The “no reflow” phenomenon following acute myocardial infarction: Mechanisms and treatment options

AbstractIf ‘no reflow’ is observed within 45min of reperfusion using balloon angioplasty or stent, it is probably related to microthromboemboli, which may also contribute to the extension of the ‘no reflow’ zone by converting ‘low reflow’ areas into necrotic ones even when reperfusion is achieved more than 45min after the onset of coronary occlusion. Since ‘no reflow’ is noted when 45min of coronary occlusion has elapsed even in the absence of a thrombus, ‘no reflow’ late after reperfusion is predominantly due to tissue necrosis and unlikely to be resolved unless methods to reduce infarct size are used.Attempts at reducing the intracoronary thrombus burden during a coronary procedure for acute myocardial infarction (AMI) have been shown to reduce ‘no reflow’ and improve clinical outcome, as has the use of potent antithrombotic agents. Drugs that can reduce infarct size, when given intracoronary or intravenous in conjunction with a coronary intervention during AMI can also reduce ‘no reflow’ and improve outcomes in patients with AMI.The prognostic importance of ‘no reflow’ post-AMI is related to its close correspondence with infarct size. Although several imaging and non-imaging methods have been used to assess ‘no reflow’ or ‘low reflow’ myocardial contrast echocardiography remains the ideal method for its assessment both in and outside the cardiac catheterization laboratory

Contrast echocardiography in acute myocardial ischemia. II. The effect of site of injection of contrast agent on the estimation of area at risk for necrosis after coronary occlusion

Myocardial contrast echocardiography has been shown to accurately assess the area at risk for necrosis after acute coronary occlusion in the experimental model. The area at risk as determined by this method, however, has been defined in different ways depending on the model used. Some investigators have injected the contrast agent proximal to the site of coronary occlusion (left main coronary artery or aorta) and defined the area at risk as the segment of myocardium not showing a contrast effect (negative risk area). Others have injected the contrast agent directly into the occluded vessel and have defined the area at risk as that showing contrast enhancement (positive risk area).To evaluate whether the areas at risk determined by these two techniques are identical, six open chest dogs were studied using both methods. The area at risk was slightly but significantly larger when the contrast agent was injected into the occluded vessel than when it was injected proximally into the left main coronary artery (4.98 ± 1.69 versus 3.97 ± 1.27 cm2, p < 0.01). It is concluded that the site of injection of the contrast agent significantly influences the determination of area at risk. Therefore, data obtained by the two techniques should not be used interchangeably, and in a given study the area at risk should be measured consistently using one technique

Do false positive thallium-201 scans lead to unnecessary catheterization? outcome of patients with perfusion defects on quantitative planar thallium-201 scintigraphy

AbstractObjectives. We postulated that artifactually abnormal thallium-201 scans are well identified at the time of initial clinical interpretation by experienced readers and do not lead to unnecessary coronary angiography.Background. Exercise thallium-201 scintigraphy employing quantitative imaging techniques has yielded sensitivity and specificity values of 80% to 90%. There are image artifacts, such as breast shadows, and variants of normal that, if not correctly identified, can lead to a high false positive rate for detection of coronary artery disease.Methods. Data from 338 consecutive patients with one or more focal thallium-201 defects on quantitative planar Images were reviewed. All patients had undergone symptom-limited exercise scintigraphy and were classified as having either artifactual or nonartifactual thallium-201 defects after review of clinical reports.Results. Of the 265 patients with defects judged to be nonartifactual on clinical readings, 167 underwent coronary angiography, which demonstrated significant coronary artery disease (≥50% stenosis) in 161 (96%) and normal findings in 6. Four of the latter six had documented prior myocardial infarction. The remaining 73 patients (85% female) had thallium-201 defects deemed to be artifactual on clinical readings, chiefly as a result of breast (66%) and diaphragmatic (8%) attenuation or variants of normal (26%). Only 4 (5%) of the 73 patients underwent subsequent coronary angiography; none had coronary artery disease. One had aortic stenosis and two had variant angina. Follow-up (mean 20 ± 2 months) of the 69 patients in this group who did not undergo coronary angiography revealed no deaths and one nonfatal non-Q wave myocardial infarction.Conclusions. Artifactual defects on quantitative planar thallium-201 scintigraphy are well recognized by experienced interpreters and do not result in a high false positive rate leading to unnecessary cardiac catheferization. The incidence of coronary artery disease is high in patients with thallium-201 defects judged to be nonartifactual, and many patients with perfusion defects and angiographically normal coronary arteries have organic heart disease

971-56 Myocardial Contrast Echocardiography can be Used to Assess Dynamic Changes in Microvascular Function In-Vivo

