Myocardial contrast two-dimensional echocardiography: experimental examination at different coronary flow levels

Contrast Sonography, Video Densitometry and Intervillous Blood Flow: A Pilot Project

PURPOSE

To examine the feasibility of constructing time-intensity (TI) curves from the intervillous space with an intravascular ultrasound contrast agent and computer assisted video densitometry.

STUDY DESIGN

We sedated nine pregnant baboons, optimized the grey scale and color Doppler images of their placentas, and then fixed the transducers in place. For each injection of contrast, we recorded images on videotape without changing the ultrasound image processing functions. Video images were captured using a Macintosh personal computer equipped with a video-capture board using image analysis software (Image 1.4, W Rasband, NIH). For each injection, we sampled digitized images of a fixed region of interest at regular intervals. After computing the mean video density of each image, we used the sampling frequency to construct TI curves depicting any change over time as the contrast agents washed into and out of the intervillous space.

RESULTS

Three of four agents tested produced changes in the video density of the placenta. TI curves were established using both grey scale and color Doppler signal augmentation. As expected, intra-arterial agents produced rapid accumulation and decay. Intravenous agents produced more protracted effects secondary to bolus dilution and transit through the right heart and pulmonary vascular bed.

CONCLUSION

TI curves may be generated from the intervillous space with the use of a transpulmonary ultrasound contrast agent and video densitometry. If validated by further study, this may allow investigators to apply ultrasound and indicator-dilution theory to intervillous blood flow.

Novel method for assessing myocardial perfusion: visualization and measurement of intramyocardial coronary blood flow in the entire left ventricular wall using contrast enhanced, high frequency Doppler echocardiography

Using a high frequency ultrasonic transducer, intramyocardial coronary blood flow (IM-CBF) can be visualized and evaluated during hemodynamic changes in the anterior wall and septum of the left ventricle (LV). We tested the hypothesis that detection and quantitative measurement of IM-CBF of entire LV segments are feasible using a high frequency ultrasonic transducer in conjunction with intravenous contrast injection in vivo. A 3 - 8 MHz transducer was used to image and measure IM-CBF in 10 anesthetized dogs. We obtained a color Doppler image of IM-CBF in the LV short-axis view after intravenous Levovist injection (25 mg/ml). The IM-CBF velocity was recorded using spectral Doppler in the antero-septal and infero-posterior wall of closed chest dogs and in the entire LV after opening the chest. A significant increase in IM-CBF velocity was observed in all LV regions after adenosine 5'- triphosphate (ATP) administration. After Levovist(TM) injection, the visualization of IM-CBF was improved and the spectral Doppler pattern of coronary flow velocity was clarified compared to baseline. IM-CBF was assessed in the antero-septal region of the LV before and after left anterior descending coronary artery occlusion. A high frequency ultrasonic transducer in conjunction with intravenous contrast injection improved IM-CBF visualization, enabling quantitative evaluation of the intramyocardial coronary circulation in the entire LV after coronary occlusion and hyperemia. This study may represent a step towards noninvasive assessment of myocardial perfusion before and after coronary reperfusion.

Abnormal myocardial blood flow distribution in patients with angina pectoris and normal coronary arteriograms

To evaluate coronary microvascular function and its relation to the genesis of chest pain and ST-segment depression during exercise in patients with syndrome X, pacing-induced changes in transmural myocardial blood flow distribution were quantitatively assessed by 2-dimensional myocardial contrast echocardiography. Of 25 patients with a history of chest pain and normal coronary arteries with the negative ergonovine test, 11 had exercise-induced chest pain and ST-segment depression (syndrome X), and 14 did not (controls). Myocardial blood flow distribution before and after pacing stress was assessed by measuring the ratio of the endocardial to epicardial gray level (ie, endo/epi gray level ratio) in the territory of the left anterior descending coronary artery. Pacing-induced chest pain and ST-segment depression were observed in syndrome X, but not in controls. The endo/epi gray level ratio in syndrome X significantly decreased after pacing (from 0.98+/-0.10 to 0.76+/-0.17, p<0.01), but not in controls (from 0.97+/-0.08 to 0.99+/-0.08, NS). Abnormal myocardial blood flow distribution may play an important role in exercise-induced chest pain and ST-segment depression in these patients.

Thermographic imaging in the beating heart: a method for coronary flow estimation based on a heat transfer model

Intraoperative thermographic imaging in open-chest conditions can provide the surgeon with important qualitative information regarding coronary flow by utilizing heat transfer analysis following injection of cold saline into the aortic root. The heat transfer model is based on the assumption that the epicardial temperature changes are mainly due to convection of heat by the blood flow, which may, therefore, be estimated by measuring the temperature variations. Hearts of eight dogs were exposed and imaged by a thermographic camera. Flow in the left arterial descending (LAD) coronary branch was measured by a transit-time flowmeter. 20 ml of cold saline were injected into the aortic root (just after the aortic valve) and the epicardial temperature images were recorded at end-diastole, for 20-30 s. Different flow rates were achieved by 1 min occlusion of the LAD, which affected a reactive hyperemic response. The dynamics of the temperature in the arterial coronary tree was obtained by averaging the temperature over an edge-detected arterial segment for each frame. The heat transfer equation was curve-fitted, and the flow-dependent heat transfer index was correlated with the experimentally determined coronary flow (r = 0.69, p < 0.001). In summary: a method for quantitative estimation of coronary blood flow by thermography and heat transfer analysis was developed and tested in animal experiments. This method can provide important information regarding coronary blood flow during open-chest surgical procedures.

Estimation of coronary blood flow by ECG gated cardiac thermography in open-chest conditions

Thermography is suggested as a tool to estimate myocardial and coronary epicardial flow in open-chest heart surgery. To test the feasibility and compare various methods for coronary flow estimation in open-chest surgery, thermographic imaging was applied to eight open-chest dogs which were injected with cold saline into the aortic root. Blood flow in the left arterial descending (LAD) coronary vessel was measured by a transit-time flowmeter. ECG gated images were acquired for 20-30 s, while the cold saline (20 ml) was injected into the aortic root. Several flow levels were achieved during repeated hyperaemic response to transient occlusions of the LAD. A temperature response curve for each flow level was obtained by averaging over an edge-detected arterial segment for each image frame. Several indices were calculated from the temperature curve and correlated with the measured coronary flow. These include: an index based on a corrective heat transfer model (r = 0.69, p < 0.001), the slope of the descending part of the response curve (r = 0.76, p < 0.001), the peak temperature difference (r = 0.66, p < 0.001), and the area above the temperature response curve (r = 0.61, p < 0.01). As shown, coronary flow can be estimated quantitatively by intraoperative epicardial thermography, and may therefore provide important on-line information regarding blood flow during open-chest surgical procedures. Further studies are required for optimal application of this technique so as to increase its potential as a valid clinical tool.

