition, many small projections extend from the margins of the primary and secondary processes (arrowheads). The primary…

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  Figure 6. No Reflow Size Is Reduced With a Specific GPR39 Inhibitor No reflow areas are the black areas not stained with Thioflavin-S (Thio-S) (red arrows) and risk areas are the zones not stained blue with Evans blue (EB) (white arrows) in cross-sections of the heart in wild-type mice treated with vehicle (A) or a specific GPR39 inhibitor, VC43 (B). Extensive no reflow is visible in the former and none in the latter. Aggregate data (C) show a 2.9-fold smaller no-reflow/risk area ratio in the VC43- versus vehicle-treated animals.41

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  Figure 7. Infract Size Is Reduced With a Specific GPR39 Inhibitor Infarct size data from wild-type (WT) mice each treated with either VC43 or vehicle. A large infarct (pale tissue marked by black arrows) in a vehicle-treated animal (A) whose risk area (tissue not stained with Evans blue dye) is similar in size to a VC43-treated animal (B); the infarct size in the latter is markedly smaller. Aggregate data (C) show a 2-fold difference in the infarct size/risk area ratio in the VC43- versus vehicle-treated animals.41

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  Figure 8. Capillary Density and Diameter Are Larger During Myocardial Ischemia With a Specific GPR39 Inhibitor Capillary topography and pericyte locations in a WT vehicle-treated mouse (A) and a WT VC43-treated mouse (B) from within the risk area (top) and the normal myocardium (bottom). Bar = 10 μm. Capillaries are stained green with CD31 and pericytes are stained red with NG2. Both capillary size and density appear similar within the normal myocardium in both mice but appear more diminished in the risk area in the vehicle- versus the VC43-treated mouse. Quantitative analysis reveals the average capillary diameter (C) to be smaller in the risk area of the vehicle-treated mice compared to the VC43-treated mice, while being similar in the normal myocardium in the 2 groups of animals. Similarly, the capillary density measured as a percentage of capillaries occupying the total myocardial area viewed under the microscope (D) was less in the risk area of the vehicle- versus the VC43-treated mice. Capillary density was similar in the normal, nonischemic myocardium in the 2 groups of animals.41 DAPI = 4′,6-diamidino-2-phenylindole; other abbreviations as in Figures 5 and 7.

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  Figure 9. Spatial Extent of Perfusion During Coronary Occlusion Predicts Infarct Size 6 Hours Later In vivo myocardial contrast echocardiography–defined perfusion defect sizes at various intervals of microbubble replenishment of myocardium and ultimate infarct size on postmortem examination in a dog undergoing 6 hours of coronary occlusion without reperfusion. Although the risk area (A) 30 minutes after coronary occlusion appears large, the infarct size is modest (D) because of collateral myocardial blood flow (B and C), the absence of which clearly defines the spatial extent of necrosis.93 Thus, even at 30 minutes after coronary occlusion the worst-case scenario infarct size is defined based on myocardial perfusion. White arrows define extant of collateral myocardial blood flow laterally. Collateral flow is also present in the epicardium. Black arrows define extent of necrosis, which corresponds to area with no perfusion in C. PI = pulsing interval.

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  Central Illustration. Mechanism of No Reflow During Acute Myocardial Infarction When perfusion pressure drops during coronary occlusion, capillary hydrostatic pressure also drops. In order to keep this pressure constant pericytes contract resulting in capillary constriction and no reflow. Pericytes respond to increased 15-HETE and other vasoconstrictors by mobilizing calcium that results in their contraction. 15-HETE = 15-hydroxyeicosatetraenoic.