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Evaluation of the efficacy of in vitro emboli capture using Greenfield and Braile vena cava filters *
(Portuguese PDF version)

Domingo Marcolino Braile,¹ José Maria Pereira de Godoy², Marco Centola³, José Luiz Simon Torati⁴, Guilherme de Campos Marino⁴

1. Physician. Professor and Head of the Graduate Course, Faculdade de Medicina de São José do Rio Preto (FAMERP), SP, Brazil.
2. Physician. Professor, Department of Cardiology and Cardiovascular Surgery, FAMERP, SP, Brazil.
3. Engineer, Braile Biomédica, São José do Rio Preto, SP, Brazil.
4. Undergraduate students, FAMERP, SP, Brazil.

* The study had the support of Braile Biomédica, which provided the peristaltic pump and the Braile filter. Regarding conflict of interest, we state that Prof. Dr. Domingo Marcolino Braile is the President of Indústria Braile Biomédica and Dr. Marco Centola works for this enterprise.

Correspondence:
José Maria Pereira de Godoy
Rua Floriano Peixoto, 2950
CEP 15010-020 - São José do Rio Preto, SP, Brazil
E-mail: godoyjmp@riopreto.com.br

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ABSTRACT

Objective: The aim of this study was to compare the efficacy of the Greenfield and Braile vena cava filters for clot retention in an in vitro model.

Methods: The Greenfield and Braile filters were evaluated in an in vitro study. We used bovine blood and 3-mm diameter silicon tubes to create thrombi of 10, 15, 20, and 30 mm. The filters were installed and fastened inside a 30-mm internal diameter silicon tube, in a vertical position and connected to a pulsating flow system (peristaltic pump - Braile Biomédica, São Paulo, Brazil). A saline solution (0.9%) with 40% glycerin maintained at room temperature was used as the vehicle. The flow was adjusted to 2 l/min and 50 thrombi were released isolatedly for each embolus length, with a total of 200 events, verifying its capture in each release. Fisher's exact test was used for statistical analysis, considering an alpha error of 5%.

Results: The capture efficacy using the Greenfield filter was 78.5% of events and 92% for the Braile filter, a statistically significant difference.

Conclusion: We concluded that the Braile filter proved to be more efficient for thrombi capture in an in vitro study than the Greenfield filter.

Key-words: in vitro, vena cava filters, embolus.

J Vasc Br 2005;4(1):42-6

Pulmonary thromboembolism (PTE) is still one of the main causes of morbidity and mortality all over the world and it represents one of the challenges to be overcome by medicine.¹ The main form of prevention after deep venous thrombosis is the anticoagulation. However, in case it fails or it is contraindicated, vena cava filters represent a new alternative. One of the advantages of anticoagulation is that it does not have the invasive character of the filters and it prevents the secondary thrombosis. Bleeding, however, is its great complication.^1-3 The filters protect the lungs, but affect the vessels since they use an invasive method in its implantation and are foreign bodies in the vena cava.¹ Several filters have been developed over the past years, but they have not reached the status of an ideal filter,^4,6 thus justifying the development of new models that meet these requirements. Temporary vena cava filters were developed as an alternative for permanent filters,^7-9 but they bring similar problems.

The available vena cava filters can be differentiated by design (cone, basket, types of net), by material and for being removable or not. New filters should aim at reducing the complications of the existing filters and improve their efficacy.

The aim of the study was to study a new low-profile vena cava filter for clot retention in an in vitro model, comparing it to the Greenfield filter.

MATERIAL

The efficacy of in vitro emboli capture was assessed using two models of vena cava filters, the Greenfield filter and the Braile filter. The Greenfield filter is made of stainless steel, with six legs that are joined together at the top forming a cone. It is 5-cm long, as can be seen in Figure 1.

Figure 1 - Schematic drawing illustrating the cone-shaped Greenfield vena cava filter with six legs joined together at the top.

The Braile vena cava filter consists of two cones opposed by the apex. The distal cone is formed by eight stainless steel legs, shaped like a "web", 5-cm long with the objective of clot retention. The proximal cone is formed by four legs, 2-cm long made of the same material, playing the role of an anchor and filter centralization, as can be seen in Figure 2.

Figure 2 - Schematic drawing illustrating the Braile vena cava filter with two cones opposed by the apex.

We used bovine blood for the tests, which was injected into 3-mm diameter silicon tubes to create thrombi of 10, 15, 20, and 30 mm. The vena cava filters were installed and fastened inside a 30-mm internal diameter silicon tube, in a vertical position and connected to a pulsating flow system (peristaltic pump - Braile Biomédica, São Paulo, Brazil), as can be seen in Figure 3. The Greenfield filter centralization was assured during all experiment. A parallel device was interconnected to a three-stage valve system with a syringe for clot introduction in the circuit. A saline solution (0.9%) with 40% glycerin maintained at room temperature was used as the vehicle. The flow was adjusted to 2 l/min for both models and 50 thrombi were released isolatedly for each embolus length, with a total of 200 events for each filter, verifying its capture in each release.