Contrast echocardiography: review and future directions

Recent developments and advances in contrast echocardiography have been made to improve the diagnosis and evaluation of cardiac structures and function. By coupling new developments in acoustic instrumentation with new contrast agents, information that was previously difficult or impossible to gather by standard 2-dimensional echocardiography can now be obtained. Numerous studies have been published confirming the advantages of using contrast during echocardiographic studies, particularly with stress testing and myocardial perfusion. This review aims to summarize (1) the various contrast agents that are available or being developed; (2) factors that have been found to affect the strength of enhanced signals; (3) the new developments in instrumentation that improve the ability of scanners to differentiate echo contrast from cardiac tissue; and (4) the documented and possible future uses of contrast echocardiography.

Contrast echocardiography: influence of ultrasonic machine settings, mixing conditions, and pressurization on pixel intensity and microsphere size of Albunex solutions in vitro

To use Albunex as a blood-flow tracer, the stability and consistency of microspheres under mixing conditions must be known. This study examined the effects of mixing conditions and machine settings on the size and echogenicity of Albunex solutions in vitro. Acoustic power, log compression, time-gain compensation, and transducer frequency were varied as Albunex solutions were imaged after mixing with magnetic stirring and pressurized. Higher acoustic power and lower transducer frequency decreased mean pixel intensity of Albunex solution images over time. Intensity, size, and number of Albunex microspheres were not significantly different between stirring speeds. The echogenicity of the Albunex solutions decreased with pressurization, and the critical pressure necessary to reduce the intensity to half its initial value increased with the logarithm of concentration (r = 0.91; p < 0.001). The microsphere size decreased with pressurization and remained smaller after pressure release (3.66 +/- 2.13 versus 1.47 +/- 0.95 microns; p < 0.01). These data indicate that acoustic power and transducer frequency may affect the physical properties of Albunex microspheres, decreasing mean videointensity. Pressure sensitivity of Albunex caused the decrease of videointensity and microsphere size.

Myocardial contrast echocardiography: reliable, safe, and efficacious myocardial perfusion assessment after intravenous injections of a new echocardiographic contrast agent

Reliable and reproducible myocardial opacification after intravenous administration of echocardiographic contrast agents has remained elusive. This study was performed to determine whether a new agent, FS069, a suspension of perfluoropropane-filled albumin microspheres (3.6 microns average microbubble size, concentration 8 x 8(8)/ml), could achieve safe and successful myocardial opacification in open-chest dogs. Seventeen dogs (group 1, n = 7, group 2, n = 10) underwent two-dimensional echocardiography before, during, and after the administration of intravenous FS069. Safety was evaluated by measuring arterial and pulmonary artery pressures, heart rate, blood gases, systolic function, myocardial blood flow, and postmortem analysis of myocardial viability by triphenyl-tetrazolium chloride staining. Efficacy to detect changes in regional myocardial perfusion was assessed by injecting FS069 at baseline, after sequential coronary occlusions and reperfusion, and during intravenous vasodilators with and without coronary occlusions. Results were compared with radiolabeled microspheres. FS069 was found to be safe and effective. In the absence of coronary occlusions, uniform myocardial opacification was observed in all dogs. A perfusion defect was observed in all dogs during coronary occlusions. Background-subtracted peak contrast intensity in the myocardium correctly identified regional myocardial blood flow changes and showed a significant correlation with radiolabeled microspheres (r = 0.65, p = 0.0001).

Second harmonic imaging of an intravenously administered echocardiographic contrast agent: Visualization of coronary arteries and measurement of coronary blood flow

OBJECTIVES

This study sought to evaluate the potential of second harmonic contrast echocardiography to assess coronary vasculature.

BACKGROUND

Newer transpulmonary ultrasound contrast agents capable of resonance phenomena detected by harmonic imaging may theoretically be able to demonstrate blood flow in the myocardium.

METHODS

Transthoracic B-mode images and Doppler were obtained using a prototype second harmonic ultrasound system after femoral vein injection of AF0145 (10 to 40 mg) in 13 closed chest dogs (mean weight 25.6 kg). Coronary Doppler flow was simultaneously invasively measured using an intracoronary flow wire and visually compared with transthoracic Doppler flow. "Noninvasive" coronary vasodilator reserve was determined by measuring the ratio of the Doppler time velocity integral after adenosine to the baseline value and compared with the "invasive" intracoronary determination.

RESULTS

Harmonic imaging showed heterogeneous opacification of the myocardium characterized by linear branching structures consistent with intramyocardial coronary arteries, which were not clearly visible during conventional ultrasound imaging. In nine dogs, transthoracic Doppler was performed, and characteristic coronary Doppler flow was observed, identical to the simultaneously observed intracoronary Doppler flow. Intracoronary adenosine (120 to 150 microgram) equally increased intracoronary and transthoracic Doppler flow velocities. The calculated "noninvasive" and "invasive" coronary vasodilator reserve ratios were similar ([mean +/- SD] 3.3 +/- 1.0 and 3.6 +/- 1.2, p = NS), with excellent correlation (r = 0.95, p = 0.0012).

CONCLUSIONS

These findings indicate that noninvasive assessment of intramyocardial coronary vasculature and measurement of coronary blood flow reserve are possible using second harmonic contrast echocardiography.

Effects of shadowing on the time-intensity curves in contrast echocardiography: a phantom study

Literature states that the inability of myocardial contrast echocardiography to assess differences in flow between regions could be related to myocardial shadowing. In this study, we evaluate this effect on the time-intensity curves. In a perfusion phantom, shadowing was induced by high concentrations of a contrast agent (approximately 1 x 10(8) bubbles/mL), and evaluated for different flows (50-270 mL/min). The high concentration resulted in an increase of the video-intensity in regions of interest close to the transducer (2 mm) and a marked reduction in remote areas (20 mm). The peak intensity of the time-intensity curves did not correlate with flow (range r = 0.1, 0.37), while the inverse area under the curve correlated strongly (r = 0.98). The inverse curve width and the decay after peak intensity also correlated excellently with flow (r = 0.99 and range r = 0.97, 0.99). We opt for the decay as transmural flow indicator using contrast echocardiography, since this parameter is least affected by shadowing.

