Noninvasive transthoracic low frequency ultrasound augments thrombolysis in a canine model of acute myocardial infarction--evaluation of the extent of ST-segment resolution

BACKGROUND

Recently it has been demonstrated that transcutaneous delivery of ultrasound combined with tissue plasminogen activator (tPA) is more effective than tPA alone in recanalizing acutely thrombosed canine coronary arteries. In the present study, we investigated the incidence of partial (> or =50%) and complete (> or =70%) ST-segment elevation resolution in the precordial leads of dogs with experimental acute myocardial infarction that were treated with tissue plasminogen activator (tPA) alone or in combination with noninvasive transcutaneous delivery of high-intensity low frequency (27[emsp3 ]kHz) ultrasound.

METHODS

Thrombotic coronary occlusions were induced in the midportion of left anterior descending (LAD) coronary artery by electrical injury in 24 dogs. All dogs were given intravenous heparin and tPA. Dogs were randomized to tPA alone (n=12) or combined tPA and adjunctive transcutaneous ultrasound (US) delivery (n=12). Electrocardiograms were recorded at 1) baseline, 2) after coronary occlusion just before initiation of therapy, 3) when coronary angiography showed recanalization of the coronary artery (or at 90 minutes after initiation of therapy if reperfusion did not occur before then) and 4) 90 minutes later. ST amplitude was measured in all 6 precordial leads.

RESULTS

ST-segment amplitude at baseline was comparable between the tPA and the US group. Before initiation of therapy, sum of ST-segment elevation tended to be higher in the US group. At reperfusion and 90 minutes thereafter, sum of ST-segment amplitude tended to be smaller for the US group than in the tPA group (p<0.001 for the time effect; p=0.118 for the time x group interaction). Up to 90 minutes after initiation of therapy >/=50% resolution of the sum of precordial ST elevation was detected in 7 out of 11 dogs (63.6%) in the tPA group versus 10 out of 11 dogs (90.9%) in the US group. Ninety minutes thereafter, 3 out of 7 dogs in the tPA group (42.9%) versus 9 of 11 dogs in the US group (81.8%) had >/=50% resolution of the sum of precordial ST elevation.

CONCLUSIONS

The combination of tPA with noninvasive transcutaneous delivery of low frequency high-intensity ultrasound resulted in greater resolution of ST-segment elevation when reperfusion occurs and 90 minutes thereafter, as well as a higher rate of epicardial coronary artery reperfusion.

Noninvasive transcutaneous low frequency ultrasound enhances thrombolysis in peripheral and coronary arteries

Previous studies have shown that external ultrasound with low frequencies and high intensities can enhance thrombolytic drug-induced clot dissolution during in vitro experiments. In this series of studies, we evaluated the efficacy of peripheral and coronary thrombolysis in vivo in animals by using noninvasive transcutaneous ultrasound combined with thrombolytic drugs (streptokinase and tPA) and/or microbubbles agents (dodecafluoropentane [DDFP] and perfluorocarbon-exposed sonicated dextrose albumin [PESDA]). Thrombotic occlusions were induced in 74 rabbit iliofemoral arteries and 24 canine left anterior descending (LAD) coronary arteries in this in vivo study. By using the combination of transcutaneous ultrasound and streptokinase, the angiographic patency rate in rabbit iliofemoral arteries was higher (56%-100%) than with ultrasound (6%; P < or = 0.0036) and streptokinase alone (6%; P < or = 0.0012). Also, with transcutaneous ultrasound and microbubbles, the angiographic patency rates were 76%-100% as compared with ultrasound alone (0%, P < or = 0.0003) or microbubbles alone (9%, P < or = 0.0001). In the canine study of acute myocardial infarction, thrombolysis in myocardial infarction (TIMI) grade flow at 90 minutes in the tPA alone group was 0.92 +/- 1.4 as compared with 2.42 +/- 1.9 in the tPA plus transthoracic ultrasound group (P = 0.006). There was much improved reperfusion with tPA plus ultrasound as compared with tPA alone. In vivo animal studies demonstrate that noninvasive transcutaneous ultrasound can greatly enhance the effect of clot dissolution with thrombolytic drugs and/or microbubbles, and has the potential for clinical application as an adjunctive method to improve arterial thrombolysis.

Contrast-enhanced B-mode US angiography in the assessment of experimental in vivo and in vitro atherosclerotic disease

RATIONALE AND OBJECTIVES

This study was performed to (a) test the hypothesis that filling the arterial lumen with echoes at B-mode ultrasound (US) enables the assessment of wall and luminal abnormalities and (b) compare contrast material-enhanced B-mode US with color and power Doppler US angiography.

