William P. Glasheen

In vivo myocardial kinetics of air-filled albumin microbubbles during myocardial contrast echocardiography. Comparison with radiolabeled red blood cells.

Myocardial contrast echocardiography (MCE) is a new technique for assessing myocardial perfusion that uses intracoronary injections of microbubbles of air. Because these microbubbles have a mean diameter of 4.3 +/- 0.3 microns and an intravascular rheology similar to that of red blood cells (RBCs), we hypothesized that their mean myocardial transit rates recorded on echocardiography would provide an estimation of regional myocardial blood flow in the in vivo beating heart. Accordingly, blood flow to the left anterior descending coronary artery (LAD) of 12 open-chest anesthetized dogs (group I) was adjusted to 4 to 6 flows (total of 60 flows), and microbubbles and radiolabeled RBCs were injected into the LAD in a random order at each stage. The mean myocardial RBC transit rates were measured by fitting a gamma-variate function to time-activity plots generated by placing a miniature CsI2 probe over the anterior surface of the heart, and the mean myocardial microbubble transit rates were measured from time-intensity plots derived from off-line analysis of MCE images obtained during the injection of microbubbles. An excellent correlation was noted between flow (measured with an extracorporeal electromagnetic flow probe) and mean myocardial RBC transit rate (y = 2.83 x 10(-3)x + 0.01, r = .96, SEE = 0.02, P < .001). A close correlation was also noted between mean RBC and microbubble myocardial transit rates (y = 1.01x + 0.01, r = .89, SEE = 0.02, P < .001). Despite its theoretical advantages, a lagged normal density function did not provide a better fit to the MCE data than the gamma-variate function.(ABSTRACT TRUNCATED AT 250 WORDS)

Assessment of regional myocardial blood flow with myocardial contrast two-dimensional echocardiography

It was hypothesized that regional myocardial blood flow could be measured using myocardial contrast echocardiography. Accordingly, arterial blood was perfused into the coronary circulation in 16 dogs. In Group 1 dogs (n = 8), blood flow to the cannulated left circumflex artery was controlled with use of a roller pump, whereas in Group 2 dogs (n = 8) blood flow to the left anterior descending coronary artery was controlled by a hydraulic occluder placed around it. Sonicated microbubbles (mean size 4 microns) were used as the contrast agent. In Group 1 dogs the microbubbles were injected subselectively into the left circumflex artery, whereas in Group 2 dogs they were injected selectively into the left main coronary artery and two-dimensional echocardiographic images were recorded. Computer-generated time-intensity curves were derived from these images and variables of these curves correlated with transmural blood flow measured with radiolabeled microspheres. A gamma-variate function (y = Ate-alpha t) best described the curves, and alpha (a variable of curve width) correlated well with transmural blood flow at different flow rates in all Group 1 and Group 2 dogs (mean r = 0.81 and 0.97, respectively). Other variables of the curve width also correlated well with myocardial blood flow, but peak intensity had a poor correlation with myocardial blood flow in both groups of dogs (r = 0.39 and r = 0.63, respectively). When data from all dogs were pooled, Group 1 dogs still showed good correlation between variables of curve width and myocardial blood flow (r = 0.81); Group 2 dogs did not (r = 0.45). The difference between the two sets of dogs was related to the site of contrast agent injection. It is concluded that measurement of the transit time of microbubbles through the myocardium with two-dimensional echocardiography accurately reflects regional myocardial blood flow. Although injection of contrast agent selectively into the left main coronary artery only allows measurement of relative flow, it may be feasible to measure absolute flow by injecting contrast agent subselectively into a coronary artery. Myocardial contrast echocardiography may, therefore, offer the unique opportunity of simultaneously assessing regional myocardial perfusion and function in vivo.

Myocardial contrast echocardiography in humans. II. Assessment of coronary blood flow reserve