Intravenous Albunex during transesophageal echocardiography: quantitative assessment by videodensitometry and integrated backscatter analysis from unprocessed radiofrequency signals

This study investigates the comparative sensitivity of video and radiofrequency imaging to detect changes of the myocardial acoustic properties after intravenous Albunex. Thirty-six patients received Albunex, 0.08 and 0.22 ml/kg intravenously, during transesophageal imaging of the ventricular short axis. Analysis of video images was performed in all patients and of radiofrequency data in 20 patients. Although myocardial videointensity remained unchanged, 57% of the myocardial backscatter plots demonstrated significant contrast enhancement. The study demonstrates that intravenous Albunex is capable of myocardial contrast enhancement and proves the diagnostic superiority of radiofrequency compared with video imaging. Ultrasonic radiofrequency imaging may provide a technical basis for future noninvasive assessment of myocardial perfusion.

Myocardial contrast echo effect: the dilemma of coronary blood flow and volume

Despite the useful information provided by myocardial contrast echocardiography, the meaning of myocardial contrast intensity remains elusive. This review is meant to define the contribution of physical and biologic factors in producing myocardial contrast and to elucidate the relative roles of coronary blood flow and intramyocardial blood volume in determining contrast effect. The main physical factors influencing the contrast echo effect include the properties of microbubbles as scattering elements (mainly their radius, compressibility, stability and concentration), electronic signal processing, instrument setting and contrast-induced signal attenuation. The effect of these factors can be limited by an appropriate experimental or clinical setup. Biologic factors are less easily controllable, and changes in coronary blood flow and alterations in myocardial blood volume appear to be the main determinants of myocardial contrast intensity. Moreover, these factors influence contrast intensity in opposite directions. Both the area under the time-intensity curve and the mean transit time of myocardial contrast are inversely related to coronary blood flow but directly related to myocardial vascularity and blood volume. Therefore, an increase in coronary flow not accompanied by an increase in myocardial vascularity and volume is accompanied by a decrease in the area under the curve and mean transit time of contrast. Conversely, an increase in coronary flow mediated by augmented myocardial vascularity and volume will produce an increase in the area under the curve and mean transit time. A better understanding of the physical and biologic determinants of contrast echo intensity will be fundamental in the clinical application of new agents and technologies.

Improvement of subendocardial myocardial perfusion after percutaneous transluminal coronary angioplasty. A myocardial contrast echocardiography study with correlation between myocardial contrast reserve and Doppler coronary reserve

BACKGROUND

After angioplasty coronary reserve improves but does not normalize in most patients. The purpose of this study was to examine before and after angioplasty coronary reserve and transmural myocardial blood flow distribution using myocardial contrast echocardiography.

METHODS AND RESULTS

Twelve patients with left anterior descending coronary artery stenosis were investigated before and immediately after angioplasty. A Doppler catheter was placed in the proximal segment. Myocardial contrast echocardiography was performed by imaging the septum in M mode in a parasternal view using a 3.0-mL bolus of sonicated amidotrizoate sodium meglumine through the guiding catheter. The gray level before injection was subtracted from the gray level after injection to maximize contrast time-intensity curves. The area under the curve was used as an indicator of myocardial blood flow, and subendocardial/subepicardial ratios were measured. After baseline measurements were obtained, Doppler and echographic data were recorded after a bolus infusion of papaverine into the left main coronary artery. The same protocol was performed in patients after angioplasty and in five control subjects with normal coronary arteries. Before angioplasty, echocardiographic and Doppler coronary reserve were 2.57 +/- 0.48 and 2.54 +/- 0.57, respectively. Both increased after angioplasty to 3.65 +/- 0.57 and 3.36 +/- 0.70, respectively (P < .05). Coronary reserve values obtained in patients with these two methods under the different conditions and in control subjects were correlated (r = .81; P = .0001). Before angioplasty, subendocardial/subepicardial septal ratios decreased from 0.80 +/- 0.48 to 0.60 +/- 0.27 after papaverine (P < .05). However, after angioplasty, these ratios tended to increase, from 0.72 +/- 0.27 to 0.92 +/- 0.45 after papaverine, but they did not change in control subjects (1.11 +/- 0.23 to 0.92 +/- 0.11).

CONCLUSIONS

These results show that myocardial contrast echocardiography yields flow reserve values that correlate with values obtained using intracoronary Doppler. This technique may be considered as an accurate tool to assess coronary reserve in humans.

Acoustic lability of albumin microspheres

The sonication of human serum albumin produces air-filled microspheres that are used in echocardiographic studies of myocardial perfusion. Recent studies suggest that the microspheres disappear when high pressures are applied, altering the relationship between the administered microsphere dose and the echocardiographic response. Because an ultrasound pulse generates a pressure wave in insonified medium, we hypothesized that with increasing acoustic pulse pressure, the microsphere concentration decreases, hence ultrasonic backscatter decreases. We measured relative integrated backscatter from albumin microspheres diluted in normal saline solution (6152 microspheres/ml) and 5% human plasma protein fraction (24,608 microspheres/ml), with increasing acoustic pulse pressures at the transducer's focus. Backscatter was also measured in normal saline solution with increasing concentrations (up to 15,380 microspheres/ml) of albumin microspheres at an acoustic pulse pressure of 0.11 MPa (1.1 atm). Backscatter and microsphere concentration were related logarithmically: y = 3.38 x 0.32; r = 0.93. Backscatter was unchanged over time at acoustic pulse (peak compression) pressures less than 0.15 MPa (1.5 atm). However, backscatter decreased readily at acoustic pulse pressures greater than 0.33 MPa (3.3 atm), which included any mixing effects. Thus, albumin microspheres are acoustically labile.

Digital subtraction myocardial contrast echocardiography: design and application of a new analysis program for myocardial perfusion imaging

Myocardial contrast echocardiography may provide important physiologic information on myocardial perfusion. Most current analysis programs use manual frame grabbing and selecting of the area of interest. This is time-consuming and not highly reproducible. A system for automatic analysis of myocardial contrast echocardiographic studies was developed and evaluated. The program acquires an electrocardiographically gated sequence of end-diastolic images with a frame grabber in a personal computer. The baseline image is subtracted and the videodensity versus time contrast curve parameters are calculated on-line. Fast color-coded analysis is done automatically with a running square window that covers the entire image. A second mode of contrast analysis allows manual selection of multiple regions of interest. The program was evaluated with contrast echo data from open-chest dogs and two demonstrative patients. This myocardial contrast analytic package is an inexpensive, rapid, flexible, convenient, and reproducible on-line method that facilitates myocardial contrast echocardiographic analysis.