MATERIALS AND METHODS

Atherosclerotic lesions were created in 14 rabbit aortas and imaged with color Doppler and B-mode US before and after the intravenous administration of 0.3 mL of AF0150, a US contrast agent. In addition, four replicas of diseased human carotid arteries were immersed in a tissue-mimicking phantom and imaged with B-mode and color and power Doppler US before and after the administration of 1 mL of AF0150 per liter of porcine blood. Radiopaque plastic casts of the rabbit aortas and contact radiographs of the plastic replicas served as standards.

RESULTS

Although color and power Doppler US allowed immediate localization of the lumen, precise estimation of stenoses and reliable visualization of surface irregularities were not possible. After AF0150 administration, angiogram-like images of the lumen were created with B-mode US, allowing rapid assessment of the entire vessel lumen and wall. Consequently, luminal stenoses were more accurately measured than with unenhanced B-mode US (r2 = 0.94, P < .0001 vs r2 = 0.21, P = .25) or Doppler (r2 = 0.42, P < .03). In addition, plaques and ulcerations were visible only with contrast-enhanced B-mode US.

CONCLUSION

Microbubbles fill the arterial lumen with echoes at B-mode US, creating an angiogram-like image. The ability to visualize the inner and outer surfaces of the vascular wall improved the evaluation of luminal and wall abnormalities.

Noninvasive, transthoracic, low-frequency ultrasound augments thrombolysis in a canine model of acute myocardial infarction

BACKGROUND

Limitations of coronary thrombolysis include the time to reperfusion, patency rate, and bleeding. We evaluated the use of noninvasive transcutaneous ultrasound to augment coronary thrombolysis.

METHODS AND RESULTS

In 24 dogs, a thrombotic occlusion of the left anterior descending coronary artery was induced and documented by 12-lead ECG and coronary angiography. After >/=60 minutes of occlusion, tissue-type plasminogen activator (t-PA; 1.42 mg/kg) was given intravenously over 90 minutes. A total of 12 of the 24 dogs had concomitant transcutaneous application of low-frequency ultrasound (27 kHz) over the chest. At 90 minutes, the mean TIMI grade flow in the t-PA alone group was 0.92+/-1.4 compared with 2. 42+/-1.9 in the t-PA plus ultrasound group (P=0.006). TIMI 2 to 3 flow was present in 4 of 12 cases receiving t-PA alone compared with 10 of 12 cases receiving t-PA plus ultrasound (P=0.003). At 180 minutes, mean TIMI grade flow was 0.75+/-1.4 in the t-PA alone group versus 2.58+/-0.9 in the t-PA plus ultrasound group (P=0.001). Pathological examination confirmed the angiographic patency rate and did not reveal injury secondary to ultrasound in the skin, soft tissues, heart, or lungs.

CONCLUSIONS

In vivo, the noninvasive transthoracic application of low-frequency ultrasound (1) greatly augments the efficacy of t-PA-mediated thrombolysis, (2) seems safe, and (3) has substantial potential as a noninvasive adjunct to improve coronary patency without increasing the risk of bleeding.

Assessment of liver and kidney enhancement with a perfluorocarbon vapor-stabilized US contrast agent

RATIONALE AND OBJECTIVES

The authors evaluated the time-echogenicity response of liver, kidney, and implanted VX2 tumor after injection of a microbubble contrast medium and assessed use of an avascular lesion as an internal standard.

MATERIALS AND METHODS

Twenty-one New Zealand White rabbits were studied. To evaluate use of an internal standard and the dose-response relationship, nine rabbits with 7-day-old avascular liver lesions created by alcohol ablation received 0.1, 0.25, 0.5, and 1.0 mL of AF0145, a microbubble contrast agent. To evaluate tumor echogenicity, 12 rabbits implanted with VX2 tumor in the liver (six also underwent alcohol ablation) received 0.5 mL of AF0145. Videodensitometry was used to analyze echogenicity changes over 10 minutes.

RESULTS

Echogenicity of the alcohol-ablated liver was not affected by contrast material administration. Liver and kidney echogenicity relative to ablation increased linearly with dose, peaking 1 minute after injection and decaying to baseline over 9 minutes. Contrast material administration defined the size and margins of VX2 lesions more clearly. In the arterial phase, the tumor rim was hyperechoic relative to surrounding liver, becoming isoechoic during the portal venous phase then hypoechoic during the late phase parenchymal phase.

CONCLUSIONS

Lesions created by alcohol ablation can be used as an internal standard for quantitative analysis of adjacent tissues. AF0145 enhances perfused tissues, including vascular tumors, at gray-scale, real-time ultrasonography and enhances the liver.

