A porcine deep vein thrombosis model for magnetic resonance-guided monitoring of different thrombectomy procedures

Venous thromboembolism swine model with reflux-induced venous hypertension

Objective

This study describes a novel swine model of venous thromboembolism (VTE) with reflux-induced venous hypertension.

Methods

Six pigs underwent disruption of the tricuspid chordae tendineae to create reflux and venous hypertension in the femoral vein. The vein was traumatized 2 to 3 weeks later by repeated withdrawal of a slightly overinflated occlusion balloon across the lumen, followed by balloon occlusion of the outflow. A small amount of thrombin was injected into the traumatized vein segment immediately after outflow occlusion. Thrombosis of the traumatized vein evolved into an organized thrombus seven weeks later. The histological features of the harvested post-thrombotic femoral vein were studied with hematoxylin and eosin and Trichrome stains.

Results

In all six pigs, initial disruption of the chordae tendineae was successfully performed to create tricuspid reflux and venous hypertension. After two-stage sequential procedures, a thrombus formed in the target femoral vein segment. Histology of the harvested thrombotic vein showed features of an organizing thrombus with collagen formation and fibrosis.

Conclusions

The novel swine VTE model may serve as a platform for developing and testing human-sized therapeutic procedures and devices in translational venous research.

Clinical Relevance

This study describes a swine model of VTE created by incorporating all three elements of Virchow's triad. The model uniquely incorporates reflux-induced venous hypertension, which may be used in studying venous insufficiency and VTE in those with systemic venous hypertension. Likewise, this model may serve as a platform for development and evaluation of diagnostic imaging or therapeutic procedures and devices in subjects with systemic venous hypertension.

Thrombosis Models: An Overview of Common In Vivo and In Vitro Models of Thrombosis

Occlusions in the blood vessels caused by blood clots, referred to as thrombosis, and the subsequent outcomes are leading causes of morbidity and mortality worldwide. In vitro and in vivo models of thrombosis have advanced our understanding of the complex pathways involved in its development and allowed the evaluation of different therapeutic approaches for its management. This review summarizes different commonly used approaches to induce thrombosis in vivo and in vitro, without detailing the protocols for each technique or the mechanism of thrombus development. For ease of flow, a schematic illustration of the models mentioned in the review is shown below. Considering the number of available approaches, we emphasize the importance of standardizing thrombosis models in research per study aim and application, as different pathophysiological mechanisms are involved in each model, and they exert varying responses to the same carried tests. For the time being, the selection of the appropriate model depends on several factors, including the available settings and research facilities, the aim of the research and its application, and the researchers' experience and ability to perform surgical interventions if needed.

Effect of Thrombin and Incubation Time on Porcine Whole Blood Clot Elasticity and Recombinant Tissue Plasminogen Activator Susceptibility

Deep vein thrombosis is a major source of morbidity and mortality worldwide. Catheter-directed thrombolytics are the frontline approach for vessel recanalization, though fibrinolytic efficacy is limited for stiff, chronic thrombi. Although thrombin has been used in preclinical models to induce thrombosis, the effect on lytic susceptibility and clot stiffness is unknown. The goal of this study was to explore the effect of bovine thrombin concentration and incubation time on lytic susceptibility and stiffness of porcine whole blood clots in vitro. Porcine whole blood was allowed to coagulate at 37°C in glass pipets primed with 2.5 or 15 U/mL thrombin for 15 to 120 min. Lytic susceptibility to recombinant tissue plasminogen activator (rt-PA, alteplase) over a range of concentrations (3.15-107.00 µg/mL) was evaluated using percentage clot mass loss. The Young's moduli and degrees of retraction of the clots were estimated using ultrasound-based single-track-location shear wave elasticity and B-mode imaging, respectively. Percentage mass loss decreased and clot stiffness increased with the incubation period. Clots formed with 15 U/mL and incubated for 2 h exhibited properties similar to those of highly retracted clots: Young's modulus of 2.39 ± 0.36 kPa and percentage mass loss of 8.69 ± 2.72% when exposed to 3.15 µg/mL rt-PA. The histological differences between thrombin-induced porcine whole blood clots in vitro and thrombi in vivo are described.

Endovascular Venous Stenosis and Thrombosis Large Animal Model: angiographic, histological, and biomechanical characterization

PURPOSE

Characterize an ovine endovascular radiofrequency ablation based venous stenosis and thrombosis model for studying venous biomechanics and response to intervention.

