Ultrasonic clot disruption: an in vitro study

Does external ultrasound accelerate thrombolysis? Results from a rabbit model

BACKGROUND

Prior in vitro and in vivo studies have reported that external ultrasound accelerates thrombolysis at intensities too low to have a direct effect on clot dissolution in the absence of a thrombolytic agent. The present study was undertaken to examine the ultrasound effect on thrombolysis and reocclusion in a rabbit thrombosis model.

METHODS AND RESULTS

Blood clots were produced in a femoral artery segment with endothelial damage and distal stenosis. Recombinant tissue-type plasminogen activator (rTPA) was infused at 30 micrograms.kg-1.min-1 for 60 minutes. Femoral artery flow was measured every 5 minutes for 2 hours. Rabbits were randomized to four groups with continuous wave ultrasound on or off with or without intravenous injection of 17 mg/kg aspirin (+US/-US/+Asp/-Asp). Ultrasound frequency and intensity were 1 MHz and 6.3 W/cm2. In seven of eight and five of five rabbits given rTPA and -US/-Asp or -US/+Asp, respectively, reflow was observed, persisting to the end of the observation period. In five of nine and four of five rabbits given rTPA and +US/-Asp or +US/+Asp, reflow was achieved, but persistent reocclusion was subsequently observed in five of five and two of four of these rabbits, respectively. Overall, femoral artery patency was worse and reocclusion occurred more often when ultrasound was added to rTPA (P = .002 by nonparametric ANOVA). However, initial reflow occurred more rapidly with ultrasound exposure (21 +/- 10 and 33 +/- 6 minutes for the +US/+Asp and +US/-Asp groups, respectively) compared with without ultrasound (46 +/- 13 and 74 +/- 14 minutes for the -US/+Asp and -US/-Asp groups, respectively) (P = .03 by ANOVA).

CONCLUSIONS

Although time to initial reflow was shortened by ultrasound, it was associated with less reperfusion and more reocclusion in this model. A possible explanation for these results is ultrasound-induced platelet activation counterbalancing its thrombolysis-accelerating effect.

Enhancement of thrombolysis by ultrasound

Drug-induced early reperfusion in acute myocardial infarction reduces myocardial damage and decreases mortality. A further beneficial effect may be achieved if the time from start of thrombolytic treatment to reperfusion, on average 45 min, can be shortened. With this purpose in mind, we have analysed the effect of ultrasound on the reperfusion time in an experimental model in vitro. A cylindrical fibrin thrombus with a 2 mm diameter and a 20 microL volume was made by thrombin activation of a pure 0.5% fibrinogen solution in a soft silicone tube. The tube was placed in a low pressure perfusion system and maintained at 37 degrees C. The thrombi were then exposed to hydrostatic loading with a streptokinase concentration of 5000 units/mL. Reperfusion times (RT) were measured from time of Streptokinase exposure to fluid passage, identified by the photoelectric technique. RT increased significantly with increasing thrombus age (r = 0.92, p < 0.05) and was 34-45 min (95% confidence limits) at a thrombus age of 1 h and 102-122 min at a thrombus age of 2 h. RT was unaffected by temperatures between 33 and 45 degrees C but increased with higher temperatures. All investigations of ultrasound effects were performed with 1 h old thrombi and at 37 degrees C. RT decreased by 49% (p < 0.01) as an effect of exposure to 1 MHz ultrasound at 1 W/cm2 SATA. Intermittent ultrasound exposure for 10 microseconds/ms with the same intensity and frequency shortened RT by 54% (p < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Clinical trial of percutaneous peripheral ultrasound angioplasty

OBJECTIVES

The purpose of this study was to present follow-up data as well as short-term results on a larger clinical series of patients undergoing ultrasound angioplasty.

BACKGROUND

Previous pilot studies have demonstrated the feasibility of peripheral arterial ultrasound angioplasty.

METHODS

We performed percutaneous ultrasound angioplasty on 50 arterial lesions in 45 patients. Our ultrasound ablation system had a frequency of 19.5 kHz. A fixed-wire probe with 2- or 3-mm ball tips and a 3-mm over-the-wire probe were used to treat 40 femoral, 7 popliteal and 3 tibioperoneal lesions. Seventeen (34%) of the lesions were calcific. Thirty (86%) of 35 occluded segments, 0.5 to 28 cm long (mean 6.2 +/- 5.7), were recanalized.

