Post-ischaemic myocardial dysfunction (stunning) results from myofibrillar oedema

OBJECTIVE

To test the hypothesis that myocardial stunning is due to myofibrillar oedema.

METHODS

Experiments were performed in anaesthetised closed-chest pigs. In 15 pigs (group 1), myocardial stunning was produced by repetitive ischaemia and reperfusion; 5 pigs each were studied at 2 hours, 2 days, and 5 days later. Circumferential left ventricular (LV) mid-wall myocardial strain (E(cc)) was estimated in vivo using tagged magnetic resonance imaging. Myocardial water content (MWC) was measured post mortem, from which interfilament lattice distance (d) was calculated. In 6 pigs (group 2), myocardial dysfunction was produced by intracoronary administration of a mast cell degranulator. Animals were euthanised immediately upon induction of regional LV dysfunction to avoid development of inflammation. In 4 pigs (group 3), transmission electron microscopy (EM) was performed to quantify d in stunned versus normal myocardium.

RESULTS

In group 1 pigs, MWC was raised in the stunned compared with normal myocardium (p<0.02) and decreased over time. An inverse relation was found between E(cc) and MWC in the stunned myocardium (r = -0.81) and between E(cc) and d (r = -0.90). A similar relation was noted between wall thickening and increase in MWC in group 2 (r = -0.84) pigs. In group 3 pigs, d on EM was significantly lower (40 (3) nmol/l) in normal myocardium than in stunned myocardium (46.4 (4) nmol/l), p<0.001.

CONCLUSIONS

Ischaemia-reperfusion results in myocardial oedema, with consequent myocyte swelling and myofibrillar oedema. The latter leads to an increase in d, causing myosin heads to either fail to latch, or to latch improperly, onto the actin filament with poor force generation, leading to myocardial dysfunction. As the myocardial oedema abates, myocyte function improves.

Changes in myocardial blood volume over a wide range of coronary driving pressures: role of capillaries beyond the autoregulatory range

OBJECTIVE

To determine whether, when the vasomotor capacity of the coronary arterioles is exhausted at rest, myocardial blood volume decreases in order to maintain a normal capillary hydrostatic pressure, even at the expense of myocardial oxygen delivery.

METHODS

18 dogs were studied. In group 1 (n = 9), coronary driving pressure (CDP) was reduced by 10-80 mm Hg below normal by a stenosis; in group 2 (n = 9), it was increased 20-80 mm Hg above baseline by increasing aortic pressure with phenylephrine. Myocardial contrast echocardiography (MCE) was undertaken to measure the myocardial blood volume fraction and myocardial blood flow (MBF).

RESULTS

In group 1 dogs, as CDP was reduced, both coronary blood flow (CBF) and MBF decreased. Myocardial blood volume fraction also decreased and myocardial vascular resistance increased, while coronary sinus PO2 decreased. In group 2 dogs, as CDP was increased, epicardial CBF increased but MBF remained unchanged because of a decrease in myocardial blood volume fraction. Myocardial vascular resistance decreased, however, implying the presence of coronary arteriovenous shunting, which was supported by a progressive increase in the coronary sinus PO2.

CONCLUSIONS

When arteriolar tone is exhausted so that CBF becomes dependent on CDP, myocardial blood volume decreases in order to maintain a constant capillary hydrostatic pressure, which takes precedence over myocardial oxygen delivery. These novel findings implicate capillaries in the regulation of CBF beyond the autoregulatory range.

Quantification of mitral regurgitation in the cardiac catheterization laboratory with contrast echocardiography

BACKGROUND

There is no method of quantifying the severity of mitral regurgitation (MR) from injection of tracer directly into the left ventricular (LV) cavity, a method commonly used in the cardiac catheterization laboratory.

METHODS AND RESULTS

We used a previously validated mathematical model that derives regurgitant fraction (RF) from the relative tracer washout from the left atrial (LA) and LV cavities. Thirty-nine patients referred for diagnostic cardiac catheterization with clinical evidence of possible MR were included in the study. Five milliliters of a microbubble mixture was power-injected into the LV during simultaneously performed contrast echocardiography. Relative changes in background-subtracted video intensity were measured from the LV and LA, and the resultant model-derived RF was correlated with the severity of MR on cineangiography. The severity of MR ranged from 0 to 4+ on cineangiography with corresponding model-derived RF of 0 to 0.69 on contrast echocardiography. A close linear relation was noted between angiographic severity of MR and model-derived RF on contrast echocardiography (y = 0.1x + 0.03, r = 0.89, P <.001). Contrast echocardiography was more sensitive than cineangiography for detecting mild MR.

CONCLUSIONS

We describe a new method of measuring the severity of MR in the cardiac catheterization laboratory. Apart from being quantitative, this method can be safely used during cardiac catheterization in patients in whom iodinated contrast agents may be potentially harmful.

Role of capillaries in determining CBF reserve: new insights using myocardial contrast echocardiography

To define the role of capillaries in the control of coronary blood flow (CBF) reserve, we developed a model of the coronary circulation and evaluated experimental data in its context. Our model comprised three compartments connected in series (arterial, capillary, and venous), each with its own resistance. The resistance in each vascular compartment was derived from the model based on hemodynamic data obtained in nine dogs during baseline and stenosis, both at rest and during hyperemia. The capillary hydrostatic pressure was assumed to be constant in all stages. Although in the absence of stenosis, the contribution of capillaries to total myocardial vascular resistance was only 25 +/- 5% at rest, it increased to 75 +/- 14% during hyperemia, despite the total myocardial vascular resistance decreasing by 51 +/- 13%. In the presence of a noncritical stenosis, total myocardial vascular resistance decreased by 22 +/- 10% at rest, with no change in capillary resistance. During hyperemia, total myocardial vascular resistance increased by 58 +/- 50% in the presence of the noncritical stenosis. In this situation, because arteriolar and venular resistances were already minimal, the increase in myocardial vascular resistance was due to increased capillary resistance, making it the predominant source (84 +/- 8%) of total myocardial vascular resistance. Myocardial video intensity (VI) on myocardial contrast echocardiography (MCE), which reflects capillary blood volume, decreased distal to the stenosis during hyperemia. In the presence of a flow-limiting stenosis at rest, myocardial VI also decreased, indicating that decrease in CBF was associated with an increase in capillary resistance. Our findings also provide an alternative explanation for the critical coronary closing pressure. Thus, contrary to previously held notions, capillaries play a vital role in the regulation of CBF.

Albumin microbubble persistence during myocardial contrast echocardiography is associated with microvascular endothelial glycocalyx damage

BACKGROUND

We hypothesized that the persistence of albumin microbubbles within the myocardium during crystalloid cardioplegia (CP) infusion and ischemia-reperfusion (I-R) occurs because of endothelial injury.

