A new generation of polyurethane vascular prostheses: rara avis or ignis fatuus?

Three polyurethane (PU) vascular grafts with novel designs were investigated and compared in terms of the microporous structure, reinforcement technology, polymer chemistry, microphase separation, and mechanical properties. The Corvita graft, composed of a poly(carbonate urethane) polymer, displayed a helically wound filament structure with communicating inter-fiber spaces. The reinforced model contained an external PET mesh impregnated with a protein sealant, and displayed good microphase separation, the highest Young's modulus in the longitudinal direction, and the second highest in the radial direction. The Thoratec graft was made of a polyetherurethaneurea with an average micropore size of 15 microns. Silicone was observed on both surfaces of the graft. The Thoratec device displayed a low degree of hydrogen-bonding among the urethane groups and had no well-organized hard-segment domains. Its mechanical strength was superior to that of the Pulse-Tec graft. A solid PU layer underneath the luminal surface precluded any communication between the luminal and adventitial sides. The Pulse-Tec prosthesis was composed of polyetherurethane, with an average micropore size of 28 microns. It offered the highest radial compliance, a high degree of hydrogen-bonding, a narrow molecular weight distribution, and a certain degree of microphase separation. Its tensile strength and hysteresis loss were inferior to those of the other two grafts.

Optimal central trapping (OPCETRA) vena caval filter: results of experimental studies.

The authors present the in vitro and in vivo results of use of a new vena caval filter, the optimal central trapping (OPCETRA) filter. The in vitro study was designed to compare the clot-trapping effectiveness of three filters: the OPCETRA, the stainless steel Greenfield, and the original Vena Tech-LGM. Standardized 3-mm, 5-mm, and 7-mm clots were captured in 66%, 100%, and 100% of cases, respectively, with the OPCETRA filter; in 34%, 82%, and 100% of cases with the Greenfield filter; and in 50%, 100%, and 100% of cases with the LGM filter. When filters were tilted 15 degrees, 100% of the 5-mm clots were trapped with the OPCETRA filter versus 50% with the Greenfield (P < 1.04 x 10(-9)) and 70% with the LGM (P < 1.78 x 10(-5)) filters. The in vivo animal study was designed to confirm ease of placement, tolerance, and effectiveness of the OPCETRA filter in sheep. Animals were separated into two groups: Group 1 underwent embolization through the femoral vein 40 days after filter implantation and were killed immediately; group 2 underwent embolization at 90 days and were killed 8 days later. In all cases pathologic analysis on the vena cava wall was performed after resection. The in vivo study demonstrated no tilting or migration with the OPCETRA filter. These encouraging results can be explained by the filter's hourglass shape, the number of arms, and the slow release of the filter at insertion.(ABSTRACT TRUNCATED AT 250 WORDS)

Conical endocaval filters with metallic struts: search for a new model.

The multitude of caval filters now available and conflicting experimental and clinical findings indicate that no one model can be considered to be perfect. The principal drawbacks of conical endocaval filters, such as the Greenfield model, are the possibility of migration and tilting, often related to failure to open, or untimely ejection of the filter. In this study, we analyzed the geometrical aspects of these filters, leaving aside all concerns of biocompatibility and thrombogenicity, as well as the hemodynamic or clinical aspects. The goal was to determine an ideal geometrical model, with optimal filtration of clots and low risk of tilting.

Collagen coatings as biological sealants for textile arterial prostheses.

Two collagen-coated grafts were studied: Hemashield (bovine collagen cross-linked with formaldehyde vapours and softened by exposure to glycerol) and Tascon (collagen fibres cross-linked with glutaraldehyde solution). The weight of the coating was 310 +/- 5 mg/g for Hemashield and 45 +/- 2.5 mg/g for Tascon. However, notwithstanding these differences, both coatings were efficient in making the walls of the grafts impervious to blood. The water permeabilities for the Hemashield and the Tascon were 8.7 and 5.9 ml.min-1.cm-2 at 120 mmHg respectively. The Hemashield collagen coating was rapidly eroded in vitro (4 h) after exposure to buffer, trypsin or pancreatin solutions, whereas the Tascon collagen coating remained well preserved after 7 d incubation. Both coatings were safe and did not interfere with the physical properties of the graft which was used as a skeleton. The healing properties of the Hemashield were similar to that observed with preclotted polyester prostheses, except in the early hours following graft implantation. On the other hand, the absence of erosion in the coating of the Tascon seemed to contribute to early antithrombogenicity. It also induced marked inflammatory reactions in the surrounding tissues and thus the healing appeared to be delayed.

Textile arterial prostheses: is water permeability equivalent to porosity?

Porosity and water permeability are two distinct terms that describe different characteristics of vascular prostheses. The porosity is a measure of the void fraction within the prosthesis wall and is believed to give a rough prediction of the capacity of the graft to anchor newly formed surrounding tissue after implantation, whereas the water permeability indicates the rate at which water can flow through the prosthesis wall and, when measured under physiological pressure conditions, provides the surgeon with information about the need for preclotting prior to implantation. The literature has not always clearly distinguished between these two terms, and some authors in fact have suggested that they both refer to the same property of a prosthesis. In an attempt to clarify the issue, porosity and water permeability measurements were made on 34 commercial vascular prostheses having different textile constructions. Linear regression analysis demonstrated that these two characteristics are only weakly related (r = 0.59). It is therefore recommended that the current draft standards for such devices reference both properties: porosity and water permeability.