RESULTS

In the 45 patent arteries, the stenosis decreased from 94 +/- 10% to 55 +/- 23% after ultrasound angioplasty and to 12 +/- 8% after balloon angioplasty. Mechanical arterial dissections (n = 4) and perforations (n = 4) without clinical consequence occurred only with the fixed non-over-the-wire probes. No evidence of embolism or vasospasm was detected; in fact, vasodilation occurred. There were no clinical manifestations of acute reocclusion. At 24 h, ankle-brachial indexes increased by 0.23 +/- 0.21 (range -0.27 to 0.72). Six- to 12-month clinical and ankle-brachial index follow-up data for 35 patients treated with ultrasound and adjunctive balloon angioplasty were indicative of restenosis in 7 patients (20%).

CONCLUSIONS

Our findings indicate that percutaneous peripheral ultrasound angioplasty 1) is useful for recanalization of fibrous, calcific and thrombotic arterial occlusions; 2) reduces arterial stenoses; and 3) has clinical and ankle-brachial index data indicative of a restenosis rate of 20% at 6 to 12 months in a small cohort of patients. A larger randomized series of patients will need to be studied to assess the impact of ultrasound ablation on restenosis.

Effects of ultrasound energy on total peripheral artery occlusions: initial angiographic and angioscopic results

Ultrasonic energy has been shown to ablate atherosclerotic plaques and arterial and venous thrombi. We used an ultrasonic angioplasty device developed by our group in ten patients with totally occluded femoral artery during surgical bypass. Ultrasonic angioplasty was performed with a 130-cm long and 0.8-cm diameter titanium probe with a 2- or 2.5-mm titanium ball-tip. In one patient, angioplasty could not be performed. Angiographic and angioscopic examination were performed before and after angioplasty in nine patients. Before ultrasound recanalization, angioscopic examination showed that the proximal end of the occlusion was formed by atheromatous material in 3 cases, red thrombus in 3 cases, amd white thrombus in 3 cases. After ultrasound recanalization, angioscopy showed residual stenosis at the site of entry in only one case. In three other cases, the artery was free of residual stenosis without persistent clot. In the five other patients, a residual stenosis was present beyond the proximal occlusion point with some fibrin mesh and small clots. At angiography, flow was restored in 4 cases; in 4 patients flow rate of entry was slow in the distal segment; and in 1 patient, the distal arterial bed could not be opacified. Altogether, ultrasonic angioplasty was able to recanalize a complete occlusion in nine out of ten patients, with partial or complete dissolution of clots and with no complication. At its present stage of development, adjunctive balloon angioplasty would be needed in most cases to obtain unrestricted flow and unsignificant residual stenosis.(ABSTRACT TRUNCATED AT 250 WORDS)

Enhancement of thrombolysis by external ultrasound

The purpose of our study was to assess the efficacy of external ultrasound to enhance in vitro thrombolysis with urokinase or streptokinase. One-hour-, 1-day-, 4-day-, and 6-day-old human blood thrombi (n = 366) were incubated in normal saline solution with three different concentrations of streptokinase (50, 250, and 2000 mu/ml) or urokinase (200, 2000, and 5000 mu/ml). Thrombi were exposed to pulsed ultrasound of 1 MHz at 1.0, 1.5 and 2.2 W/cm2 at different exposure times. The combination of ultrasound (2.2 W/cm2, 30 min) and urokinase or streptokinase enhanced lysis rate by an average of 25% compared with lysis with thrombolytic agents alone (p < 0.05). The enhancement was greater at higher ultrasound power outputs (2.2 W/cm2 > 1.5 W/cm2 > 1.0 W/cm2). At higher-power outputs there was no increase of temperature in the solution containing the thrombus. The extent of lysis was higher with longer ultrasound exposure time and with fresh thrombi. These data suggest that use of external ultrasound has the potential to increase both efficacy and rate of thrombolysis.