METHODS AND RESULTS

The myocardial transit rate of albumin microbubbles was measured in 18 dogs perfused with different CP solutions and in 12 dogs undergoing I-R. Electron microscopy with cationized ferritin labeling of the glycocalyx was performed in 9 additional dogs after CP perfusion and in 3 additional dogs undergoing I-R. Microbubble transit was markedly prolonged during crystalloid CP perfusion. The addition of whole blood to the CP solution accelerated the transit rate in a dose-dependent fashion (P<0.05), which was greater with venous than with arterial blood (P<0.05). The addition of plasma or red blood cells to CP solutions was less effective in improving transit rate than addition of whole blood (P<0.05). Microbubble transit rate was independent of the temperature, K+ content, pH, PO2, osmolality, viscosity, and flow rate of the perfusate. Similarly, a proportion of microbubbles persisted in the myocardium after I-R, which was related to the duration of ischemia (P<0.01) but not of reflow. Crystalloid CP perfusion and I-R resulted in extensive loss of the endothelial glycocalyx without other ultrastructural changes. This effect was partially reversed in the case of crystalloid CP when it was followed by blood CP.

CONCLUSIONS

Sonicated albumin microbubbles persist within the myocardium in situations in which the endothelial glycocalyx is damaged. The measurement of the myocardial transit rate of albumin microbubbles may provide an in vivo assessment of endothelial glycocalyx damage.

Assessment of transmural distribution of myocardial perfusion with contrast echocardiography

BACKGROUND

We hypothesized that by using our newly defined method of destroying microbubbles and measuring their rate of tissue replenishment, we could assess the transmural distribution of myocardial perfusion.

METHODS AND RESULTS

We studied 12 dogs before and after creation of left anterior descending coronary artery stenoses both at rest and during hyperemia (n=62 stages). Microbubbles were administered as a constant infusion, and myocardial contrast echocardiography (MCE) was performed with the use of different pulsing intervals. The video intensity versus pulsing interval plots derived from each myocardial pixel were fitted to an exponential function: y=A(1-ebetat), where A reflects microvascular cross-sectional area (or myocardial blood volume), and beta reflects mean myocardial microbubble velocity. The product A . beta represents myocardial blood flow (MBF). Average values for these parameters were derived from the endocardial and epicardial regions of interest placed over the left anterior descending coronary artery bed. Radiolabeled microsphere-derived MBF was also measured from the same regions. There was poor correlation between radiolabeled microsphere-derived MBF and A-endocardial/epicardial ratios (EER) (r=0.46). The correlation with beta-EER was better (r=0. 69, P<0.01). The best correlation with radiolabeled microsphere-derived MBF-EER was noted with A . beta-EER (r=0.88, P<0. 01).

CONCLUSIONS

The transmural distribution of myocardial perfusion can be accurately assessed with MCE with the use of our newly described method of tissue replenishment of microbubbles after their ultrasound-induced destruction. In the model studied, an uncoupling of the transmural distribution of MBF and myocardial blood volume was observed during reversal of the MBF-EER.

Basis for detection of stenosis using venous administration of microbubbles during myocardial contrast echocardiography: bolus or continuous infusion?

OBJECTIVES

This study sought to determine the basis of detection of stenosis by myocardial contrast echocardiography using venous administration of microbubbles and to define the relative merits of bolus injection versus continuous infusion.

BACKGROUND

The degree of video intensity (VI) disparity in myocardial beds supplied by stenosed and normal coronary arteries can be used to quantify stenosis severity after venous administration of microbubbles. However, the comparative merits of administering microbubbles as a bolus injection or continuous infusion has not been studied.

METHODS

Coronary stenoses of varying severity were created in either the left anterior descending or the left circumflex coronary artery in 18 dogs. Imagent US (AF0150) was given as a bolus injection in 10 dogs (Group I) and as both a bolus injection and a continuous infusion in 8 dogs (Group II). For bolus injections, peak VI was derived from time-intensity plots. During continuous infusion, microbubble velocity and microvascular cross-sectional area were derived from pulsing interval versus VI plots. Myocardial blood flow (MBF) was determined using radiolabeled microspheres.

RESULTS

During hyperemia, VI ratios from the stenosed versus normal beds correlated with radiolabeled microsphere-derived MBF ratios from those beds for both bolus injections (r = 0.81) and continuous infusion (r = 0.79). The basis for detection of stenosis common to both techniques was the decrease in myocardial blood volume distal to the stenosis during hyperemia. The advantage of continuous infusion over bolus injection was the abolition of posterior wall attenuation and the ability to quantify MBF.

CONCLUSIONS

Both bolus injection and continuous infusion provide quantitative assessment of relative stenosis severity. Compared with bolus injection, continuous infusion also allows quantification of MBF and data acquisition without attenuation of any myocardial bed.

Quantification of myocardial blood flow with ultrasound-induced destruction of microbubbles administered as a constant venous infusion

BACKGROUND

Ultrasound can cause microbubble destruction. If microbubbles are administered as a continuous infusion, then their destruction within the myocardium and measurement of their myocardial reappearance rate at steady state will provide a measure of mean myocardial microbubble velocity. Conversely, measurement of their myocardial concentration at steady state will provide an assessment of microvascular cross-sectional area. Myocardial blood flow (MBF) can then be calculated from the product of the two.

METHODS AND RESULTS

Ex vivo and in vitro experiments were performed in which either flow was held constant and pulsing interval (interval between microbubble destruction and replenishment) was altered, or vice versa. In vivo experiments were performed in 21 dogs. In group 1 dogs (n=7), MBF was mechanically altered in a model in which coronary blood volume was constant. In group 2 dogs (n=5), MBF was altered by direct coronary infusions of vasodilators. In group 3 dogs (n=9), non-flow-limiting coronary stenoses were created, and MBF was measured before and after the venous administration of a coronary vasodilator. In all experiments, microbubbles were delivered as a constant infusion, and myocardial contrast echocardiography was performed using different pulsing intervals. The myocardial video intensity versus pulsing interval plots were fitted to an exponential function: y=A(1-e[-betat]), where A is the plateau video intensity reflecting the microvascular cross-sectional area, and beta reflects the rate of rise of video intensity and, hence, microbubble velocity. Excellent correlations were found between flow and beta, as well as flow and the product of A and beta.

CONCLUSIONS

MBF can be quantified with myocardial contrast echocardiography during a venous infusion of microbubbles. This novel approach has potential for measuring tissue perfusion in any organ accessible to ultrasound.

Modeling the myocardial dilution curve of a pure intravascular indicator

The dispersion and dilution of contrast medium through the myocardial vasculature is examined first with a serial model comprised of arterial, capillary, and venous components in series to determine their time-concentration curves (TCC) and the myocardial dilution curve (MDC). Analysis of general characteristics shows that the first moment of the MDC, adjusted for that of the aortic TCC and mean transit time (MTT) from the aorta to the first intramyocardial artery, is one-half the MTT of the myocardial vasculature and that the ratio of the area of the MDC and aortic TCC is the fractional myocardial blood volume (MBV). The use of known coronary vascular morphometry and a set of transport functions indicates that the temporal change in MDC is primarily controlled by the MTT. An analysis of several models with heterogeneous flow distributions justifies the procedures to calculate MTT and MBV from the measured MDC. Compared with previously described models, the present model is more general and provides a physical basis for the effects of flow dispersion and heterogeneity on the characteristics of the MDC.