Intraoperative perfusion echocardiography

The ability to assess perfusion intraoperatively should enable end-organ evaluation of the effects of therapeutic choices and provide a basis for understanding the mechanisms of disease. Several experimental techniques for assessment of tissue perfusion are being evaluated; contrast echocardiography appears to be adaptable to the perioperative setting because of its portability and relatively modest cost. With further improvements in commercial ultrasound imaging devices and ultrasonic contrast agents, intraoperative contrast echocardiography may prove to be a technique for quantitation of tissue perfusion. Contrast echocardiography is currently being used intraoperatively to assess cardioplegia distribution, coronary bypass graft patency, and coronary artery collateral vessel distribution. In addition, relative change in renal blood flow can be assessed during renal transplant surgery. With continued advancement of ultrasound technology providing linear (or known) acoustic signal response and wider dynamic range for detection of small and large concentrations of contrast agents, tissue blood flow may soon be evaluated with even greater precision.

Assessment of regional myocardial perfusion with myocardial contrast echocardiography in a canine model of varying degrees of coronary stenosis

Previous studies showed that myocardial contrast echocardiography can be used to detect relative regional underperfusion in a canine model of critical coronary stenoses. The aim of this study was to determine if myocardial contrast echocardiography performed with a sterile sonicated human serum albumin preparation (Albunex, Molecular Biosystems, San Diego, Calif.) could detect the presence of coronary stenoses of variable degrees of severity. The results were compared with myocardial blood flow as assessed by radiolabeled microspheres in 16 open-chest dogs during baseline, following the creation of a coronary stenosis of variable severity and at the peak of dipyridamole-induced hyperemia. When flow ratios (that is, flow in the control area/ischemic area) were examined, a good correlation was observed between the area under the time-intensity curve and radioactive microsphere blood flow (r = 0.94, p < 0.0001), which allowed distinction of mild from severe stenosis. However, a consistent underestimation of the dipyridamole-induced hyperemia by myocardial contrast echo was seen in the control zone. This could be partially explained by a simultaneous increase in coronary blood flow and blood volume with dipyridamole and by attenuation of the ultrasound signal because of the high concentration of microbubbles at high flows. Myocardial contrast echocardiography can quantitate the amount of jeopardized myocardium during various degrees of coronary stenoses.

Safety and efficacy of a new transpulmonary echo contrast agent in echocardiographic studies in patients

OBJECTIVES

This study was designed to investigate in patients the effect of a new transpulmonary echo contrast agent, made from 5% human serum albumin (Albunex), on systemic and pulmonary hemodynamics and the influence of the contrast doses on left ventricular opacification.

BACKGROUND

New intravenous transpulmonary echo contrast agents are promising, allowing contrast stress echocardiography and myocardial contrast echocardiography. Nevertheless, some shortcomings still remain. Thus, the pulmonary hypertension observed in pigs after Albunex injection should be investigated in humans, and the optimal dose of contrast agent remains to be determined because previous experiments indicated that the left ventricular opacification and attenuation are dose dependent.

METHODS

Albunex in doses of 0.08 and 0.22 ml/kg was successively injected intravenously in 20 catheterized patients; in 11 of them, anti-inflammatory drugs were withdrawn to avoid the blocking of an eventual thromboxane-mediated pulmonary artery hypertension. Systemic blood pressure and pulmonary artery, capillary wedge and right atrial pressures were continuously monitored. Cardiac output, left ventricular fractional shortening and blood gases were determined 5 min before and 5 and 10 min after each injection. The left ventricular opacification was qualitatively assessed by three independent observers using a grading scale from 0 to 3, with 0 indicating an absence of contrast effect and 3 indicating full opacification.

RESULTS

No clinical, hemodynamic or respiratory adverse reactions were observed in any patient. Irrespective of doses, a left ventricular opacification grade > or = 2 was observed in 74% of the 35 injections that could be evaluated. This percentage increased to 94% when the higher dose group was considered alone.

CONCLUSIONS

This first report of the effect of Albunex injected intravenously on pulmonary artery pressures in humans demonstrates that this contrast agent appears to be safe. The significant left ventricular opacification obtained in a majority of patients without an important increase in attenuation supports the use of the higher dose of the contrast agent.

Myocardial contrast echocardiography: relation between on-line and off-line assessment of myocardial perfusion

BACKGROUND

Quantitative assessment of myocardial perfusion by myocardial contrast echocardiography has been made possible by the use of custom-made off-line video-intensity programs. A standardized program that could be used by all investigators would improve the reproducibility of results and enhance its clinical utility.

METHODS AND RESULTS

The purpose of this study was to determine if the assessment of myocardial perfusion by contrast echocardiography using a new commercially available, quantitative on-line software program correlates with an off-line custom-made video-intensity program previously validated by our laboratory and with radiolabeled microspheres, under various experimental myocardial perfusion conditions. Two of the measured myocardial contrast echocardiographic parameters (peak intensity, area under the time-intensity curve {area}) correlated well among on-line and off-line methods and radiolabeled microspheres, especially when the data were "normalized" by comparing percent change from baseline or a ratio of ischemic to nonischemic myocardium. The third myocardial contrast echocardiographic parameter examined, half-time of the peak intensity on the washout limb of the curve (t 1/2), correlated only when the percent change from baseline was compared between the two methods or when the off-line method was compared with radiolabeled microspheres.