Ultrasound enhancement of thrombolysis and reperfusion in vitro

OBJECTIVES

The aims of this study were 1) to develop an in vitro flow system in which reperfusion mediated by ultrasound-accelerated thrombolysis could be studied, and 2) to test whether ultrasound-accelerated thrombolysis could hasten reperfusion in this system.

BACKGROUND

Ultrasound has been shown to increase tissue plasminogen activator (t-PA)-induced thrombolysis in vitro as assessed by radioactive fibrinogen release from labeled clots and in an animal in vivo model.

METHODS

To test whether reperfusion is accelerated, we created obstructive whole blood clots in an in vitro flow system. Four control clots were exposed to ultrasound only without any thrombolytic agent (group 1). Sixteen clots were exposed to continuous infusion of recombinant tissue-type plasminogen activator rt-PA and randomized to either continuous wave ultrasound exposure at a frequency of 0.5 MHz and an intensity of 8 W/cm2 (group 2) or to no ultrasound (group 3). Flow distal to the clot and the rate of release of radiolabeled fibrin products were used as an index of reperfusion and thrombolysis, respectively. Samples were obtained for measurements of lytic variables such as plasminogen, fibrinogen and rt-PA concentrations.

RESULTS

Flow was significantly higher in the rt-PA-treated clots within 10 min of exposure to ultrasound than in those without such exposure (9.4 +/- 9.9% of maximal flow in group 2 vs. 0.5 +/- 1.5% in group 3, p < 0.05). The maximal difference in flow between groups 2 and 3 was achieved at 25 min (61.0 +/- 30.4% vs. 14.2 +/- 14.7%, p = 0.03). Thrombolysis was significantly higher after 15 min of ultrasound exposure (12.8 +/- 9.1% in the ultrasound-treated group 2 vs. 4.0 +/- 3.9% in group 3, p < 0.05). The maximal difference between groups 2 and 3 occurred at 25 min (26.7 +/- 13.1% vs. 7.24 +/- 5.7%, p < 0.004). Neither flow nor clot lysis occurred in group 1. Plasminogen and fibrinogen concentrations and rt-PA antigen concentrations were consistent with those observed during fibrinolytic therapy in vivo.

CONCLUSIONS

Continuous wave ultrasound at 0.5 MHz and an intensity of 8 W/cm2 accelerates rt-PA-induced thrombolysis and reperfusion in vitro.

Effects of ultrasound energy on thrombi in vitro

Ultrasonic energy may be used for dissolution of venous or arterial thrombi. However, its effects may depend on the mode of ultrasonic vibration and on the length of the probe. We investigated the in vitro effects of an ultrasonic angioplasty device coupled with a 130 cm-long flexible titanium probe, with an incorporated automatic optimal frequency of resonance scanning function and continuous mode of emission. Sixteen clots were treated of which eight were whole blood and eight cell-free. In each of these groups, four were treated in association with streptokinase and four by ultrasound alone. The ages of the clots in these subgroups of four were 1, 3, 7, and 15 days. All thrombi were dissolved in 6 min or less (3'15" +/- 1'35") at a mean optimal frequency of resonance of 19,444 Hz. Ninety-six percent of the debris were less than 10 mu. Fewer than 1% of the particulates were larger than 100 mu. These large particulates were observed in disrupted whole blood clots and were almost non-existent in disrupted cell-free clots. They were very fragile. Clot dissolution was not speeded by adding streptokinase to ultrasound. Ultrasound did not induce D-Dimer production, and its effect was most likely to be due to cavitation. Ultrasound energy could represent an advance for thrombotic vascular occlusion therapy, provided that more flexible probes can be devised.