MATERIALS AND METHODS

Unilateral short-segment (n= 2) or long-segment (n = 6) iliac vein stenoses were created in eight adult sheep using an endovenous radiofrequency (RF) ablation technique. Angiographic assessment was performed at baseline, immediately after venous stenosis creation, and after 2-week (n = 6) or 3-month (n = 2) survival. Stenosed iliac veins and contralateral healthy controls were harvested for histological and biomechanical assessment.

RESULTS

At follow-up, the short-segment RF ablation group showed stable stenosis without occlusion. The long-segment group showed complete venous occlusion/thrombosis with formation of collateral veins. Stenosed veins showed significant wall thickening (0.28 mm vs 0.16 mm; p = 0.0175) and confluent collagen deposition compared to healthy controls. Subacute non-adherent thrombi were apparent at 2 weeks, which were replaced by fibrous luminal obliteration with channels of recanalization at 3 months. Stenosed veins demonstrated increased longitudinal stiffness (448.5 ± 5.4 kPa vs. 314.6 ± 1.5 kPa, p < 0.0001) and decreased circumferential stiffness (140.8 ± 2.6 kPa vs. 246.0 ± 1.6 kPa, p < 0.0001) compared to healthy controls.

CONCLUSION

Endovenous radiofrequency ablation is a reliable technique for creating venous stenosis and thrombosis in a large animal model with histological and biomechanical attributes similar to those seen in humans. This platform can facilitate understanding of venous biomechanics and testing of venous specific devices and interventions.

Critical Review of Large Animal Models for Central Deep Venous Thrombosis

OBJECTIVE

To review the existing literature on large animal models of central venous thrombosis (CVT) and to evaluate its relevance in regard to the development and testing of dedicated therapeutics applicable to humans.

METHODS

A systematic literature search was conducted in PubMed and Embase. Articles describing an in vivo experimental protocol of CVT in large animals, involving the iliac vein and/or the vena cava and/or the brachiocephalic vein, were included. The primary aim of the study, animal characteristics, experimental protocol, and thrombus evaluation were recorded.

RESULTS

Thirty-eight papers describing more than 30 different protocols were included. Animals used were pigs (53%), dogs (21%), monkeys (24%), and cattle (3%). The median number of animals per study was 12. Animal sex, strain, and weight were missing in 18 studies (47%), seven studies (18%), and eight studies (21%), respectively. CVT was always induced by venous stasis: solely (55%), or in addition to hypercoagulability (37%) or endothelial damage (10%). The size of the vessel used for thrombus creation was measured in four studies (10%). Unexpected animal death occurred in nine studies (24%), ranging from 3% to 37% of the animals. Twenty-two studies (58%) in the acute phase and 31 studies in the chronic phase (82%) evaluated the presence or absence of the thrombus created, and its occlusive characteristic was reported, respectively, in five and 17 studies. Histological examination was performed in 24 studies (63%) with comparison to human thrombus in one study.

CONCLUSION

This review showed advantages and weaknesses of the existing large animal models of CVT. Future models should insist on more rigour and consistency in reporting animal characteristics, as well as evaluating and comparing the thrombus created to human thrombus.

Animal models of venous thrombosis

Venous thrombosis (VT) is a prevalent clinical condition with significant adverse sequela or mortality. Anticoagulation and pharmacologic or pharmacomechanical thrombolytic therapies are the mainstays of VT treatment. An understanding of thrombosis biology will allow for more effective VT-tailored diagnosis and therapy. In vivo models of thrombosis provide indispensable tools to study the pathogenesis of thrombus formation and to evaluate novel therapeutic or preventive adjuncts for VT management or prevention. In this article, we review the most prominent in vivo models of VT created in rodents and swine species and outline how each model can serve as a useful tool to promote our understanding of VT pathogenesis and to examine novel therapies.

Mechanical thrombectomy of iliac vein thrombosis in a pig model using the Rotarex and Aspirex catheters

PURPOSE

To investigate the efficacy and safety of mechanical thrombectomy for iliac vein thrombosis using Rotarex and Aspirex catheters in a pig model.

MATERIALS AND METHODS

Iliac vein thrombosis was induced in six pigs by means of an occlusion-balloon catheter and thrombin injection. The presence of thrombi was verified by digital subtraction angiography (DSA) and computed tomography (CT). Thrombectomy was performed using 6F and 8F Rotarex and 6F, 8F, and 10F Aspirex catheters (Straub Medical AG, Wangs, Switzerland). After intervention, DSA and CT were repeated to evaluate the efficacy of mechanical thrombectomy and to exclude local complications. In addition, pulmonary CT was performed to rule out pulmonary embolism. Finally, all pigs were killed, and iliac veins were dissected to perform macroscopic and histological examination.