Figure 3 - Schematic drawing showing the circuit used for assessing the in vitro efficacy of the Greenfield and Braile vena cava filters.

Fischer's exact test was used for statistical analysis, considering an alpha error of 5%.

RESULT

The Greenfield vena cava filter captured 78.5% of the emboli, while the Braile filter captured 92% (Table 1).

Table 1 - Efficacy of Greenfield and Braile vena cava filters in capturing four different lengths of emboli

Diameter vs. length	Braile filter capture (%)	Greenfield filter capture (%)	P
3 vs. 10 mm	84	72	0.2
3 vs. 15 mm	92	76	0.053
3 vs. 20 mm	96	84	0.09
3 vs. 30 mm	96	82	0.051
Average capture	92	78.5	0.0002

 

Fischer's exact test showed a significant difference (P < 0.0002) concerning the efficacy of the two models of filters. In the general evaluation, the Braile filter was more efficient in terms of emboli capture than the Greenfield filter. The difference of sizes was nor statistically significant, as shown in Table 1.

DISCUSSION

The present study showed that the Braile vena cava filter was more efficient in terms of emboli capture in an in vitro experiment, when compared to the Greenfield filter. However, there was no difference when the different sizes were compared separately. This can be a consequence of the lower number of events analyzed. Although the in vitro test models use the same principle (fluid flow of saline, Dextran, or glycerin solution, circulating by means of pumps in a translucent tubulation),^7,10 there are no study reports in the literature using the same model of this experiment. Nevertheless, studies show that several factors can influence the results, such as the number of released thrombi, thrombi size, tube diameter used for filter fixation,^11-13 as well as the horizontal or vertical placement of the test tube. As to the number of the test sample used in the present study, it was enough for the statistical analysis to show a significant difference. The thrombi size used for testing is variable: 4 x 4, 2 x 4 x 3, 5 x 5, 3 x 6, until 9 x 20.^7,10,11 The choice for thrombi size in the present study was a consequence of a pilot study that showed the interference of the embolus diameter and size. The in vitro study, in this case, made possible to identify important data concerning the filter before moving on to the following phase.

The Greenfield filter, for being a pioneer and an efficient filter, with one of the lowest frequencies of long-term vena cava occlusion, was the most used and analyzed in the literature,² and, therefore, motivated its selection for evaluating a new filter. It also served as a model for the development and improvement of the new filter. The development of the Braile filter was based on the cuneiform structure of the Greenfield filter, identifying its flaws and seeking solutions. The first objective was to improve the efficacy of in vitro emboli capture. In order to do that, the number of filter legs was increased. This procedure improved the capture performance, but caused an increase in the material volume: a 48-cm steel leg against 30 cm of the Greenfield filter. The solution for this increase was to reduce the leg length to 32 cm. The higher number of legs reduced the space among them, allowing smaller emboli to be captured. The number of legs is one of the differentials between the two filters concerning the mechanism of emboli capture. Another aspect that contributed to the increase in efficacy of the Braile filter is the centralization mechanism. This filter design, with two cones opposed by the apex, allowed the creation of the centralization mechanism. One of the cones captures the emboli and the other one helps a better placement and device centralization. Thus, it accomplishes one of the objectives of the development of new products, that is, the improvement and better efficacy of the filters. The emboli tendency is the centralized displacement, and thus the cone apex region allows the smaller emboli capture. The filter decentralization widens the distance between the legs in the vessels central region and cause more capture failure, since the emboli tend to move to the center.

The few clinical studies assessing this aspect showed that around 5% of implanted Greenfield filters were decentralized. However, they did not report if this fact would affect its efficacy.^13-15

The reduction of the filter profile was another emphasized aspect. The Braile filter can be implanted with a 7 F introducer, which causes fewer vessel lesions during implantation. Thus, two great advantages were identified on the Braile filter compared to the Greenfield filter: more efficacy in emboli capture and the reduction of the introducer. These advantages are also present in the Simon nitinol filter and the Bird's nest filter.¹² However, the long-term behavior of this new filter in the vena cava should be assessed.

CONCLUSION

Considering the conditions of this in vitro model, the Braile vena cava filter proved to be more efficient in emboli capture than the Greenfield filter.

ACKNOWLEDGEMENT

The undergraduate students received the support of the BIC Scholarship of scientific initiation at the Faculdade de Medicina de São José do Rio Preto (FAMERP) for this study.

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