CONCLUSION

The results of this investigation add further support to the potential use of myocardial contrast echocardiography to evaluate serial changes in myocardial perfusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Pitfalls in quantitative contrast echocardiography: the steps to quantitation of perfusion

Current methods used clinically to assess myocardial perfusion are invasive and expensive. As the technology of ultrasound imaging improves, CE may provide a relatively inexpensive, noninvasive means of quantitating myocardial perfusion. Issues regarding stability of microbubble contrast agents must be studied more closely under physiologic conditions. As such, encapsulated microbubbles may provide more stability under physiologic pressures than free gas microbubbles. Introducing high concentrations of contrast, either by hyperconcentrating the contrast agent or by increasing the injection rate, may provide greater stability under physiologic conditions. Further, before quantitative statement of tissue perfusion can be made, the relationship between tracer concentration and system response must be established. Further, a "linear" postprocessing ultrasound setting does not eliminate this requirement as data must still undergo nonlinear transformation during log compression and time-gain compensation. Additionally, issues regarding "electronic thresholding" must be explored more extensively in vivo. Commercial ultrasound scanners, in their present form, may not offer adequate sensitivity for absolute quantitative studies. Further development of modified ultrasound systems may provide sufficient sensitivity for quantitative perfusion imaging. CE offers a potentially powerful tool in the clinical management of patients with ischemic heart disease. Conventional coronary angiography provides information on the size of a lesion, but accompanying tissue perfusion distal to the lesion cannot be determined. Doppler ultrasonography determines velocity of blood flow in large vessels but does not offer the potential to quantitate tissue perfusion. Clearly, CE has a place in the future of diagnostic imaging. The recent work of Ito et al. demonstrated the qualitative potential of CE in the identification of "areas at risk" in patients who had undergone thrombolysis or percutaneous transluminal coronary angioplasty after an acute myocardial infarction. With further improvement in the ultrasound imaging techniques and microbubble stability, CE may offer an inexpensive, noninvasive means of assessing myocardial perfusion.

In vitro calculation of flow by use of contrast ultrasonography

Contrast echocardiography has been used for qualitative assessment of cardiac function, and its potential for quantitative assessment of blood flow is being explored. With the development of an ultrasound contrast agent capable of passage through the microcirculation, a mathematical model based on classic dye dilution theory, and a digital ultrasound acquisition system, absolute quantitation of myocardial perfusion may be feasible. This study validates the mathematical model in a simple in vitro tube system. Flow was delivered at variable rates through an in vitro tube system while a longitudinal section was imaged with a modified commercial ultrasound scanner. Albunex contrast agent was injected, and videointensity data were captured and analyzed off line. Time-intensity curves were generated, and flow was calculated by use of a mathematical model derived from classic dye dilution mathematics. For 39 different flow rates, ranging for 9.2 to 110 ml/seconds, a correlation coefficient of r = 0.928 (p < 0.001) with a slope of 0.97 was calculated. We conclude that (1) contrast ultrasonography is capable of quantitative determination of flow in an in vitro system, and (2) a mathematical model based on dye dilution theory can be used to calculate flow with accuracy and precision.

Doppler quantification of echo-contrast injections in vivo

It is difficult to quantify myocardial perfusion using contrast echocardiography because the echogenicity of injected contrast is unknown. We propose that a measurement of Doppler amplitude from blood in a systemic artery during the passage of contrast could define the needed input function. Time-amplitude curves from pulsed Doppler cuffs on coronary and carotid arteries of 7 dogs were analyzed during aortic root and left atrial injections of Albunex. We found in individual animals that the areas under the Doppler time-amplitude curves were correlated to the amount of Albunex injected (R = 0.87-0.99), inversely correlated to cardiac output (R = 0.83), and uncorrelated to coronary flow (R = 0.18). Due to better mixing, the coronary and carotid response areas correlated better for left atrial injections (R = 0.96) than for aortic root injections (R = 0.56). We conclude that Doppler amplitude detection can be used to quantify the passage of echo-contrast agents, that the measurements comply with indicator-dilution principles, and that systemic measurements in the carotid artery could be used to predict the coronary input function for injection sites with good systemic mixing.

Quantification of transpulmonary echocontrast effects

Videodensity of left heart and right heart were studied after intravenous injection of increasing dosages of 0.01-0.02 and 0.04 mL/kg bodyweight of Albunex in 10 healthy volunteers. The increase in videodensity in the left ventricle was always lower than in the right ventricle. Possible explanations are diffusion of gases caused by ambient pressures changes and change in microspheres distribution due to the sieving effect of the lung capillary bed. These phenomena were studied in vitro and were consistent with clinical observations. These limitations restrict a quantitative assessment of left heart echocontrast after intravenous injection.

Myocardial transit time of the echocardiographic contrast media

The mean transit time of a tracer through a sample of tissue is a quantitative marker most closely related to regional tissue blood flow. Therefore, an accurate estimation of the mean time of transit of an ultrasonic tracer through a sample of myocardial tissue, obtained by contrast echocardiography, may provide a quantitative noninvasive estimate of myocardial perfusion. We hereby present an algorithm for the determination of the mean transit time by computerized analysis of a series of contrast-enhanced echocardiographic images. The algorithm comprises the evaluation of the echocardiographic impulse response function of a selected region of interest, using a deconvolution technique based on a fast Fourier transform and a frequency domain division of the videointensities measured in the sample, by that measured in a predetermined reference region. An extensive computer simulation study was designed to facilitate the optimization of the steps of analysis. We present the results of the evaluation study performed in order to assess the accuracy of the procedure in computer-simulated echocardiographic images. Within a wide range of parameters chosen to define these functions, the analysis is shown to be essentially independent of the rise and decay times of the impulse response function of the tissue sample as well as of the simulated intensities. The effects of random noise introduced into the simulated intensity curves and of their variable width were investigated. The mean transit time was found to be accurately evaluated within about 10% of error for the variety of widths and noise levels permitted. The reconvolution error did not correlate with the accuracy of the evaluation of the mean transit time, indicating that the reconvolution error cannot be used as an estimate of the accuracy of the procedure. The numerical methods and the results of the computer study are discussed in detail. The approach is proposed to be used as part of a more general technique for the quantitative measurement of regional myocardial tissue blood flow.

Myocardial washout of sonicated iopamidol reflects coronary blood flow in the absence of autoregulation

OBJECTIVES

The aim of the study was to evaluate the relation between measurements derived from myocardial contrast echocardiography and coronary blood flow.

BACKGROUND

Contrast echocardiography has the potential for measuring blood flow.

METHODS

In six open chest anesthetized dogs, the left circumflex coronary artery was cannulated and perfused with blood drawn from the left femoral artery. While adenosine was infused into the circuit, circumflex flow was generated by a calibrated roller pump to the point of abolishing coronary autoregulation. At each of 25 levels of coronary blood flow, paired bolus injections of sonicated iopamidol were performed proximal to a mixing chamber. The perfused area of the left circumflex coronary artery was labeled by radioactive microspheres injected into the perfusion line. Two-dimensional echocardiographic images of the left ventricular short axis were digitized off-line, and myocardial videodensity was measured in the area perfused by the left circumflex coronary artery to generate time-intensity curves.