Enhancement of thrombolysis in vivo without skin and soft tissue damage by transcutaneous ultrasound

Previous studies have shown that transcutaneous ultrasound enhances thrombolysis by streptokinase in animals in vivo; however, skin and soft tissue damage induced by ultrasound energy has been a major limitation. The objective of this study was to examine the efficacy of thrombolysis and damage to skin and soft tissues using a newly designed concentrated ultrasound system with a cooling manifold. Using a rabbit model with iliofemoral arterial thrombotic occlusions, 15 pairs of arteries were randomized to receive ultrasound treatment or no ultrasound treatment. Streptokinase (25,000 unit/kg) was given intravenously. Skin temperature was maintained at 25-33 degrees C when ultrasound energy was applied. The serum level of creatine kinase, lactate dehydrogenase, red blood cell counts, and platelet counts were checked at baseline, after thrombus induction, and after ultrasound treatment. Fifteen of fifteen (100%) iliofemoral arteries were angiographically recanalized after ultrasound treatment. In contrast, only 1/15 (6.7%) contralateral arteries were patent after 1 hour. After the subsequent hour with heparin the patency was 14/15 in the ultrasound treated group and 3/15 in the control group. Histologically, the patent arteries had only minimal focal mural thrombus, whereas the angiographically occluded arteries had occlusive thrombi. There was no histologic evidence of ultrasound induced damage to overlying skin, soft tissues, or arteries. In addition, there was no significant rise of creatine kinase, lactate dehydrogenase, or decrease in red blood cell counts and platelet counts induced by ultrasound. In conclusion, transcutaneous concentrated ultrasound which significantly enhances streptokinase induced thrombolysis in vivo can be delivered without concomitant tissue damage. This simple combination therapy has clinical potential for safely treating patients with arterial or venous thromboses.

Noninvasive in vivo clot dissolution without a thrombolytic drug: recanalization of thrombosed iliofemoral arteries by transcutaneous ultrasound combined with intravenous infusion of microbubbles

BACKGROUND

Previous in vivo studies have shown that microbubbles not only enhance the effectiveness of thrombolytic agents in the presence of ultrasound but may also augment clot dissolution without thrombolytic drugs.

METHODS AND RESULTS

The objective of this study was to examine the efficacy of arterial clot disruption by a noninvasive, nonlytic approach with intravenous administration of perfluorocarbon-exposed sonicated dextrose albumin (PESDA) and transcutaneous delivery of ultrasound alone. Pairs of iliofemoral arteries in 10 rabbits were randomized to receive transcutaneous ultrasound treatment or no ultrasound treatment after an acute artery thrombotic occlusion and intravenous PESDA infusion. Five arteries from 3 additional rabbits served as controls (ultrasound alone). All 10 iliofemoral arteries treated with PESDA + ultrasound were recanalized by angiography after ultrasound treatment. None of the 10 contralateral arteries treated with PESDA alone and none of the 5 arteries treated with ultrasound alone were patent after 1 hour. D-Dimer levels did not change after intravenous PESDA + ultrasound-mediated reperfusion.

CONCLUSIONS

In vivo arterial clot dissolution can be achieved with intravenous microbubbles and transcutaneous ultrasound delivery alone. This technique has potential for clinical application in patients with acute arterial and venous thrombotic occlusions.

Pressure-volume characteristics of aortas of harbor and Weddell seals

The mechanical properties of the radially enlarged proximal segment of the aorta of diving marine mammals was studied on 15 excised aortas of harbor seals and five aortas of Weddell seals. This was done by recording static pressure-volume relationships for the whole thoracic aorta, the aortic bulb, and the descending thoracic aorta and passive length-tension measurements of aortic strips. Aortic bulb volume distensibility was found to be much greater than that of the descending thoracic aorta or of an equivalent aortic segment of terrestrial mammals. The consequences were that the total potential energy and volume that may be stored within the aortic bulb is very large, with a capacity for storage of the stroke work of more than two normal heart beats and a volume of more than three times normal stroke volume. The aortic bulb has an average radius and wall thickness twice that of the descending aorta, but at any level of distension the wall stress (g/cm2) is the same throughout. The static mechanical properties of aortic strips from the bulb and descending thoracic aortas were not markedly different, so that the differences in the pressure-volume relationships are explained by differences in geometry of the two sections. The expanded aortic bulb functions through energy and volume storage actions and through uncoupling actions to maintain arterial pressures and stroke volume at near predive levels during a dive.