Enhancement of left ventricular cavity opacification by harmonic imaging after venous injection of Albunex

Venous injection of Albunex does not consistently produce left ventricular (LV) cavity opacification during conventional echocardiography. We postulated that by increasing the signal-to-noise ratio, harmonic imaging will result in more successful LV cavity opacification and provide a better assessment of regional LV systolic function. Forty-two patients with poor baseline endocardial delineation were given 10 ml intravenous injections of Albunex during continuous fundamental and harmonic imaging. Change in segmental wall-thickening scores and the confidence levels for these scores were assessed for 3 observers with various levels of experience. Compared with fundamental imaging, harmonic imaging significantly improved the success of LV cavity opacification (83% vs 62%, p <0.05). The background-subtracted video intensity within the central two thirds of the LV cavity increased threefold (from 10 +/- 15 to 31 +/- 29, p <0.05) with harmonic imaging. The spatial extent of opacification increased from 40% of the LV cavity during fundamental imaging to 65% with harmonic imaging (p <0.001). The confidence level for assessing regional LV systolic function improved (p <0.05) after contrast administration, particularly when observer experience was limited. We conclude that in patients with poor endocardial definition, injection of intravenous Albunex should be combined with harmonic imaging to improve LV cavity opacification.

Interactions between microbubbles and ultrasound: in vitro and in vivo observations

OBJECTIVES

We attempted to examine the interactions between ultrasound and microbubbles.

BACKGROUND

The interactions between microbubbles and ultrasound are poorly understood. We hypothesized that 1) ultrasound destroys microbubbles, and 2) this destruction can be minimized by limiting the exposure of microbubbles to ultrasound.

METHODS

We performed in vitro and in vivo experiments in which microbubbles were insonated at different frequencies, transmission powers and pulsing intervals. Video intensity decay was measured in vitro and confirmed by measurements of microbubble size and concentrations. Peak video intensity and mean microbubble myocardial transit rates were measured in vivo.

RESULTS

Imaging at lower frequencies and higher transmission powers resulted in more rapid video intensity decay (p = 0.01), and decreasing exposure of microbubbles to ultrasound minimized their destruction in vitro. Although these effects were also noted in vivo with venous injections of microbubbles, they were not seen with aortic root or direct coronary artery injections.

CONCLUSIONS

Ultrasound results in microbubble destruction that is more evident at lower frequencies and higher acoustic powers. Reducing the exposure of microbubbles to ultrasound minimizes their destruction. This effect is most marked in vivo with venous rather than aortic or direct coronary injections of microbubbles. These findings could lead to effective strategies for myocardial perfusion imaging with venous injections of microbubbles.

Changes in myocardial blood volume with graded coronary stenosis

Vasodilation of microvessels distal to a stenosis results in an increase in myocardial blood volume (MBV). The purpose of this study was to examine the changes in MBV induced by graded coronary artery stenoses by using myocardial contrast echocardiography (MCE). Accordingly, 21 dogs underwent progressive stenosis of a coronary artery in a random order, the severity of which was judged by the pressure distal to it. Total myocardial blood flow (MBF) to the bed distal to the artery (both anterograde and collateral) was measured by injection of radiolabeled microspheres into the left atrium. In seven dogs, anterograde and total MBF were measured at each stenosis stage by injection of different microspheres into the left atrium and directly into the coronary artery, respectively. MBV was calculated by dividing MBF by the mean transit rate of microbubbles injected directly into the coronary artery during MCE. The perfusion bed size of the artery was also measured by MCE. Our major findings are as follows: 1) there is a nonlinear increase in MBV with increasing degrees of coronary stenosis until the coronary stenosis becomes critical; 2) at moderate levels of coronary stenosis, MBV remains constant despite ongoing autoregulation because of reduction in the size of the perfusion bed supplied by the stenotic vessel; and 3) after exhaustion of autoregulation, a decrease in MBV is noted with increasing levels of stenosis. We conclude that assessment of MBV provides insights into myocardial perfusion distal to a coronary stenosis above and beyond that provided by the measurement of MBF alone.

Contractile versus microvascular reserve for the determination of the extent of myocardial salvage after reperfusion. The effect of residual coronary stenosis

BACKGROUND

We hypothesized that microvascular reserve is a better indicator of the extent of viable myocardium postinfarction than contractile reserve, especially in the presence of a residual stenosis of the infarct-related artery.

METHODS AND RESULTS

Fifteen dogs with various infarct sizes were studied after reperfusion. Contractile reserve, studied by use of dobutamine echocardiography, and microvascular reserve, studied by use of myocardial contrast echocardiography, were measured both before and after creation of a stenosis. In the absence of a stenosis, the relation between infarct size, expressed as percent of risk area, and wall thickening improved with increasing doses of dobutamine (r = .41, .71, and .90 for 5, 10, and 15 micrograms.kg-1.min-1, respectively; P < .01 for dobutamine 15 micrograms.kg-1.min-1). In the presence of a stenosis, however, the relation was poor for all doses of dobutamine (r = .22, .57, and .32 for 5, 10, and 15 micrograms.kg-1.min-1, respectively; P < .01 for 15 micrograms.kg-1.min-1 dobutamine in the absence of a stenosis). There was a fair correlation between infarct size and perfusion defect size on myocardial contrast echocardiography after reperfusion (r = .82), with the defect size underestimating infarct size by approximately 20%. This relationship improved (P < .01) during infusions of both adenosine (r = .99) and dobutamine (r = .94) in the absence of a stenosis. The correlations between infarct size and perfusion defect on myocardial contrast echocardiography also remained good in the presence of a stenosis (r = .95 and .81 for adenosine and dobutamine, respectively; P = NS compared with stenosis).

CONCLUSIONS

Microvascular reserve is superior to contractile reserve for definition of the spatial topography of necrosis and hence the extent of viable myocardium within the infarct bed after reperfusion, particularly when a residual stenosis is present in the infarct-related artery.

Relation between air-filled albumin microbubble and red blood cell rheology in the human myocardium. Influence of echocardiographic systems and chest wall attenuation

BACKGROUND

We have previously shown that the intravascular rheology of sonicated air-filled albumin microbubbles is similar to that of red blood cells (RBCs) and that their myocardial transit rate is also similar to that of RBCs in the beating canine heart. In the present study, we tested the hypothesis that the myocardial transit rates of these microbubbles reflect those of RBCs in humans at different coronary flow rates.