RESULTS

Thrombus induction was successfully achieved in all animals as verified by DSA and CT. Subsequent thrombectomy lead to incomplete recanalization of the iliac veins with residual thrombi in all cases. However, the use of the 6F and 8F Rotarex catheters caused vessel perforation and retroperitoneal hemorrhage in all cases. Application of the Aspirex device caused one small transmural perforation in a vessel treated with a 10F Aspirex catheter, and this was only seen microscopically. Pulmonary embolism was detected in one animal treated with the Rotarex catheters, whereas no pulmonary emboli were seen in animals treated with the Aspirex catheters.

CONCLUSION

The Aspirex catheter allowed subtotal and safe recanalization of iliac vein thrombosis. In contrast, the use of the Rotarex catheter caused macroscopically obvious vessel perforations in all cases.

Noninvasive treatment of deep venous thrombosis using pulsed ultrasound cavitation therapy (histotripsy) in a porcine model

PURPOSE

This study evaluated histotripsy as a noninvasive, image-guided method of thrombolysis in a porcine model of deep vein thrombosis. Histotripsy therapy uses short, high-intensity, focused ultrasound pulses to cause mechanical breakdown of targeted soft tissue by acoustic cavitation, which is guided by real-time ultrasound imaging. This is an in vivo feasibility study of histotripsy thrombolysis.

METHODS AND MATERIALS

Acute thrombi were formed in the femoral vein of juvenile pigs weighing 30-40 kg by balloon occlusion with two catheters and thrombin infusion. A 10-cm-diameter 1-MHz focused transducer was used for therapy. An 8-MHz ultrasound imager was used to align the clot with the therapy focus. Therapy consisted of five cycle pulses delivered at a rate of 1 kHz and peak negative pressure between 14 and 19 MPa. The focus was scanned along the long axis of the vessel to treat the entire visible clot during ultrasound exposure. The targeted region identified by a hyperechoic cavitation bubble cloud was visualized via ultrasound during treatment.

RESULTS

Thrombus breakdown was apparent as a decrease in echogenicity within the vessel in 10 of 12 cases and in 7 cases improved flow through the vein as measured by color Doppler. Vessel histology found denudation of vascular endothelium and small pockets of hemorrhage in the vessel adventitia and underlying muscle and fatty tissue, but perforation of the vessel wall was never observed.

CONCLUSIONS

The results indicate histotripsy has potential for development as a noninvasive treatment for deep vein thrombosis.

Molecular magnetic resonance imaging of deep vein thrombosis using a fibrin-targeted contrast agent: a feasibility study

PURPOSE

To evaluate the value of a fibrin-specific MR contrast agent (EP-2104R; EPIX Pharmaceuticals) for detection of deep vein thrombosis (DVT) and monitoring of percutaneous intervention for treatment.

MATERIALS AND METHODS

In 6 domestic swine, DVT was induced in an iliac/femoral vein using an occlusion-balloon catheter and subsequent injection of thrombin. The occluded vessels were recanalized by mechanical thrombectomy using a Fogarty catheter and an Arrow rotating thrombectomy device. Magnetic resonance imaging of the pelvis and lung was repeated 4 times (before and after DVT induction, after contrast agent administration, and after intervention) using a 1.5-T whole-body XMR system (ACS-NT, Philips Medical Systems, Best, NL). The visualization of the thrombi and contrast-to-noise ratio (CNR) was assessed.

RESULTS

EP-2104R allowed selective visualization of thrombi with accurate determination of the extent of DVT with high contrast (CNR: 65.3 +/- 17.2). After intervention, dislodged thrombus fragments were selectively visualized in the lung (CNR: 27.9 +/- 9.3).

CONCLUSIONS

Molecular magnetic resonance imaging using fibrin-specific MR contrast agent EP-2104R allowed for selective visualization of DVT and monitoring of percutaneous intervention.

Advances in magnetic resonance (2008)

Current advances in magnetic resonance, as a diagnostic modality, are discussed in the context of publications from Investigative Radiology during 2007 and 2008. The articles relating to this topic, published during the past 2 years, are reviewed by anatomic region. The discussion concludes with a consideration of magnetic resonance contrast media, focusing on studies published in the journal, and examining in particular the potential impact of nephrogenic systemic fibrosis.