RESULTS

The washout slope of curves showed a good correlation with coronary blood flow, ranging from 0.5 to 12.5 ml/min per g of tissue. This correlation was good both in individual dogs (correlation coefficient [r] ranging from 0.78 to 0.96) and in the group of animals as a whole (r = 0.85). Washout slope also showed a good correlation with coronary diastolic pressure (r = 0.80), which ranged from 23 to 114 mm Hg, suggesting a possible primary effect of pressure on contrast washout. However, coronary blood flow appeared to be a stronger predictor of washout slope (partial F = 26.5, p < 0.001) than did perfusion pressure (partial F = 5.9, p < 0.05 by multiple regression). The injection to injection variability in myocardial washout slope appeared to be high (24%). The gamma variate fitting of curves did not improve the correlation with coronary flow (r = 0.78).

CONCLUSIONS

Myocardial washout of sonicated iopamidol reflects coronary blood flow in a model in which coronary autoregulation is abolished.

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.

Myocardial perfusion and contrast echocardiography: review and new perspectives

Myocardial perfusion can be assessed qualitatively and quantitatively with new ultrasound contrast techniques. This article reviews progress and problems in this area, discussing intracoronary and aortic root injections in animals and humans. The technique has great potential clinical application for the identification of coronary flow reserve, and the assessment of the need for and outcome of coronary revascularization procedures. It may allow direct measurements of regional myocardial perfusion.

Intraoperative myocardial contrast echocardiography for assessment of regional bypass perfusion

The intraoperative determination of the success of surgical myocardial revascularization remains problematic because of major limitations in all currently used methods. To assess the regional blood flow of the bypass graft-dependent myocardial segments, 2 ml of sonicated iopromid (a nonionic x-ray contrast medium) was injected into the bypass graft in the beating heart. Simultaneously electromagnetic flow measurements were performed. Eleven graft injections in 8 men (mean age +/- standard deviation 60 +/- 4 years) were performed without any adverse effects. Excellent 2-dimensional cross-sectional views of the left ventricle were obtained in all cases. Ten of 11 injections resulted in adequate myocardial opacification. Computer-assisted evaluation by videodensitometry resulted in time-intensity curves with contrast decay half-times (T1/2) from 2.2 to 6.9 seconds (mean 4.3 +/- 1.4). The corresponding electromagnetic flow ranged from 55 to 100 ml/min (mean 80.0 +/- 16.2). there was no correlation between contrast 2-dimensional echocardiography-derived T1/2 and electromagnetic flow (r = 0.32; p = 0.38). Thus, myocardial contrast echocardiography is a feasible and safe method for intraoperative evaluation of the success of bypass graft surgery. It offers online visualization of perfusion of revascularized myocardium and may allow immediate intraoperative revision of unsuccessful bypass graft placement.

Safety and efficacy of a new transpulmonary ultrasound contrast agent: initial multicenter clinical results

Myocardial contrast echocardiography has been found to be a safe and useful technique for evaluating relative changes in myocardial perfusion and delineating areas at risk. Although earlier contrast agents required direct delivery into the coronary arteries or aortic root, a new echocardiographic contrast agent, sonicated albumin microspheres (Albunex), has been found to cross the pulmonary circulation in experimental models. To determine the safety and preliminary efficacy of intravenous injections of Albunex in humans, 71 patients at three independent medical institutions underwent two-dimensional echocardiographic examination before, during and after the administration of three intravenous doses of Albunex, ranging from 0.01 to 0.12 ml/kg body weight. All patients provided a complete history and underwent physical and neurologic examination and laboratory and electrocardiographic evaluation before the injections; all evaluations (except for the history) were repeated at 2 h and 3 days after the injections of Albunex. The efficacy of the injections was qualitatively assessed by two independent blinded observers using a grading system of 0 to +3, with 0 indicating an absence of contrast effect and +3 indicating full opacification of the cavities examined. All injections were well tolerated and no serious side effects were noted in any of the patients. Irrespective of dose group, a cavity opacification greater than or equal to +2 was seen in the right ventricle in 212 (88%) of 240 injections and in the left ventricle in 151 (63%) of 240 injections as judged by the independent observers. The degree of ventricular cavity opacification appeared to be dose and concentration related.(ABSTRACT TRUNCATED AT 250 WORDS)

Reproducibility of quantitative myocardial contrast echocardiography

To determine whether myocardial contrast echocardiography is quantitatively reproducible, repeated intracoronary injections of sonicated albumin (5%) were performed in eight open chest dogs. Paired injections were performed at baseline, during ischemia produced by ligation of a coronary artery, and during hyperemia induced by intravenous infusion of 0.75 mg/kg body weight of dipyridamole. Contrast washout curves were generated for the left anterior descending coronary artery territory (ischemic area) and left circumflex coronary artery territory (nonischemic area) by beat per beat analysis of frozen end-diastolic frames of left ventricular short-axis views. Peak contrast intensity, contrast washout half-time and area under the curve were derived from these curves. A total of 75 contrast washout curves were analyzed for the study of interinjection, intraobserver and interobserver reproducibility. The correlation coefficients between measurements obtained from paired injections of the echocardiographic contrast agent (interinjection reproducibility) ranged from 0.78 for peak contrast intensity to 0.87 for area under the curve. Percent error varied between 14.7% and 24.7%. The intraobserver variability in measurements was less than the interinjection variability, with a cumulative mean percent error of 17.8% and correlation coefficients of 0.72 (peak contrast intensity), 0.95 (area under the curve) and 0.96 (washout half-time). Interobserver correlation for all indexes was high (r = 0.92 to 0.96). It is concluded that peak contrast intensity, contrast washout half-time and the area under the curve derived from myocardial contrast washout curves can be measured reproducibly from videotapes. In addition, the variability between two injections attempted under identical conditions is greater than reader variability from videotapes.