METHODS AND RESULTS

RBC and microbubble transit rates were measured in 17 patients undergoing coronary angiography: in 8, measurements were made only at rest, whereas in 9, they were performed both at rest and during a pacing-induced increase in coronary blood flow. A gamma-variate function was used to derive mean RBC and microbubble transit rates from the time-activity and time-intensity plots after the left main injection of RBCs and microbubbles, respectively. There was linear correlation between the myocardial transit rates with both tracers with the slope of the correlation determined by the specific echocardiographic system that was used. Microbubble transit rate consistently overestimated RBC transit rate due to artificial narrowing of the time-intensity curves caused by chest wall attenuation of the echocardiographic signal, which was confirmed through in vitro experiments.

CONCLUSIONS

There is close correlation between air-filled albumin microbubbles and RBC rheology in the human myocardium. The use of these microbubbles in the cardiac catheterization laboratory could, therefore, provide further insights into myocardial blood flow/myocardial blood volume relations in humans.

Mechanism of mitral leaflet excursion

The factors that influence the extent of mitral leaflet opening (MLO) and closure (MLC) have not been defined. We hypothesized that left ventricular (LV) systolic function determines the rate of increase of the early diastolic left atrial (LA)-LV pressure gradient, which is responsible for the extent of MLO, and also the rate of change of the early systolic LV-LA pressure gradient, which determines the degree of MLC. Accordingly, global LV function was changed by altering left main coronary artery flow with LA pressure held relatively constant. LV end-systolic dimension and peak positive LV rate of pressure development (dP/dt) correlated best with the degrees of MLO and MLC, with average correlation coefficients of 0.88 and 0.68, and 0.86 and 0.72, respectively. Although transsecting the submitral apparatus resulted in flailing of the mitral leaflets during normal LV systolic function, the extents of MLO and MLC during LV systolic dysfunction were still influenced by LV systolic function. It is concluded that LV systolic function determines the extent (both opening and closure) of mitral leaflet excursion.

Integrated backscatter and digital acquisition during myocardial contrast echocardiography: is there an advantage over conventional echocardiography for intracoronary injections?

This study was designed to answer the question of whether, despite their theoretic superiority, integrated backscatter imaging (IBS) and digital data acquisition (DA) offer any advantage over conventional echocardiography (CE) during quantitative myocardial contrast echocardiography. In vitro experiments were performed (1) to determine the microbubble concentration versus videointensity relationships for CE and IBS and (2) to define the relationship between flow through and microbubble transit rates for CE and IBS. These data were stored on videotape. In vivo experiments were performed whereby microbubbles were injected into the left anterior descending artery at different flow rates in 14 dogs and IBS and CE data were stored both in digital format and on videotape. Although the level of compression did not affect the microbubble concentration versus videointensity plots during IBS compared with CE, in practical terms the mean transit rate, peak intensity, and area under the curve were not affected by the level of compression for both forms of imaging as long as the postprocessing used for CE imaging was linear and the microbubble dose was small. In addition, although DA resulted in higher peak intensity and area under the curve compared with storage on videotape because of its broader dynamic range, the correlation between these measurements was excellent with both forms of image storage. We conclude that, although differences exist between CE and IBS and between Da and analog acquisition, these differences do not significantly affect the derivation of parameters from time-intensity plots during myocardial contrast echocardiography when contrast material is injected into a coronary artery.

New insights into the physiology of retrograde cardioplegia delivery

Although retrograde cardioplegia (RC) is being increasingly used in clinical practice, its physiology is unclear. Because the microvascular architecture of the coronary venous system is different from that of the arterial system, we hypothesized that myocardial perfusion would be different during RC compared with anterograde cardioplegia (AC) delivery. To better understand these differences, three groups of dogs were studied during similar RC and AC flow rates. Radiolabeled microsphere-derived microvascular flow underestimated total cardioplegia flow by 66% during RC. For the same flows, the first-pass extraction fractions of 201Tl and 99mTc were significantly less during RC compared with AC despite adjusting for microsphere loss. Myocardial contrast echocardiography (MCE), however, provided an accurate estimation of AC and RC flow rates. In addition, the rate of myocardial cooling for most of the left ventricular myocardium was similar for AC and RC at the same flow rates, as long as the flow rates were brisk. It is concluded that microvascular and nutrient flows are significantly lower at the same flow rates during RC compared with AC due to loss of RC at different microvascular sites. Unlike microspheres and diffusible radioisotopes, MCE can provide a reliable measure of myocardial flow during RC delivery. Furthermore, myocardial cooling is similar in most of the myocardium during high-flow RC and AC, which suggests that the clinical benefits of RC are probably related to myocardial cooling and that substrate replenishment may be better achieved at the same flow rates and myocardial temperatures with AC rather than RC.

Technical factors that influence the determination of microbubble transit rate during contrast echocardiography

The calculation of mean microbubble transit rate (MMTR) during contrast echocardiography provides information regarding the flow through and blood volume of a vascular system. Several technical factors can impact on the accuracy of MMTR determination. In this study, with computer simulation and in vitro and in vivo data, we systematically examined four such factors: the relationship between microbubble concentration versus videointensity, the size of the region of interest and the sampling rate used to derive time-intensity plots, and the method of deriving MMTR (direct numeric calculation vs use of a mathematic model). The results indicate that the effect of these technical factors on the estimation of MMTR can be significant.

Detection of coronary stenoses and quantification of the degree and spatial extent of blood flow mismatch during coronary hyperemia with myocardial contrast echocardiography

BACKGROUND

We hypothesized that the degree and spatial extent of blood flow mismatch in beds supplied by stenoses that are not flow-limiting at rest can be quantified with myocardial contrast echocardiography (MCE) using left atrial (LA) and right atrial (RA) injections of contrast during pharmacologically induced coronary hyperemia.

METHODS AND RESULTS

In 12 open-chest dogs, MCE was performed and myocardial blood flow (MBF) was measured by use of radiolabeled microspheres at baseline and during phenylephrine-induced coronary hyperemia. In the presence of this drug, stenoses were placed during different stages on the left anterior descending (LAD) and left circumflex (LCx) coronary arteries, and MCE and MBF assessments were performed. LA injections of 2 mL of 0.5 billion/mL microbubbles (mean diameter, 4.3 microns) were performed at each stage in all 12 dogs, and RA injections of 10 mL of 6 billion/mL microbubbles (mean diameter, 3.7 to 5.3 microns) were administered in 7 dogs. MCE images in which the contrast disparity between the LAD and LCx beds was maximal were digitally subtracted from precontrast images, and mean videointensities in these beds were measured after the dynamic range of gray-scale intensities was increased in the subtracted image and the image was color coded. The region showing hypoperfusion during LAD stenosis was planimetered and expressed as a percentage of the myocardial area in the short-axis slice. There was an excellent correlation between the LAD/LCx bed videointensity ratio and LAD/LCx bed MBF ratio (y = 0.5x + 0.44, r = .91, P < .001) during 57 LA injections. There was also an excellent correlation between the hypoperfused bed size on MCE during LA injection of contrast in the presence of LAD stenosis and the hypoperfused myocardium as determined by radiolabeled microspheres (y = 0.8x + 4.2, r = .90, P < .001, SEE = 2.4, n = 11). The anterior myocardium was opacified in 6 dogs receiving RA injections of contrast, and the hypoperfused area during LAD stenosis correlated closely with that determined by radiolabeled microspheres (y = 0.86x + 3.4, r = .93, P < .01).