Enhancement of fibrinolysis in vitro by ultrasound

The effect of ultrasound on the rate of fibrinolysis has been investigated using an in vitro system. Plasma or blood clots containing a trace label of 125I fibrin were suspended in plasma containing plasminogen activator and intermittently exposed to continuous wave 1-MHz ultrasound at intensities up to 8 W/cm2. Plasma clot lysis at 1 h with 1 microgram/ml recombinant tissue plasminogen activator (rt-PA) was 12.8 +/- 1.2% without ultrasound and was significantly (P = 0.0001) increased by exposure to ultrasound with greater lysis at 1 W/cm2 (18.0 +/- 1.4%), 2 W/cm2 (19.3 +/- 0.7%), 4 W/cm2 (22.8 +/- 1.8%), and 8 W/cm2 (58.7 +/- 7.1%). Significant increases in lysis were also seen with urokinase at ultrasound intensities of 2 W/cm2 and above. Exposure of clots to ultrasound in the absence of plasminogen activator did not increase lysis. Ultrasound exposure resulted in a marked reduction in the rt-PA concentration required to achieve an equivalent degree of lysis to that seen without ultrasound. For example, 15% lysis occurred in 1 h at 1 microgram/ml rt-PA without ultrasound or with 0.2 microgram/ml with ultrasound, a five-fold reduction in concentration. Ultrasound at 1 W/cm2 and above also potentiated lysis of retracted whole blood clots mediated by rt-PA or urokinase. The maximum temperature increase of plasma clots exposed to 4 W/cm2 ultrasound was only 1.7 degrees C, which could not explain the enhancement of fibrinolysis. Ultrasound exposure did not cause mechanical fragmentation of the clot into sedimentable fragments, nor did it alter the sizes of plasmic derivatives as demonstrated by SDS polyacrylamide gel electrophoresis. We conclude that ultrasound at 1 MHz potentiates enzymatic fibrinolysis by a nonthermal mechanism at energies that can potentially be applied and tolerated in vivo to accelerate therapeutic fibrinolysis.

Ultrasound recanalization of diseased arteries. From experimental studies to clinical application

At present, percutaneous peripheral ultrasound angioplasty should be considered in those patients with symptoms of claudication or resting limb ischemia. With the development of an over-the-wire system, we treat patients with suprageniculate or infrageniculate lesions. It is expected that the over-the-wire probe will allow application of ultrasound angioplasty not only to lesions below the knee but to contralateral vascular occlusions as well. An intraoperative device for plaque ablation and arterial recanalization is in development for use in less accessible sites such as the coronary arteries. Experimental studies have shown that catheter-delivered therapeutic ultrasound recanalizes complete occlusions, reduces stenoses, dissolves thrombus, vasodilates, and enhances arterial distensibility. The potential clinical applications of therapeutic ultrasound include recanalizing total arterial occlusions, dissolving thrombi, facilitating balloon angioplasty by increasing arterial compliance, and as a stand-alone angioplasty device.

Dissolution of peripheral arterial thrombi by ultrasound

BACKGROUND

We have previously shown that continuous-wave ultrasound can rapidly dissolve human thrombi in vitro, with 99% of all residual particles measuring less than 10 microns in diameter. To assess the effects of pulsed-wave ultrasound energy on whole blood clots, 1) in vitro studies were preformed to assess precisely the rates of clot disruption and to quantify particulate size, and 2) in vivo studies were performed to assess the efficacy and safety of catheter-delivered ultrasound for intra-arterial thrombus dissolution.

METHODS AND RESULTS

In vitro, we studied 50 samples of human whole blood clots and using an 89-cm-long wire probe, applied pulse-wave energies from 8 to 23 W. The corresponding peak-to-peak tip displacement range was 63.5 - 102 microns. We studied arterial thrombosis in vivo in 21 canine superficial femoral arteries. To produce an acute thrombosis, 200 units of thrombin followed by 2 ml of 72-hour-old autologous clot were injected into a 5-7-cm segment of femoral artery and left to coagulate for 2 hours. Ultrasound energy was intermittently applied at a frequency of 20 kHz with a prototype ultrasound wire ensheathed in a catheter and directed to clots by fluoroscopy. In nine cases, angioscopic guidance was used to put the probe into direct contact with the intra-arterial thromboses. In vitro clot dissolution times were inversely related to the ultrasound power output (r = 0.95). All in vivo canine thromboses were disrupted in 4 minutes or less. All successful recanalizations were confirmed by angiography and in nine cases by angioscopy as well. Angioscopy demonstrated that probe activation caused rapid clot disruption. Histological studies of the vessels showed no evidence of thermal or cavitation injury, occlusive distal embolization, or perforation.

CONCLUSIONS

Our findings in this experimental canine model suggest that ultrasound clot dissolution has the potential to be an effective and safe alternative to current treatment modalities for peripheral arterial thrombosis.