Assessment of regional myocardial perfusion by contrast echocardiography. II. Detection of changes in transmural and subendocardial perfusion during dipyridamole-induced hyperemia in a model of critical coronary stenosis

Measurements of myocardial contrast (sonicated meglumine diatrizoate) intensity were compared with myocardial flow by radioactive microspheres before and after administration of dipyridamole (0.5 mg/kg body weight intravenously) in 10 open chest dogs with a critical stenosis in the left circumflex coronary artery. Computer measurements of contrast time-intensity curves corrected for background myocardial intensity were made along 12 transmural segments of the left ventricle at mid-papillary level and for the subendocardial and subepicardial half of each segment. After administration of dipyridamole, transmural flow in the control region increased significantly (p less than 0.001), resulting in a dipyridamole/baseline flow ratio (i.e., coronary reserve ratio) of 2.54 +/- 0.95. Similar changes (p less than 0.001) were seen by contrast echocardiography; the coronary reserve ratio was 2.10 +/- 0.60 with use of peak intensity and 3.48 +/- 1.58 with use of area under the time-intensity curve. In contrast, no significant changes were observed in myocardial flow, peak contrast intensity or area under the curve in the ischemic region after dipyridamole. In the control region the ratio of subendocardial to subepicardial flow was similar at baseline and after dipyridamole administration as assessed by microspheres (1.08 +/- 0.24 versus 1.17 +/- 0.25) or by area under the time-intensity curve (1.11 +/- 0.45 versus 1.11 +/- 0.56). In the ischemic region, the subendocardial/subepicardial flow ratio decreased significantly after dipyridamole administration as measured by microspheres (1.15 +/- 0.19 to 0.82 +/- 0.25; p less than 0.001) or by area under the curve (1.10 +/- 0.28 to 0.70 +/- 0.47; p less than 0.01). Thus, myocardial contrast echocardiography appears to be a sensitive technique with which to detect changes in myocardial flow induced by dipyridamole in the various myocardial layers of normal segments as well as of segments supplied by a critically stenotic coronary artery.

Quantitative assessment of the immediate results of coronary angioplasty by myocardial contrast echocardiography

A low pressure gradient across the residual lesion and a minimal percent residual stenosis are markers of a successful coronary angioplasty. A more physiologic method of assessing the results of coronary angioplasty would involve assessment of myocardial perfusion in the affected coronary bed. Contrast two-dimensional echocardiography provides information about regional myocardial perfusion. To assess the correlation between pre- to postcoronary angioplasty changes in gradient or percent stenosis and the increase in peak contrast intensity, 23 consecutive patients were studied during coronary angioplasty. In 19 of the 23 patients, the coronary angioplasty was successful and in 15 (79%) of the 19, an adequate echocardiographic study was obtained. Mild and transient side effects of echo contrast were observed in 3 of the 15 patients. The gradient across the residual lesions decreased from 52 +/- 12 to 11 +/- 4 mm Hg (mean +/- SD), the diameter of the stenotic lesion decreased from 89 +/- 10 to 25 +/- 16% and corrected peak contrast intensity (peak contrast - baseline contrast in gray level U/pixel) increased from 15 +/- 16 to 50 +/- 26. All these differences were significant at the p less than 0.001 level. Corrected peak contrast intensity correlated exponentially with the decrease in pressure gradient (r = 0.82, p less than 0.001). The correlation curve had a greater increase in peak contrast intensity at gradient decreases greater than 45 mm Hg. Corrected peak contrast intensity did not correlate with decrease in diameter of the stenotic lesion (r = 0.19).

Sonicated echocardiographic contrast agents: reproducibility studies

This article describes the production, analysis, and reproducibility of forming microbubbles for contrast ultrasound imaging. The sonication method used to generate microbubbles was tested by four independent observers, and a subsequent laser particle counter analysis of microbubble size and concentration determined the reproducibility of the method. The results indicated that the mean bubble size was 3.3 +/- 1.2 microns for the entire group, based on three trials of each of the four participants. The characteristics of the bubble size of the microbubbles between observers were assessed with a Poisson distribution with the reproducibility based on the sample mean for each observer's trials. Standardization and calibration of the laser particle counter was accomplished with commercially available latex spheres, sonicated albumin microspheres, and a Coulter counter analysis. Our results indicate that the sonication technique generates small microbubbles with a reproducible uniform size distribution. The method of microbubble production is reproducible and can be widely applied for use in contrast echocardiographic perfusion imaging of tissue in a variety of research and clinical studies.

Frequency domain analysis of contrast echocardiographic images

Sequences of echocontrastographic images of the dog's myocardium are analysed in the frequency domain for the identification of underperfused areas. Owing to the fact that echo images are contaminated by noise and artifacts, we have applied filtering techniques based on Fourier's methods in sequences of raw frames recorded during perfusion with a contrast agent. Thus we have been able to assess correctly the spatial and temporal distribution of the contrast, i.e. the agent kinetics which parallel the distribution of coronary blood flow, in terms of the time necessary for the flow gradient to reach its maximum value, and to describe such a distribution by a functional image obtained by means of an original procedure.

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.

Visualization of subendocardial myocardial ischemia with myocardial contrast echocardiography in humans

Previous studies indicate the degree of myocardial echo contrast enhancement may be related to regional myocardial perfusion. In this study, myocardial contrast echocardiography was used to characterize changes in the transmural myocardial blood flow distribution that were provoked by rapid atrial pacing in 11 patients with one-vessel coronary artery disease. Ten patients without coronary artery disease served as controls. Myocardial contrast echocardiography was performed by intracoronary injection of 2 ml hand-agitated amidotrizoate sodium meglumine (Urografin-76) and by imaging a short-axis view of the left ventricle with two-dimensional echocardiography before and during injection of the contrast agent. The two-dimensional echocardiographic images at end diastole, before and after injection of the contrast agent, were digitized off-line into a 512 x 512 pixel matrix with 256 gray levels/pixel to quantify the degree of the enhancement of the peak gray level after injection. Transmural myocardial blood flow distribution was evaluated by measuring the ratio of the enhanced gray level in the endocardial half (endo) to that in the epicardial half (epi) (endo:epi gray level ratio) in the anteroseptal, posterolateral, and inferior segments before and just after rapid atrial pacing in each patient. In patients without coronary artery disease, there were no differences in the endo:epi gray level ratio between any of the three segments both before and after pacing. Mean values of the three segments were 0.95 +/- 0.08 before pacing and 0.90 +/- 0.13 after pacing, respectively. In contrast, in patients with coronary artery disease, the endo:epi gray level ratio for the segment supplied with stenotic coronary artery decreased after pacing (0.40 +/- 0.21 vs. 0.93 +/- 0.18, p less than 0.01), probably reflecting subendocardial myocardial ischemia, whereas that for the segment supplied with nonstenotic coronary artery remained unchanged (0.88 +/- 0.20 vs. 0.99 +/- 0.23, NS). Thus, changes in transmural myocardial blood flow distribution with rapid pacing, which may be due to transient subendocardial ischemia, are visualized with myocardial contrast echocardiography.