CONCLUSIONS

Coronary stenoses, which are not flow limiting at rest, can be detected and the degree and spatial extent of blood flow mismatch during pharmacologically induced coronary hyperemia can be quantified with MCE using LA and RA injections of contrast. Thus, it is possible that the severity of coronary stenoses and the quantum of myocardium in jeopardy could be quantified in the future with MCE using venous injection of contrast.

Low yield of transthoracic echocardiography for cardiac source of embolism

To test the hypothesis that the yield for a cardiac source of embolism is very low using transthoracic echocardiography, we reviewed the echocardiographic reports of 1,010 consecutive patients with cerebrovascular accidents who had undergone echocardiography to rule out a cardiac source of embolism; risk factor information was also available in 493 patients. We also used 325 controls who had undergone echocardiography for other reasons during the same period. Each report was examined for the presence of predefined findings depending on their propensity for causing cerebrovascular accidents via an embolic process. The prevalence of a highly probable source of embolism was low in cases (< 3%) and no different from controls after adjusting for age and other risk factors. The prevalence of a possible cardiac source of embolism was also low (< 5%) and similar in cases and controls. The presence of definite or possible thrombus on echocardiography resulted in alteration in therapy in only 2% of cases, of whom 77% had either heart failure, atrial fibrillation, or Q waves on the electrocardiogram. We conclude that the yield of highly probable or possible cardiac source of embolism in patients with cerebrovascular accidents is very low with transthoracic echocardiography, and is no higher than that noted in similar patients without cerebrovascular accidents.

Relationship between left ventricular wall thickness and left atrial size: comparison with other measures of diastolic function

We postulated that in patients with essential hypertension and normal left ventricular (LV) systolic function, left atrial (LA) size correlates with LV wall thickness by better reflecting the chronicity and duration of LA hypertension than the commonly used hemodynamic and Doppler measures of LV diastolic function. Accordingly, hemodynamic, Doppler, and two-dimensional echocardiographic measurements were performed in 30 subjects with no cardiovascular abnormalities other than essential hypertension (mean systolic blood pressure of 150 +/- 29 mm Hg). The mean LV wall thickness was 0.57 +/- 0.14 cm/m2 and the mean LV ejection fraction was 0.62 +/- 0.12. Hemodynamic and Doppler measures including pulmonary capillary wedge and LV end-diastolic pressures, isovolumic LV pressure relaxation, LV chamber elastic stiffness, and E/A ratio (E and A waves on the pulsed Doppler signal of the mitral valve) correlated poorly (r = 0.01 to -0.52) with LV wall thickness. Both E/A ratio and isovolumic LV pressure relaxation correlated better (p = 0.05) with patient age than with LV wall thickness. In contrast, LA area (in the apical four-chamber view) had a good correlation (r = 0.77 for LA area in atrial diastole and r = 0.86 for LA area in atrial systole) with LV wall thickness. Multiple regression analysis revealed LA area in atrial systole to be the best correlate of LV wall thickness. We conclude that because the left atrium is a thin-walled structure, its size may increase with an increase in LA pressure. In the absence of mitral valve disease and atrial fibrillation, LA size may reflect the chronicity and duration and thus the history of LA hypertension. LA size in the apical four-chamber view may, therefore, provide a simple noninvasive assessment of the degree of LV diastolic dysfunction.

Attenuation deforms time-intensity curves during contrast echocardiography: implications for assessment of mean transit rates

The mean transit rate of microbubbles of air obtained from time-intensity curves during contrast echocardiography can be used to evaluate flow through a vascular system, provided the volume of distribution of the system remains constant. We hypothesized that attenuation commonly associated with contrast echocardiography distorts the time-intensity curves, producing an error in the estimation of the mean transit rate of the microbubbles. The purpose of this study was to characterize this distortion with computer simulations and to study the effect of attenuation on the estimation of mean transit rate in an in vitro experiment. We also sought to determine if removing the data points from the time-intensity plots that visually can be attributed to attenuation before curve fitting can minimize the error in the estimation of mean microbubble transit rate. In both computer simulations and experimentally acquired in vitro data, attenuation distorted the time-intensity curves, producing an underestimation of mean microbubble transit rates. The mean microbubble transit rate decreased with an increase in microbubble concentration. Removing the points that visually were attributed to attenuation before curve fitting did not correct this error completely in the computer-simulated data and failed to correct it at all in the experimental data. These results have major practical implications in assessing mean microbubble transit rates during contrast echocardiography.

Quantification of myocardial perfusion with myocardial contrast echocardiography during left atrial injection of contrast. Implications for venous injection

BACKGROUND

The purpose of this study was to determine whether myocardial perfusion can be quantified with myocardial contrast echocardiography using left atrial (LA) injection of contrast.

METHODS AND RESULTS

Based on a series of in vitro and in vivo experiments, the optimal dose of sonicated albumin microbubbles injected into the LA for establishing a linear relation between video intensity and blood volume in the anterior myocardium was determined. In 10 open-chest dogs, myocardial blood flow (MBF) was augmented by increasing myocardial blood volume (MBV) with an intravenous infusion of phenylephrine HCl. In the presence of this drug, left anterior descending artery stenosis was produced, followed by release of stenosis, to change MBF within the anterior myocardium. MBV was calculated by dividing radiolabeled microsphere-derived MBF by microbubble transit rate. There was close coupling between MBF and MBV in the anterior myocardium during LA injection of contrast (y = 1.0x-0.03, SEE = 1.07, r = .92, P < .001). An excellent correlation was also noted between background-subtracted peak video intensity and MBV (y = 0.24x + 0.73, SEE = 0.36, r = .88, P < .001). On multivariate analysis, background-subtracted peak video intensity correlated best with MBV.

CONCLUSIONS

Myocardial perfusion can be quantified from time-intensity curves derived from the anterior myocardium after LA injection of contrast. Background-subtracted peak video intensity in this situation correlates closely with MBV. When MBV and MBF are closely coupled, such as during inotropic stimulation of the heart, background-subtracted peak video intensity also correlates closely with MBF. Since there are similarities in the models of LA and venous injections, these data indicate that it may be feasible to quantify myocardial perfusion with myocardial contrast echocardiography after venous injection of contrast.

Quantification of images obtained during myocardial contrast echocardiography

This article describes currently used quantitative methods for analysis of data obtained during myocardial contrast echocardiography. The specific issues addressed are: obtaining time-intensity curves from the myocardium in order to derive transit rates of microbubbles through the myocardium; defining spatial distribution of flow within a myocardial segment; and color-coding algorithms used to define the extent and magnitude of hypoperfusion within a cross-section of the heart. These methods are being adopted by several companies dealing with acquisition and analysis of echocardiographic data and should become available soon for clinical use.