The initial blood retention properties of arterial prostheses.

The initial contact between blood and vascular grafts may be a determinant of the fate of the implants. They behave in different ways depending upon their nature: the processed human umbilical vein and the bovine heterograft just lead to minimal thrombotic retention; the expanded PTFE is even more antithrombogenic; the porous polyesters which are porous textiles, should be totally impregnated with a thrombotic matrix to make the wall impervious. These observations were clearly determined by exposing the grafts to blood, incorporating either labeled platelets or labeled fibrinogen, and they were correlated by SEM examination.

[First contact between blood and the arterial prosthesis. Study of the retention of the blood elements]. / Premier contact sang/prothèse artérielle. Etude de la rétention des éléments sanguins.

In order to characterize the nature of the interaction between arterial prostheses and their initial contact with blood, we propose the use of an in vitro test using labelled platelets and fibrinogen to measure the amount of the thrombotic matrix that is entrapped by the wall of the graft. The results have been confirmed by scanning electron microscopy. Polyester prostheses, whose healing process depends on the reorganisation of the thrombotic matrix, retains considerably more blood cells than the polytetrafluoroethylene microporous prostheses or the treated biological devices, whose fate, following implantation, maintains almost complete passivity.

[Radioisotope evaluation of the interaction between blood and vascular prostheses. Preliminary results]. / Evaluation radioisotopique de l'interaction sang-prothèses vasculaires. Résultats préliminaires.

In order to illustrate the phenomena which occur at the first contact between arterial prostheses and blood, an in vitro test is suggested which may be used to quantify the retention of the cruoric matrix using labelled compounds and confirming the nature by scan electron microscopy. Reorganisation of the cruoric matrix is dependent upon the scarring of polyesters which retain considerably more blood substances than microporous polytetrafluoroethylenes or the biological models dealt with, the course of which after implantation is closer to passivation.

[Evolution of myocardial temperature following cardioplegia with and without complementary pericardial cooling. Experimental study]. / Evolution des températures myocardiques après cardioplégie avec et sans refroidissement péricardique complémentaire. Etude expérimentale.

Myocardial temperatures were measured after cardioplegia during cardiac surgery with and without pericardial cooling by a cold bath. Eight animals (50 kg pigs) were placed on cardiopulmonary bypass using a protocol reproducing clinical operating conditions as closely as possible: myocardial mass, median sternotomy, general hypothermia at 25 degrees. Two injections of cardioplegic solution were administered, one at aortic clamping and the other, 30 minutes later. Four animals (Group A) were used as reference. The four animals in Group B underwent pericardial irrigation with serum at 4 degrees C. Myocardial temperatures were measured at 9 anatomical sites every 10 minutes, (a total of 1008 measurements) and compared statistically. The results in the control group showed that myocardial warming after cardioplegia was intense and rapid at all sites. The sub-epi and sub-endocardial temperatures rose from 7 to 24 degrees in 30 minutes, and in the first 10 minutes, a rewarming of 8,5 degrees was observed. When pericardial cooling was used with cardioplegia, myocardial refrigeration was improved. The temperature remained below 15 degrees C (p less than 0,05 compared with Group A). Only a 3 degrees rise in temperature was observed at the 10th minute after cardioplegia (p less than 0,001). The rise temperature was of 2 degrees at the 20th minute, and 1 degree at the 30th minute. All temperatures remained below 15 degrees C (p less than 0,01). The authors emphasise the benefits of pericardial irrigation associated with cardioplegia for constant, durable and stable myocardial protection by cooling.

[Thermal isolation of the heart by means of a plastic foam pad during surgical cooling of the heart: experimental study]. / Isolation thermique du coeur par patin de mousse plastique au cours de la réfrigération chirurgicale du coeur. Etude expérimentale.

Maintenance of myocardial cooling during aortic clamping is a problem encountered currently during heart surgery. A plastic foam isolation pad was studied. Eight animals (pigs weighing 50 kg) were placed on extracorporeal circulation, 4 of them constituting the control group (A). Two coronary injections of a cardioplegic solutions were administered, the first at the time of aortic clamping and the second 30 minutes later. The myocardium of the 4 animals in the treated group (B) was isolated with the foam pad, and this significantly prolonged the "cold" effect of the cardioplegic. Ten minutes after injection of the cardioplegic, temperature of the total myocardial mass with isolation was between 10.7 and 12 degrees C, as against 16 to 18 degrees C in the reference group (p less than 0.05). By the 20th minute, the isolated myocardium was at approximately 15 degrees C as against 22 degrees C in group A (p less than 0.01). Finally, by the 30th minute, temperature in group B was 17 degrees C as against 23 degrees C in group A (p less than 0.01). Reheating of the heart was significantly slowed by thermal isolation, though the limit of 15 degrees C was reached by the 20th minute. To ensure that myocardial temperature remains below this limit when the foam pad is employed, further injections of cardioplegic solutions are necessary every 20 minutes.