The transit rate of sonicated albumin microbubbles (Albunex®, mean size=4.3 μ) has been shown to correlate with that of radiolabelled red blood cells in the blood perfused beating heart. We have previously demonstrated that the transit rate of these microbubbles is decreased during hyperthermia-induced microvascular injury In this study, we hypothesized that microbubble transit rate could be used as a marker of reversible endothelial injury during myocardial contrast echocardiography (MCE).We produced endothelial injury by inducing different degrees of myocardial hypoxia. This was accomplished by perfusing an arrested heart with either arterial blood, venous blood, or blood diluted to different degrees with a crystalloid cardioplegia solution. The flow rate into the cross-clamped aorta was held constant in each dog (mean=170mi), as was the perfusate temperature (mean = 27°C). Perfusate hematocrit varied from 0–27%, while perfusate pO2 ranged from 15–600mmHg. MCE was performed by injecting 2ml of a 1:1 dilution of Albunex® into the perfusate line and images were acquired at a sampling rate of 30 Hz. The mean transit rate of Albunex® through the myocardium was derived by fitting a λ-variate function to the background-subtracted time-intensity plots. The mean transit rate of Albunex® decreased as the hematocrit decreased. On multivariate analysis, the hematocrit and the pO2 were the best correlates of mean microbubble transit rate (y=0.06 hematocrit-0.002 pO2+0.72, r=0.72, p&lt;0.001).We conclude that MCE, can be used to assess reversible microvascular injury. This technique, therefore, has great potential for understanding dynamic changes in microvascular function in-vivo, particularly those modulated by the endothelium

Quantification of renal blood flow with contrast-enhanced ultrasound

AbstractOBJECTIVESThe goal of this study was to determine the ability of contrast-enhanced ultrasound (CEU) to quantify renal tissue perfusion.BACKGROUNDThe kinetics of tracers used to assess renal perfusion are often complicated by countercurrent exchange, tubular transport or glomerular filtration. We hypothesized that, because gas-filled microbubbles are pure intravascular tracers with a rheology similar to that of red blood cells, CEU could be used to quantify renal tissue perfusion.METHODSDuring a continuous venous infusion of microbubbles (SonoVue), regional renal perfusion was quantified in nine dogs using CEU by destroying microbubbles and measuring their tissue replenishment with intermittent harmonic imaging. Both renal blood volume fraction and microbubble velocity were derived from pulsing-interval versus video-intensity plots. The product of the two was used to calculate renal nutrient blood flow. Renal arterial blood flow was independently measured with ultrasonic flow probes placed directly on the renal artery and was increased using dopamine and decreased by placement of a renal artery stenosis.RESULTSAn excellent correlation was found between cortical nutrient blood flow using microbubbles and ultrasonic flow probe-derived renal blood flow (r = 0.82, p < 0.001) over a wide range (2.5 fold) of flows.CONCLUSIONSUltrasound examination during microbubble infusion can be used to quantify total organ as well as regional nutrient blood flow to the kidney

Detection of peripheral vascular stenosis by assessing skeletal muscle flow reserve

ObjectivesWe sought to determine whether the severity of peripheral arterial disease (PAD) can be assessed by measuring blood flow reserve in limb skeletal muscle with contrast-enhanced ultrasound (CEU).BackgroundNoninvasive imaging of distal limb perfusion could improve management of patients with PAD by evaluating the impact of large and small vessel disease, and collateral flow.MethodsIn 12 dogs, blood flow in the quadriceps femoris was measured by CEU at rest and during either electrostimulated contractile exercise or adenosine infusion. Femoral artery blood flow was measured by Doppler ultrasound. Studies were performed in the absence and presence of either moderate or severe stenosis (pressure gradient of 10 to 20 mm Hg and >20 mm Hg, respectively).ResultsResting femoral artery blood flow progressively decreased with stenosis severity, while resting skeletal muscle flow was reduced only with severe stenosis (52 ± 21% of baseline, p < 0.05), indicating the presence of collateral flow. Skeletal muscle flow reserve during contractile exercise or adenosine decreased incrementally with increasing stenosis severity (p < 0.01). The stenotic pressure gradient correlated with skeletal muscle flow reserve for exercise and adenosine (r = 0.70 for both, p < 0.01).ConclusionsContrast-enhanced ultrasound of limb skeletal muscle can be used to assess the severity of PAD by measuring muscle flow reserve during either contractile exercise or pharmacologic vasodilation. Unlike currently used methods, this technique may provide a measure of the physiologic effects of large- and small-vessel PAD, and the influence of collateral perfusion