HeartMate II left ventricular assist system: from concept to first clinical use.

The HeartMate II left ventricular assist device (LVAD) (ThermoCardiosystems, Inc, Woburn, MA) has evolved from 1991 when a partnership was struck between the McGowan Center of the University of Pittsburgh and Nimbus Company. Early iterations were conceptually based on axial-flow mini-pumps (Hemopump) and began with purge bearings. As the project developed, so did the understanding of new bearings, computational fluid design and flow visualization, and speed control algorithms. The acquisition of Nimbus by ThermoCardiosystems, Inc (TCI) sped developments of cannulas, controller, and power/monitor units. The system has been successfully tested in more than 40 calves since 1997 and the first human implant occurred in July 2000. Multicenter safety and feasibility trials are planned for Europe and soon thereafter a trial will be started in the United States to test 6-month survival in end-stage heart failure.

Computational simulation of platelet deposition and activation: II. Results for Poiseuille flow over collagen.

We have previously described the development of a two-dimensional computational model of platelet deposition onto biomaterials from flowing blood (Sorensen et al., Ann. Biomed. Eng. 27:436-448, 1999). The model requires estimation of four parameters to fit it to experimental data: shear-dependent platelet diffusivity and three platelet-deposition-related reaction rate constants. These parameters are estimated for platelet deposition onto a collagen substrate for simple parallel-plate flow of whole blood in both the presence and absence of thrombin. One set of experimental results is used as a benchmark for model-fitting purposes. The "trained" model is then validated by applying it to additional test cases from the literature for parallel-plate Poiseuille flow over collagen at both higher and lower wall shear rates, and in the presence of various anticoagulants. The predicted values agree very well with the experimental results for the training cases, and good reproduction of deposition trends and magnitudes is obtained for the heparin, but not the citrate, validation cases. The model is formulated to be easily extended to synthetic biomaterials, as well as to more complex flows.

Decrease in red blood cell deformability caused by hypothermia, hemodilution, and mechanical stress: factors related to cardiopulmonary bypass.

During extracorporeal circulation in cardiopulmonary bypass (CPB) surgery, blood is exposed to anomalous mechanical and environmental factors, such as high shear stress, turbulence, decreased oncotic pressure caused by dilution of plasma, and moderate and especially deep hypothermia widely applied during CPB in infants. These factors cause damage to the red blood cells (RBCs), which is manifest by immediate and delayed hemolysis and by changes in the mechanical properties of RBCs. These changes include, in particular, decrease in RBC deformability impeding the passage of RBCs through the microvessels and may contribute to the complications associated with CPB surgery. We investigated in vitro the independent and combined effects of hypothermia, plasma dilution, and mechanical stress on deformability of bovine RBCs. Our studies showed each of these factors to cause a significant decrease in the deformability of RBCs, especially acting synergistically. The impairment of RBC deformability caused by hypothermia was found to be more pronounced for RBCs suspended in phosphate buffered saline (PBS) than for RBCs suspended in plasma. The decrease in RBC deformability caused by mechanical stress was significantly exacerbated by dilution of plasma with PBS. In summary, results of our in vitro study strongly point to a possible detrimental consequence of conventional CPB arising from increased RBC rigidity, which may lead to impaired microcirculation and tissue oxygen supply.

Fine trabecularized carbon: ideal material and texture for percutaneous device system of permanent left ventricular assist device.

The development of a percutaneous artificial internal organ system requires a reliable biocompatible connection between the external environment and the inside of the human body. Such is necessary for the success of a permanent left ventricular assist device. However, the search for a satisfactory interface at the epidermal level has proven to be difficult. Carbon has been proposed for this application, but its texture does not typically promote ingrowth from surrounding tissue. We have therefore employed a new processing method to produce a fine trabecularized carbon implant. The method for preparing the implant involves infiltrating low temperature pyrolytic carbon into the surface of a carbon core which is wrapped with carbon fabric. This results in a tightly woven porous structure of carbon (carbon fiber diameter: 35-50 microm, maximal pore size >200 microm) with gradually increasing porosity from 15-75%. We implanted test samples percutaneously in a calf for in vivo histological evaluation. Thirty days after implantation epidermal downgrowth was minimal. Microscopic analysis revealed that a thin fibrous capsule surrounded the implant, and mature connective tissue with accompanying blood vessels filled the pores of the fine trabecularized carbon layer. From these results we suggest that fine trabecularized carbon is ideally suited for a percutaneous device system in a permanent left ventricular assist device.

Experience with univentricular support in mortally ill cardiac transplant candidates.

Between July 1987 and March 1989, 11 patients underwent left ventricular support with the Novacor left ventricular assist system irrespective of apparent degree of right ventricular failure. The first 2 patients died of multisystem organ failure while on support. All the remaining patients survived the support period, and actuarial survival after transplantation was 100% at 6 months and 89% at 1 year. In no patient did bacterial infection develop during support or after transplantation. Right ventricular ejection fraction before implantation of the left ventricular assist system was lower than 15% in 6 of 8 patients, yet it increased twofold during left ventricular support. The need for excessive inotropic support (2 patients) or temporary (four days) mechanical right ventricular support (2 patients) while on the left ventricular support system appeared to be related to elevated pulmonary vascular resistance during support in association with large preimplantation ventricular volumes. It appears that even patients with compromised right ventricular performance can be supported long term with a left ventricular assist device. Patients with elevated pulmonary vascular resistance may require temporary right ventricular support.

Measurement of capsular contracture: the conventional breast implant and the Pittsburgh implant.

At present, there is no accurate, reliable method of experimentally measuring capsular contracture. This study had four goals: (1) to define the parameters of capsular contracture employing principles of biomechanics of soft tissues, (2) to develop laboratory techniques to measure the parameters, (3) to design an implant that mechanically impedes the process of encapsulation, and, (4) to test this implant against a conventional one. We have developed a breast implant (the Pittsburgh implant) with an altered surface topography. Its silicone shell is punctuated by projections 1 mm in height and 1 mm in diameter. Two techniques were devised to measure contracture. The first involved measuring the force deformation along a coronal axis. The second involved measuring hydrostatic pressures within the implant resulting from the injection of known quantities of saline. Measurements were performed in vivo on 36 animals. By both force and pressure measurements, the Pittsburgh implant showed less capsular contracture (p = 0.12 and 0.012, respectively). Histology revealed that the prototype surface alters the linear arrangement of myofibroblasts and redirects the laminar collagen into a waveform pattern. We conclude from this experimental study that an altered surface topography may serve as a means of rendering a capsule less mechanically effective. We feel that the proposed methods can be used in the laboratory to characterize the extent of capsular contracture.