Regional myocardial blood flow, edema formation, and magnetic relaxation times during acute myocardial ischemia in the canine

This study was designed to measure early changes in myocardial perfusion after acute coronary occlusion, and to examine the relationships among blood flow, myocardial edema, and magnetic relaxation times of ex vivo myocardial tissue. In ten dogs, the left anterior descending coronary artery was occluded for 4 hours prior to sacrifice of the animals. Regional myocardial blood flow was measured using radiolabeled microspheres (15 micron), which were injected into the left atrium 5 minutes prior to sacrifice. Multiple subendocardial tissue samples from the left ventricular free wall were obtained for measurement of magnetic relaxation times, percent water content and tissue radioactivity. Mild, moderate, and severe ischemia were defined as reductions in myocardial blood flow to 30% to 50%, 15% to 30%, and less than or equal to 15% of control, respectively. Myocardial water content was increased with mild ischemia (79.6 +/- 0.7%), moderate ischemia (79.9 +/- 0.4%), and severe ischemia (80.3 +/- 0.6%), all P less than .005 vs. control. T1 relaxation times rose with mild (544 +/- 10 msec, P less than .005 vs. control), moderate (543 +/- 11 msec, P less than .005 vs. control), and severe ischemia (574 +/- 10 msec, P less than .001 vs. control). T2 relaxation times behaved in a similar manner, being prolonged in the mildly, moderately, and severely ischemic groups (38.3 +/- 0.3, 38.1 +/- 0.3 and 38.2 +/- 0.3 msec, respectively; all P less than .001 vs. control).(ABSTRACT TRUNCATED AT 250 WORDS)

Mandibular rest position in subjects with high and low mandibular plane angles

Studies have explored the relationship between different facial morphologies and mandibular posture; however, few conclusions can be drawn. The purpose of this study was to explore the effects of contrasting facial morphologies on both clinical and electromyographic rest positions of the mandible. Electromyographic and clinical rest positions were recorded in 10 subjects whose Frankfort horizontal-to-mandibular plane angles were less than 20 degrees (low-angle group) and in 10 whose angles were greater than 30 degrees (high-angle group). The vertical dimension of clinical rest position was recorded from soft tissue markings on the nose and chin after subjects wet their lips, swallowed, and pronounced Mississippi. Electromyographic rest was defined as the interincisal vertical dimension of minimal muscle activity as determined with an Enting Model 220 EMG integrator and the Myotronics K-5 kinesiograph. Head posture was held constant to the subject's visual axis. The vertical dimension of clinical rest position was significantly greater in the low-angle group (X = 4.6 mm., S.D. = 1.4) than in the high angle group (means = 3.1 mm., S.D. = 1.1); however, no differences were observed in the EMG rest positions, which were 9.9 and 9.7 mm., respectively, for the low- and high-angle groups. These results do not support a simple mechanical explanation of jaw posture; however, these measurements need to be repeated with subjects selecting their preferred head inclination.

Characteristics of coronary blood flow and transmural distribution in miniature pigs

The relationship between phasic systolic and diastolic coronary blood flow and its transmural distribution has been studied in 29 Yucatan miniature pigs at rest and during heavy exercise, with and without adenosine infusion (1.5 mg . kg-1 . min-1) and with and without a subtotal coronary artery occlusion. Altered factors that affected coronary flow included vascular resistance, perfusion pressure, myocardial oxygen demand, and extra-vascular pressure. The data indicate that, at rest, endomural perfusion is significantly dependent on diastolic blood flow. However, the ability of the myocardial vessels to autoregulate during systole as well as during diastole was clearly shown with the use of adenosine infusion. This ability to regulate flow intrinsically appeared to transcend the endocardial dependency on diastolic perfusion under certain stressful conditions, e.g., during heavy exercise, when the diastolic duration was significantly reduced. Systolic transmural perfusion may then become a significant factor in meeting the blood flow demands of the myocardium. However, due to gradients in vascular resistance, perfusion pressure, and oxygen demand, the coronary reserve of the epicardium appears to be greater than that of the endocardium under any condition.

Effects of endurance exercise on coronary collateral blood flow in miniature swine

Coronary collateral blood flow was measured in 7 miniature pigs, exercise trained (ET) for 10 mo by running about 35 km/wk, and in 10 sedentary control pigs (SC). In acute, open-chest preparations, radiolabeled (85SR, 141CE, or 51Cr) microspheres, 15 +/- 5 micron in diameter, were injected into the left atrium during each of three conditions: control (C), total occlusion of the left circumflex artery (TO), and TO plus mechanically elevated aortic pressure (TOP). Blood flow to the circumflex bed during control condition in ET and SC was 0.36 +/- 0.07 (SE) and 0.43 +/- 0.10 ml.g-1.min-1, respectively. During TO, circumflex flow in ET and SC fell to 0.05 +/- 0.01 and 0.06 +/- 0.01 ml.g-1.min-1, respectively. In the presence of TOP, left circumflex flow in ET and SC rose to 0.11 +/- 0.04 and 0.11 +/- 0.02 ml.g-1.min-1, respectively. Blood flow to the tissue bed of the left anterior descendens was the same in both groups of pigs under all conditions. Thus, 10 mo of endurance exercise training seems to have no effect on the development of coronary collaterals in the left ventricles of pig hearts.