The hypothesis that myocardial contrast echocardiography could be used to simultaneously assess coronary blood flow reserve and the size of the perfusion bed supplied by a coronary artery was examined in nine patients and six dogs. All patients were undergoing cardiac catheterization and had single vessel coronary artery disease (greater than or equal to 85% stenosis of either the proximal left anterior descending or the left circumflex coronary artery); the six dogs had a critical stenosis of the left circumflex coronary artery. Three milliliters of sonicated Renografin-76 (mean microbubble size 6 micron) was injected into the left main coronary artery before and after intracoronary administration of papavarine, 6 to 9 mg. The beds supplied by the normal and stenotic vessels could not be differentiated during contrast echocardiography before injection of papavarine. However, after papavarine, the normal vascular bed showed significantly more contrast enhancement than did the bed supplied by the stenotic artery. This disparity in contrast enhancement made it possible to delineate the size of the bed perfused by the stenotic vessels. When quantitative analysis of the time-intensity curves obtained from the echocardiograms was performed in the dogs, the absolute values for the area under the curve, peak contrast intensity and curve width did not correlate with absolute blood flows measured with radiolabeled microspheres. However, the ratios of the areas under the curves derived from the two vascular beds before and after papavarine correlated well with the ratios of blood flows between the two beds during the same stages (r2 = 0.73 by linear regression and r2 = 0.85 by an exponential function). In comparison, the ratios of peak amplitudes and curve widths before and after papavarine had poor correlations with ratios of flows from the two beds (r2 = 0.18 and 0.02, respectively). In conclusion, myocardial contrast echocardiography can be used to simultaneously assess coronary blood flow reserve and the size of the perfusion bed supplied by a stenotic vessel.

Albunex: A Safe and Effective Commercially Produced Agent for Myocardial Contrast Echocardiography

Microbubble solutions of sonicated human serum albumin have been shown to be effective echocardiographic contrast agents free from adverse effects on coronary blood flow, left ventricular function, and systemic hemodynamics. Albunex is a commercially prepared solution of air-filled albumin microspheres prepared from sonicated 5% human serum albumin. The effects of Albunex on coronary blood flow, left ventricular function, and systemic hemodynamics, as well as contrast enhancement were evaluated in 10 open chest, anesthetized dogs. After an intracoronary injection, Albunex did not alter left atrial, left ventricular systolic or end-diastolic, or mean aortic pressures. It did not cause a coronary hyperemic response, alter left ventricular systolic thickening, or lower the peak positive left ventricular dp/dt. It did not alter these measurements even in the presence of a critical coronary stenosis. The contrast opacification produced by Albunex microbubbles was dose dependent (optimal dose range of 0.033 to 0.5 ml/kg), with attenuation occurring at higher doses. In conclusion, although Albunex provides adequate contrast enhancement, intracoronary injection of Albunex does not significantly alter coronary blood flow, left ventricular function, or systemic hemodynamics. Because of the standardized size and concentration of the microspheres, Albunex may be an ideal agent for myocardial contrast echocardiography.

Dobutamine echocardiography for determining the extent of myocardial salvage after reperfusion. An experimental evaluation.

BackgroundAlthough dobutamine echocardiography is being increasingly used to determine the presence of viable myocardium in patients who have undergone successful reper-fusion therapy, the physiological basis for such a use has not been clearly defined. Because postischemic myocardium has contractile reserve, we hypothesized that the absolute degree of wall thickening induced by dobutamine during reflow would be directly related to the amount of myocardium that has escaped necrosis. Methods and ResultsThree groups of 12 dogs each were studied at baseline and during 2 to 6 hours of coronary artery occlusion and 15 minutes of reperfusion. In group 1 dogs, which did not receive dobutamine during any of these stages, percent wall thickening at these stages was 32±6%, −2±6%, and 5 ±6%, respectively, and there was no relation between infarct size and percent wall thickening during reflow (r=.20, P=.51). In group 2 dogs, which received 15 μg/kg per minute of dobutamine at all stages, wall thickening at these stages was 40±8%, 0±8%, and 19±10%, respectively, and a good inverse correlation was noted between infarct size and percent wall thickening during reflow (r= −.81, P=.001). In group 3 dogs, in which wall thickening during reflow was measured both before and during infusion of 15 μg/kg per minute of dobutamine, it was 5±8% and 18±14%, respectively, at these stages. Although the correlation between infarct size and percent wall thickening was poor in the absence of dobutamine (r=.36, P=.26), an excellent inverse correlation was noted between the two in the presence of dobutamine (r= −.93, P<.001). A fair inverse correlation was also noted between infarct size and the absolute change in wall thickening induced by dobutamine (r= −.72, P<.01). Maximal wall thickening was noted at a dobutamine dose of 15 μg/kg per minute, and lower doses did not elicit thickening in the presence of larger infarcts despite the presence of viable myocardium. ConclusionsWhen myocardial necrosis coexists with post-ischemic myocardial dysfunction and no residual coronary stenosis, the absolute degree of wall thickening during dobutamine can be used to determine the extent of myocardium that has escaped necrosis. The dose of dobutamine needed to elicit maximal thickening of the postischemic myocardium is related to the amount of myocardial necrosis.

Successful and reproducible myocardial opacification during two-dimensional echocardiography from right heart injection of contrast.