Effects of radiofrequency catheter ablation on regional myocardial blood flow. Possible mechanism for late electrophysiological outcome

BACKGROUND

We postulated that the late electrophysiological effects of radiofrequency (RF) ablation may be related to microvascular injury extending beyond the region of acute coagulation necrosis.

METHODS AND RESULTS

Eighteen RF lesions created in the left anterior descending coronary artery (LAD) perfusion bed of seven open chest anesthetized dogs were studied. The ablation electrode and surrounding myocardium were imaged using high-resolution two-dimensional echocardiography at x 4 magnification. After 60 seconds of RF delivery, sonicated albumin microbubbles (mean size, 4.3 microns) were injected into the LAD to measure regional myocardial perfusion, and time-intensity plots were generated from simultaneously acquired two-dimensional echocardiography images. The regions with persistent contrast effect on two-dimensional echocardiography were larger than the pathological lesions (mean cross-sectional area, 48.3 +/- 6.3 versus 19.3 +/- 4.7 mm2, respectively; P < .0001). The mean contrast transit rate in the area corresponding to the pathological lesion was 25 +/- 12% of that in the normal myocardium, but it was also reduced beyond the lesion, being 48 +/- 27% and 82 +/- 28% of normal, respectively, in the 3-mm and 3- to 6-mm circumferential rims surrounding the pathological lesion (P < .05). Electron microscopy performed in two additional dogs with similar lesions demonstrated the presence of ultrastructural damage to the microvascular endothelium well beyond the pathological lesion edge.

CONCLUSIONS

RF catheter ablation not only results in a marked reduction in blood flow within the acute pathological lesion but also causes reduced flow beyond the borders of the acute lesion because of microvascular endothelial cell injury. The progression or resolution of tissue injury within the region beyond the border of the pathological lesion may explain the late electrophysiological effects of RF ablation.

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)

Quantifying myocardial blood flow with contrast echocardiography

The principal focus of this article has been to discuss the issues relating to the quantitation of myocardial perfusion using MCE. In its current form, this technique can provide important physiological information in the cardiac catheterization laboratory and the operating room as well as in the experimental laboratory. Standardization of echo contrast agents, echo equipment, and algorithms for image analysis will make it a truly quantitative technique in the near future. MCE has the potential to offer the unique opportunity of simultaneously assessing regional myocardial flow and function in various ischemic syndromes.

Why do patients with congestive heart failure tolerate the initiation of beta-blocker therapy?

BACKGROUND

Despite its negative inotropic effects, the initiation of beta-adrenergic blockade is tolerated by patients with congestive heart failure (CHF). Accordingly, we examined the acute hemodynamic effects of beta-adrenergic blockade on systolic and diastolic left ventricular (LV) function and ventriculo-arterial coupling. In addition, isolated myocardium from patients with CHF shows selective beta 1-receptor downregulation, implying a greater role for the beta 2-receptor in maintaining in vivo LV contractility. As a secondary aim, we hypothesized that nonselective beta-adrenergic blockade would have greater negative inotropic effect than beta 1-blockade in patients with CHF.

METHODS AND RESULTS

Patients with clinical CHF (n = 24) and control patients without CHF (n = 24) were given either the nonselective beta-blocker propranolol or the beta 1-selective blocker metoprolol. LV pressure-volume relations were obtained before and after the administration of intravenous beta-blocker, and measures of LV systolic and diastolic function were examined. Patients with CHF had a deterioration in LV systolic function with a fall in LV systolic pressure (139 +/- 6 to 125 +/- 6 mm Hg), cardiac index (2.56 +/- 0.11 to 2.20 +/- 0.11 mL.min-1 x M-1), dP/dtmax (1173 +/- 63 to 897 +/- 50 mm Hg/s), and end-systolic elastance (0.88 +/- 0.10 to 0.64 +/- 0.10 mm Hg/mL), P < .05 for all. Although there was deterioration of active LV relaxation (isovolumetric relaxation 63 +/- 2 to 73 +/- 3 milliseconds, peak filling rate 543 +/- 33 to 464 +/- 28 mL/s, P < .05 for both), there was no change in passive LV diastolic function (pulmonary capillary wedge, 24 +/- 2 to 24 +/- 1 mm Hg; chamber stiffness, 0.0154 +/- 0.0005 to 0.0163 +/- 0.0005 mL-1, P = NS for both), and a decrease in afterload (arterial elastance 3.85 +/- 0.31 to 3.38 +/- 0.24 mm Hg/mL, P < .05). Control patients had no change in these parameters other than a prolongation of isovolumetric relaxation (48 +/- 1 to 55 +/- 2 milliseconds, P < .05). The effects of propranolol (n = 12) versus metoprolol (n = 12) on these parameters in patients with CHF were similar.

CONCLUSIONS

These data do not support a greater in vivo physiological role of the myocardial beta 2-receptor in CHF. The preservation of passive diastolic function and ventriculo-arterial coupling provide possible explanations of why beta-adrenergic blockade is tolerated by patients with CHF.

Transpulmonary transit of microbubbles during contrast echocardiography: implications for estimating cardiac output and pulmonary blood volume

We postulated that the pulmonary transit rate of sonicated albumin microbubbles, which have an intravascular rheology similar to that of red blood cells, would be directly proportional to cardiac output (CO) and inversely proportional to pulmonary blood volume (PBV). Accordingly, 4 ml of Albunex ultrasound contrast agent (0.5 billion/ml of 4.3 mu bubbles) was injected into the right atrium of six dogs (Group I) during simultaneously performed two-dimensional echocardiography, and the time between the initial appearance of the bubbles in the right and left ventricle, respectively, was measured. CO was either increased (by intravenous infusion of 15 micrograms/kg/min of dobutamine) or decreased (by producing left ventricular ischemia or by administering 2 mg of intravenous propranolol) in a random order and microbubbles were injected again. At each stage, thermodilution CO was measured. There was a close linear relation between CO and pulmonary transit rate of Albunex in each dog with the correlation coefficient ranging from 0.79 to 0.99, with a mean of 0.92. Pulmonary blood volume was derived in each dog from the reciprocal of the slope of the regression between CO and pulmonary transit rate and varied from 106 to 261 ml in the six dogs with a mean value of 178 +/- 64 ml. There was excellent interobserver and intraobserver correlation (r = 0.99 each) for determining the pulmonary transit rate of Albunex. The reproducibility of pulmonary transit rate estimation from repeated contrast injections at the same hemodynamic state in another group of six dogs (Group II) was also good (r = 0.99). It is concluded that the pulmonary transit rate of Albunex ultrasound contrast agent can be used to assess directional changes in CO and to measure pulmonary blood volume. This method may have clinical applications.