Quantifying changes in regional myocardial perfusion with aortic contrast echocardiography

We developed a technique to assess regional myocardial perfusion by quantifying echocardiographic myocardial contrast appearance and intensity after aortic root injection of an agitated diatrizoate meglumine solution. The technique was validated by comparing digitized echocardiographic contrast parameters to regional perfusion in the circumflex bed determined by calibrated Doppler flow probe and antemortem monastral blue staining. Regional perfusion was altered by circumflex stenosis, occlusion, and reactive hyperemia. Contrast effects were measured in an initial subset of six dogs by peak intensity change, time to peak intensity, maximal rate of intensity rise, and mean intensity change integrated over 1, 2, or 3 seconds after contrast appearance (MI1, MI2, MI3). MI2 and MI3 best predicted regional perfusion (r = 0.93, standard error of the estimate [SEE] 0.38 ml/gm/min for each). These findings were confirmed in a second subset of six dogs (r = 0.84, SEE = 0.70 ml/gm/min). Although there was a relatively broad standard error for the prediction of absolute perfusion for the pooled data, for individual dogs data were internally consistent so that each had r greater than 0.88 for its varied flow states. The hyperemic ratio calculated by contrast echocardiography correlated well with the Doppler value (r = 0.85). Observer and study-to-study predictive variabilities were small (SEE 0.19 to 0.32 ml/gm/min). No alterations were seen in hemodynamics or reactive hyperemia after 25 consecutive injections over a 90-minute period. Contrast echocardiography with aortic root contrast injection tracks changes in regional blood flow. This approach can assess regional coronary reserve and detect changes in regional myocardial perfusion during acute ischemia and drug intervention.

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)

Myocardial contrast two-dimensional echocardiography: dose-myocardial effect relations of intracoronary microbubbles

It is controversial whether echocardiographic contrast agents prepared by sonication cause transient myocardial depression beyond that known to occur with contrast agents alone. In nine open chest dogs, contrast injections were made into the left anterior descending coronary artery during two-dimensional echocardiography. One hundred forty-four recordings were analyzed subsequently, by an observer who was unaware of other data, for percent depression of systolic wall thickening, duration of regional wall motion abnormalities, peak contrast enhancement and contrast washout. Two microbubble sizes were obtained by sonicating Renografin-76 (meglumine sodium diatrizoate): mean diameter 12 +/- 3 (SEE) and 20 +/- 6 micron. Four doses (range 0.5 to 3 ml) of each of four agents (12 and 20 micron bubbles in Renografin, nonsonicated Renografin and saline solution) were injected in random order. Significant relations were found between percent depression of systolic wall thickening, duration of regional wall motion abnormalities and contrast washout time versus microbubble size (p less than 0.001) and microbubble dose (p less than 0.01). Little increased contrast effect was found at larger doses or with larger microbubbles compared with the smaller doses and size studied. Injections of nonsonicated Renografin caused less depression of systolic wall thickening (p less than 0.05), faster resolution of wall motion abnormalities (p less than 0.05), less contrast (p less than 0.001) and more rapid contrast washout (p less than 0.001) than did 12 micron bubbles in Renografin. A significant correlation was found between the duration of regional wall motion abnormalities and contrast washout time (r = 0.93, p less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Evaluation of reperfusion hyperemia with myocardial contrast echocardiography

In this study we used myocardial contrast echocardiography to evaluate reperfusion hyperemia in an open-chest canine model of temporary coronary artery occlusion. Eight dogs had coronary occluders and electromagnetic flow probes on the left circumflex coronary artery. Aortic root injections of agitated sodium diatrizoate and saline solution were used for myocardial contrast. Data were collected at baseline (n = 16), during coronary occlusion (n = 18), immediately after coronary release (n = 18), and 5 minutes after coronary artery release (n = 12). Baseline coronary flow was 23.8 +/- 5.9 ml/min, decreasing to 0 ml/min during coronary occlusion. Immediately after coronary release flow was 96.6 +/- 41 ml/min (p less than 0.001 compared with baseline), and 5 minutes after coronary release flow was 68.2 +/- 27.9 ml/min (p less than 0.001 compared with baseline). The myocardial image intensity change after injection of contrast material was 74.25 +/- 30.6 ml/min at baseline and declined to 10.4 +/- 10.9 ml/min during coronary occlusion (p less than 0.001 compared with baseline). During reperfusion hyperemia image intensity change was 102.3 +/- 33.3 ml/min (p less than 0.001 compared with occlusion, p less than 0.02 compared with baseline, p less than 0.001 compared with remote regions). Considering all observations, myocardial image intensity change after contrast injection correlated positively with coronary flow (r = 0.67, p less than 0.001). Correlations within individual dogs ranged from r = 0.70 to 0.98. We conclude that image intensity change after aortic root injection of echocardiographic contrast correlates with coronary blood flow. Objective measurements of contrast intensity reflect increases in coronary flow associated with reactive hyperemia after coronary occlusion and release.

Success of internal mammary bypass grafting can be assessed intraoperatively using myocardial contrast echocardiography

To determine whether the success of internal mammary artery bypass grafting can be assessed intraoperatively using myocardial contrast echocardiography, sonicated Renografin-76 was injected into the aortic root of 11 dogs during the delivery of cardioplegic solution. Studies were performed with the left anterior descending coronary artery patent and totally occluded, and after internal mammary artery bypass grafting distal to the occluded vessel. Flow rate during cardioplegia was constant for all three stages in each experiment. Myocardial contrast echocardiography clearly demonstrated homogeneous myocardial perfusion with the left anterior descending coronary artery patent, lack of perfusion in the left anterior descending artery bed during its occlusion and excellent perfusion of the occluded bed after internal mammary artery bypass grafting distal to the occlusion in 10 of the 11 dogs. In one dog, the bypass graft was technically inadequate and contrast opacification was not noted in the left anterior descending artery bed after internal mammary artery bypass grafting. The exponential function C(t) = Ae-alpha t + Be-beta t was fitted to computer-derived time-intensity curves from the myocardium, where alpha denotes contrast washout and beta denotes contrast appearance. Respective values for alpha and beta (mean + 1 SD) were similar for the patent left anterior descending coronary artery and after internal mammary artery bypass grafting distal to the occluded artery (0.11 +/- 0.10 versus 0.10 +/- 0.10, and 2.5 +/- 2.4 versus 1.1 +/- 0.56). In conclusion, myocardial contrast echocardiography has potential for intraoperative assessment of the adequacy of coronary artery bypass grafting.