Background Myocardial contrast echocardiography currently involves intra-arterial injection of contrast. For this technique to have a broader application, it is necessary that myocardial opacification be achieved from a venous injection of contrast. Methods and Results To achieve myocardial opacification after right-side injection of contrast, two groups of open-chest anesthetized dogs were studied. Group 1 included nine dogs in whom microbubbles of various sizes, concentrations, and volumes were injected into the left atrium to determine microbubble characteristics that influence myocardial opacification. Group 2 included eight dogs in whom the effect of the combination of microbubble characteristics and myocardial blood flow on myocardial opacification was evaluated after right atrial injection of contrast. Background-subtracted time-intensity plots were generated from the myocardium to measure peak videointensity. In the group 2 dogs, digital subtraction and color coding were used to further highlight the contrast effect. The number, concentration, and size of the microbubbles all independently affected (p < 0.01) peak myocardial videointensity after left atrial injection of contrast on multivariate analysis. Highly concentrated microbubbles (4.4 to 5.1 billion/ml) given during dipyridamole-induced coronary hyperemia was most frequently (88%) associated with myocardial opacification after right atrial injection of contrast and was the best predictor of this result on multivariate analysis (x2=9.01, p = 0.003). No changes were noted in left atrial, left ventricular, and pulmonary artery pressures despite injection of large numbers of microbubbles into the right atrium. Conclusions Successful and reproducible myocardial opacification can be achieved during myocardial contrast echocardiography after right atrial injection of contrast. These findings could have far-reaching implications in the use of myocardial contrast echocardiography in acute and chronic ischemic syndromes in humans.

Myocardial contrast echocardiography without significant hemodynamic effects or reactive hyperemia: A major advantage in the imaging of regional myocardial perfusion☆☆☆

All agents used for myocardial contrast echocardiography to date produce adverse hemodynamic effects and alter coronary blood flow. It was hypothesized that because 5% human albumin, when sonicated for use as a contrast agent, is neither hyperosmolar nor a calcium chelator, it would not have significant effects on coronary blood flow, left ventricular function or systemic hemodynamics. Albumin microbubbles of two distinct sizes (mean size 2.9 and 5.8 micron) were produced and compared with nonsonicated albumin, nonsonicated Renografin, sonicated Renografin and hand-agitated Renografin for their effects on hemodynamics, coronary blood flow and regional left ventricular systolic thickening in 15 open chest anesthetized dogs. None of the albumin solutions significantly altered left atrial, left ventricular systolic and end-diastolic and mean aortic pressures. These agents did not cause a coronary hyperemic response or alter left ventricular systolic thickening, but slightly lowered the peak positive left ventricular maximal rate of rise in pressure (dP/dt) (-4.1 +/- 5.4%, p less than 0.01). In contrast, all the Renografin solutions caused significant changes in all these variables (p less than 0.02). In six dogs. albumin solutions did not alter these variables even in the presence of critical coronary stenosis. The contrast opacification produced by 5.8 micron albumin microbubbles was equivalent to that produced by sonicated Renografin. Compared with an equivalent amount of saline and nonsonicated albumin solutions, 10 ml of sonicated albumin did not produce any evidence of infarction, embolization or hemorrhage in the myocardium, brain or kidneys of rabbits.(ABSTRACT TRUNCATED AT 250 WORDS)

Intraoperative assessment of regional myocardial perfusion using quantitative myocardial contrast echocardiography: An experimental evaluation☆☆☆

To test the hypothesis that myocardial contrast echocardiography can be used to quantitate regional myocardial flow in the arrested heart at the time of delivery of cardioplegic solution, data were acquired in 13 dogs on cardiopulmonary bypass. Different degrees of stenosis were placed in random order on the left anterior descending coronary artery. For each stenosis, myocardial contrast echocardiography was performed by injecting sonicated albumin microbubbles into the cross-clamped aortic root at the time of delivery of cardioplegic solution. The resultant echocardiographic images were analyzed on an off-line computer. Background-subtracted time-intensity plots were generated, and an exponential function, f(t) = Ce-alpha t + De- beta t, was applied to each plot. Variables that reflected the total number of microbubbles entering the coronary artery bed, such as the area under the curve and the peak height of the curve, correlated best with radiolabeled microsphere-measured myocardial flow (r = 0.92 and r = 0.91, respectively). Variables that reflected the appearance of contrast microbubbles in the myocardium, such as the initial slope and the slope at 1 s, also had a good correlation with myocardial flow (r = 0.84 and r = 0.89, respectively). Variables that reflected the washout of contrast medium from the myocardium, such as the slope of the descending portion of the curve, had only a fair correlation with myocardial flow (r = 0.65). In six dogs, the technique of injecting contrast medium into the cross-clamped aortic root was also examined. Although continuous infusion of contrast medium produced smaller perturbations in mean aortic and distal left anterior descending artery pressures compared with a bolus injection (p less than 0.01), the correlation between the variables of the time-intensity curves and flow was equally close with both techniques. It is concluded that it is possible to quantitate myocardial flow by using myocardial contrast echocardiography at the time of delivery of cardioplegic solution in dogs on cardiopulmonary bypass. The implementation of this technique in humans might be useful in guiding the sequence of graft placement and thereby improving myocardial preservation during coronary artery bypass operations.