Myocardial contrast echocardiography has the potential for the assessment of coronary microvascular reserve

Coronary vasodilators increase coronary flow by increasing myocardial blood volume. Diseases affecting the coronary microvasculature will affect vasodilator-induced changes in coronary flow by inhibiting changes in myocardial blood volume. In such cases, when myocardial time-intensity curves after administration of a vasodilator are compared with those at baseline, a less than anticipated increase in microbubble transit rates will be noted. As long as we understand what we are measuring in the context of where and how we inject the bubbles, we can begin to define the role of myocardial contrast echocardiography in assessing changes in coronary microvascular reserve. It is also conceivable that because myocardial contrast echocardiography can assess changes in myocardial flow/volume relations rather than just changes in flow, this technique could be used to provide additional insights into the mechanisms of action of different coronary vasodilators and into the pathophysiology of various diseases affecting the coronary microvasculature. Finally, with the advent of commercially available microbubbles, robust on- and off-line analysis algorithms and intracardiac imaging, myocardial contrast echocardiography may become an invaluable adjunct to coronary angiography for determining the pathophysiologic significance of coronary disease in individual patients.

On-line intraoperative quantitation of regional myocardial perfusion during coronary artery bypass graft operations with myocardial contrast two-dimensional echocardiography

We hypothesized that the success of coronary artery bypass graft operations could be assessed by means of on-line quantitative myocardial contrast echocardiography. Accordingly, myocardial contrast echocardiography was performed at baseline and after each placement of venous graft in 21 patients undergoing coronary artery bypass graft operations. Time-intensity plots were generated on-line with the use of a dedicated computer system, and areas under the curve were assessed for each injection. Successful on-line quantitation of myocardial contrast echocardiography data was performed in 17 patients; this allowed comparison before and after coronary artery bypass graft operations for 21 grafts, with agreement between expert visual analysis and quantitative data in 91% of these cases. Three distinct perfusion patterns were noted on myocardial contrast echocardiography: (1) reduced contrast effect before coronary artery bypass graft operations with improvement after coronary artery bypass graft operations (n = 11); (2) adequate contrast effect before coronary artery bypass graft operations with no change after coronary artery bypass graft operations (n = 9) (for patients in group 2, the mean percentage of coronary stenosis was less than the mean for patients in group 1-67% +/- 25% vs. 88% +/- 20%, p = 0.05); and (3) no contrast effect either before or after coronary artery bypass graft operations in one patient with previous infarction. One third of the time (34 of 95 injections), on-line quantitation was unsuccessful. Failure was related three times more often to problems associated with myocardial contrast echocardiography, such as attenuation and inadequate quality of bubbles, than to computer failure. Despite its limitations, on-line quantitative myocardial contrast echocardiography is feasible in patients undergoing coronary artery bypass graft operations and provides important objective information regarding the success of revascularization.

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.

A mathematical model for the quantification of mitral regurgitation. Experimental validation in the canine model using contrast echocardiography

BACKGROUND

Because the clearance of contrast from the left atrium (LA) relative to the left ventricle (LV) depends on the degree of mitral regurgitation (MR), we hypothesized that a mathematical model can be developed that would provide a quantitative estimation of MR from the washout of contrast from these chambers.

METHODS AND RESULTS

After mathematically developing the model, we performed experiments in two groups of dogs with the use of contrast echocardiography. Group 1 consisted of nine dogs in which different degrees of MR were produced by creating ischemic LV dysfunction. Contrast was injected into the LV, and MR was graded visually on a scale of from 0 to 4+. Videointensity plots generated from the LA and LV were provided to the model. There was excellent correlation between visual assessment of MR and model-derived regurgitant fraction in the 33 stages: y = 0.16x + 0.002 (r = 0.97, p less than 0.001, SEE = 0.06). To obtain a more quantitative validation, we placed electromagnetic flow probes on the aorta and just cephalad to the mitral annulus in six dogs (group 2) during cardiopulmonary bypass. Different degrees of MR were produced by chordal traction and/or myocardial ischemia. Regurgitant fraction was calculated at each stage from the flow probe and videointensity data. There was excellent correlation between flow probe and model-derived regurgitant fraction (y = 0.90x + 0.03; r = 0.96, p less than 0.001, SEE = 0.06), and close interobserver and intraobserver correlations were noted using flow probe and contrast echocardiographic data.

CONCLUSIONS

A mathematical model that uses the clearance of contrast from the LA relative to the LV can be used to accurately measure the severity of MR. These findings may have important practical implications for the quantification of MR.

Functional significance of collateral blood flow in patients with recent acute myocardial infarction. A study using myocardial contrast echocardiography

BACKGROUND

We hypothesized that myocardial contrast echocardiography (MCE) can be used to both measure collateral blood flow as well as assess the functional significance of collaterals in patients with acute myocardial infarction (AMI).

METHODS AND RESULTS

MCE was performed in 33 patients with recent AMI (12 +/- 7 days) and an occluded infarct-related artery (IRA), both before and after attempted percutaneous transluminal coronary angioplasty (PTCA). The size of the occluded bed was defined in patients with successful PTCA by injecting contrast directly into the opened IRA and expressed as a percent of the myocardium in the short-axis view. The percent of the perfusion bed supplied by collaterals before PTCA was determined. Transit rates of the microbubbles within the collateralized regions were also measured and were expressed as a percent of the transit rates in the normal adjacent beds. Regional function within the occluded bed was assessed using echocardiography and was graded as 1 (normal) to 5 (dyskinetic). Collaterals were graded on coronary angiography as 0 (none) to 3 (abundant). The perfusion bed size was larger for the left anterior descending (LAD) than for the right (RCA) and left circumflex (LCx) coronary arteries (37 +/- 6% versus 27 +/- 12% of the myocardium, p = 0.02). The percent of the occluded bed supplied by collateral flow was greater for RCA and LCx compared with the LAD (87 +/- 30% versus 72 +/- 22%, p less than 0.01). There was poor correlation between MCE-defined percent of occluded bed supplied by collaterals and angiographic collateral grade (r = 0.13). Regions supplied by collaterals were less likely to show confluent hypoperfused zones after reperfusion compared with those not supplied by collaterals. Similarly, the percent of myocardium not perfused by either anterograde or collateral flow correlated well (r = 0.67, p less than 0.01) with peak creatine kinase levels and was more likely to be associated with Q waves. Finally, although there was poor correlation between angiographic collaterals and regional function (r = 0.20), there was a significant negative correlation between MCE-defined spatial extent of collateral flow and regional function (r = -0.57, p less than 0.01).

CONCLUSION

MCE can be used to measure collateral flow in patients with recent AMI and to assess the functional significance of collaterals in these patients. This technique may be ideally suited for the assessment of collateral perfusion in patients undergoing cardiac catheterization.

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

BACKGROUND

Myocardial contrast echocardiography currently involves intro-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 less than 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 (chi 2= 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.

A computer-aided approach for the quantitation of regional left ventricular function using two-dimensional echocardiography

Quantitation of regional left ventricular function is becoming increasingly important in this era of interventional cardiology. In this article we describe a computer-assisted approach for the assessment of regional left ventricular function that integrates three features. First, it measures left ventricular wall thickening and endocardial motion and curvature during an entire systolic contraction sequence. Second, it allows us to easily define endocardial and epicardial borders without actually having to manually trace the entire border. Third, it displays complex data in a simple format for easy assimilation. We believe that this approach is practical and comprehensive and can be used to quantitate regional left ventricular function with two-dimensional echocardiography.

Method for the quantitation of myocardial perfusion during myocardial contrast two-dimensional echocardiography

This article describes the hardware and software components of two systems designed for quantitative analysis of data obtained during myocardial contrast two-dimensional echocardiography. One system is meant for off-line analysis of data, whereas the other is designed for on-line analysis, especially in the operating room. The algorithms used for data transfer, selection of appropriate frames, data alignment, derivation of time-intensity plots, and curve-fitting and parameter generation are described in some detail. It is hoped that this information will be of use to others who work in the field of myocardial perfusion imaging.

Efficiency of work of inspiratory muscles in standing and in exercising dogs

The objective of this study was to elucidate the resistive, elastic, and total inspiratory muscle work and mainly the efficiency of the inspiratory muscle work in still standing and in exercising dogs. In 180 experiments with nine mongrel dogs, respiratory and circulatory functions were recorded at rest and during a 1.5-mph walk on a treadmill belt which was inclined by 9 degrees. Each dog had been trained and surgically prepared before the series of experiments started. An electronic pressure transducer together with a fluid-filled catheter was attached to the ribs at the midthoracic level. Airflow was monitored with a pneumotachygraph attached to a face mask. Aortic flow was monitored with an electromagnetic flow transducer. Catheters in the atria, the ventricles, and the pulmonary and systemic vessels were used to determine hemodynamic parameters and blood gas levels. All recorded and all derived functions were evaluated by a dedicated computer. At rest, resistive inspiratory work (0.98 kg cm) was smaller than elastic work (1.36 kg cm). Elastic work did not change with exercise but resistive work increased to 1.46 kg.cm. The increase of total work from 2.33 kg.cm at rest to 2.79 kg.cm during exercise did not reach significance. Inspiratory power increased with exercise. Estimated diaphragmatic oxygen consumption increased from 2.23 ml/min at rest to 3.64 ml/min during exercise. It was 1.6 percent of the total oxygen consumption at rest and 1.7 percent of the total oxygen consumption during exercise. The estimated efficiency of the work of inspiratory muscles was 15.8 percent at rest and 25.8 percent during exercise. These values did not vary substantially between different animals.

Changes of phasic pleural pressure in awake dogs during exercise: potential effects on cardiac output

Eighty experiments were performed with nine awake dogs to study the changes of phasic-pleural pressure with exercise. The increased minute volume with exercise was obtained by more frequent pleural pressure swings and by a substantial extension of the pressure swings in both directions. The cyclic changes of stroke volume following the pressure swings support the hypothesis that alterations of pleural pressure affect the stroke volume and can act, if necessary, as a secondary pump for the circulation. Mean pleural pressure during exercise fell by 2.5 cm H2O from the rest value of 12.1 cm H2O. The absolute right atrial pressure during exercise (-2.69 mm Hg) was not different from that at rest (-2.39 mm Hg). However, the transmural right atrial pressure of 7.6 mm Hg during exercise was higher than the pressure of 6.2 mm Hg at rest because during exercise the right atrium was perfused by 38% higher blood flow than that at rest. The phasic pattern of right atrial pressure shows that there is good reason to assume that during inspiration the extrathoracic veins are collapsed at their entrance into the chest, but this collapse is removed during expiration. There is no reason to assume an effective, sustained collapse of extrathoracic veins. Rather we can visualize a rhythmical change of flow in extrathoracic veins from transient limitation to transient acceleration.

Interaction of europium(III) with phospholipid vesicles as monitored by laser-excited europium(III) luminescence

The technique of laser-excited Eu(III) luminescence was applied to monitor Eu(III) binding to a variety of phospholipids. Eu(III) excitation spectra were similar with and without the presence of neutral phospholipids, while acidic phospholipids changed the spectrum in a concentration-dependent manner. Eu(III) appears to bind to the phosphate moiety with at least a 2:1 phospholipid:metal ion stoichiometry. Analysis of luminescence lifetimes reveals that only one or two waters of hydration are removed from Eu(III) by addition of neutral phospholipids, whereas acidic phospholipids and inorganic phosphate strip off all but one or two waters. Implications with regard to fusion and use of lanthanides as probes in membrane preparations are discussed.

Characterization of calciphorin by laser-excited europium luminescence

There is some question whether the calcium binding characteristics of calciphorin are due to contaminating phospholipids. To differentiate protein ion binding by phospholipids or contaminating detergent, we describe here the use of Eu(III) as a metal-binding-site probe, and characterize the interaction of Eu(III) with calciphorin, cardiolipin, deoxycholate, and digitonin. The luminescence excitation pattern of Eu(III) bound to the calciphorin preparation clearly differentiates it from Eu(III) interactions with the possible contaminants. In addition, the effect of the luminescence decay constant of Eu(III) bound to calciphorin on the mole fraction of H2O in a mixture of H2O/2H2O indicates that all except approximately 0.8 of the 9 to 10 water molecules coordinating Eu(III) in solution are stripped off upon binding to calciphorin. This also contrasts with the data for the possible contaminants.

Electrical conductivity, transfer of hydrogen ions in lipid bilayer membranes and uncoupling effect induced by pentachlorobenzenethiol (pentachlorothiophenol)

Pentachlorobenzenethiol (PCBT) has been considered an anomalous uncoupler. It was reported as active in mitochondria, but not effective in inducing electrical conductivity in lipid bilayer membranes. We have overcome the experimental difficulties associated with accurate determination of the induced conductivity. The main contributing factors to the difficulties, we discovered, are the photolability and the low solubility of the compound in aqueous medium. We have conclusively demonstrated that PCBT does induce conductivity in lipid bilayers and compared this conductance with its uncoupling activity reported by other investigators in the literature. We present the results of steady-state current-voltage measurements: conductance dependence on applied voltage for various values of pH, buffer strength and PCBT concentration, as well as the dependence of the conductance on pH, buffer strength and PCBT concentration in the limit of zero applied voltage. We have also compared the above results with those obtained previously with pentachlorophenol. Our experimental results on PCBT-induced membrane conductance suggest that PCBT belongs to class II uncouplers and that "disulfide dimer" of PCBT is membrane inactive. Thus the replacement of oxygen in molecular structure of pentachlorophenol (R-OH) by sulfur (R-SH) does not change the protonophoretic activity of the compound. The conductivity of a membrane is due to PCBT-induced hydrogen ion transfer and it was found to be limited by the kinetics of reactions coupled to transmembrane charge transfer.(ABSTRACT TRUNCATED AT 250 WORDS)