Effects of antifibrinolytic agents on the life span of fibrin sealant

Fibrin sealant, a biologic glue consisting of fibrinogen and thrombin, has been used in a variety of surgical procedures. The usefulness of fibrin sealant may be prolonged by the addition of antifibrinolytic agents. This study compared the efficacy of transexamic acid (30 mg/ml), epsilon-aminocaproic acid (25 mg/ml), and aprotinin (3000 KIU/ml) to provide data on the choice of an appropriate antifibrinolytic agent for use with fibrin sealant. By use of a modified in vitro plasma euglobulin lysis time (hours), all agents were found to be superior (n = 10 for each agent, P < 0.05, analysis of variance for completely randomized design followed by Dunnetts test for multiple comparisons) to control. Lysis times (mean +/- SE) were (control) 50.9 +/- 0.5, (tranexamic acid) 402.6 +/- 25.4, (epsilon-aminocaproic acid) 433.5 +/- 21.2, and (aprotinin) 393.9 +/- 26.0. Using the in vivo implantation of fibrin sealant supplemented with 125I-fibrinogen in the rat peritoneum significant improvement in percentage clot (mean +/- SE) remaining was found (P < 0.05, analysis for repeated measures followed by tests for multiple comparisons) under the following conditions: at 3 hr by weight (n = 15), tranexamic acid (70.13 +/- 2.02%) was superior to aprotinin (61.22 +/- 2.21%) and control (61.28 +/- 2.36%); at 3 hr by radioactivity counts (n = 19), tranexamic acid (76.29 +/- 0.75%) was superior to epsilon-aminocaproic acid (72.52 +/- 1.28%) and aprotinin (73.84 +/- 0.78%); at 72 hr by radioactivity counts (n = 10), aprotinin (27.30 +/- 2.45%) was superior to epsilon-aminocaproic acid 19.76 +/- 3.09% and control (20.38 +/- 3.01%). These data suggest the early (3-hr) superiority of tranexamic acid as an inhibitor of plasminogen activation and the late (72-hr) effectiveness of aprotinin as an inhibitor of plasmin. The possibility of a synergistic effect of tranexamic acid and aprotinin is suggested.

Myocardial contrast echocardiography and the transmural distribution of flow: A critical appraisal during myocardial ischemia not associated with infarction

OBJECTIVES This study was undertaken to determine whether myocardial contrast echocardiography can be used to estimate the transmural distribution of flow. BACKGROUND Myocardial contrast echocardiography has been shown to reliably measure average transmural blood flow during myocardial ischemia. However, there is controversy regarding its ability to determine the transmural distribution of flow. METHODS The transmural distribution of flow was measured in 21 open chest anesthetized dogs with use of radiolabeled microspheres and sonicated albumin microbubbles (mean size 4.5 microns). In the 11 Group I dogs, myocardial contrast echocardiography was performed at baseline and during left anterior descending artery stenosis. In five of these dogs, it was also performed during left circumflex artery stenosis. In these dogs large (mean 12 microns) hand-agitated bubbles were also used. In the five Group II dogs, myocardial contrast echocardiography was performed before and 45 s after intracoronary injection of 6 mg of papaverine in the presence of a critical left circumflex artery stenosis. The five Group III dogs were studied during cardiopulmonary bypass at baseline and during left anterior descending artery stenosis. Off-line image analysis of the echocardiographic images was performed and time-intensity curves obtained from these images were correlated with radiolabeled microsphere-derived flows. RESULTS The ratios of the parameters derived from the endocardium and epicardium during myocardial contrast echocardiography were found to correlate poorly (ranging from R2 = 0 to R2 = 0.35) with radiolabeled microsphere-derived endocardial/epicardial flow ratios over a wide range of flow ratios (0.01 to 2.58). These results were not influenced either by the location of the regions of interest (left anterior descending vs. left circumflex artery bed) or by the size of the bubbles (4.5 vs. 12 microns). CONCLUSIONS Myocardial contrast echocardiography cannot be used to assess the transmural distribution of flow during myocardial ischemia not associated with infarction.