[Models for improving the stability of unilateral external osteosynthesis (bracket fixation)].
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Biomechanical Phenomena↗
Biomedical subjects
H Schima
Publications and source records attributed to H Schima.
[Program package for evaluating experimental circulation studies].
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Animals↗
Driving the artificial heart with constant air pressure or constant flow.
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Air↗
Testing of biomaterials, accelerated ageing.
The residual elongation is a critical property of materials used for manufacturing diaphragms of artificial hearts. It is therefore important to check goods received or to control manufactured diaphragms, whether their creep properties are within the required limits. Ordinary creep tests take at least several months, while the release of goods received or diaphragms manufactured should be possible within a few days. Acceleration of the creep test by increasing the test temperature permits an estimation whether the creep properties of a material are within the required limits within a week.
Heart, Artificial↗
Value and limitations of proximal jet dimensions for the quantitation of valvular regurgitation: an in vitro study using Doppler flow imaging.
To evaluate usefulness and limitations of proximal jet dimensions for the quantitation of valvular regurgitation, fluid jets were created in a pulsatile flow model, and proximal jet width and cross-sectional area were measured by means of color Doppler flow imaging. When circular orifices with diameters from 1 to 6 mm were studied, jet width and cross-sectional jet area measured as close as possible to the orifice were directly related to the orifice diameter (r = 0.97; standard error of the estimate, 0.4 mm; y = 0.5 + 0.97x) and the orifice area (r = 0.97; standard error of the estimate, 5.7 mm2; y = 0.22 + 2.47x), respectively. No significant dependence on flow rate or pressure gradient was found for these measurements. Jet width was measured with color M-mode Doppler by use of the smallest sample volume size (1 mm) and was slightly greater than the orifice diameter (4.2 +/- 1.7 mm versus 3.8 +/- 1.7 mm). However, cross-sectional areas were approximately fourfold the orifice areas, on average (52.5 +/- 24.6 mm2 versus 12.3 +/- 9.7 mm2), with a range of twelvefold (smallest orifice) to threefold (largest orifice). When the sample volume size was reduced from 2.4 to 1.0 mm, color areas decreased by 25.6 +/- 6.0%. Slit-shaped orifices were studied with two different orientations of the slit-parallel and perpendicular to the ultrasound beam: Color M-mode measurements were again slightly greater than length and width of the slit, but cross-sectional areas were substantially larger than the orifice areas and increased between 44% and 115% when changing the orientation of the slit from perpendicular to parallel. This, again, reflected problems with lateral resolution. When cross-sectional areas were measured at increasing distances from the orifice, the cross-sectional jet area increased significantly within a few millimeters. This increase was greater with higher gradients and smaller orifice sizes. In case of a small orifice (2 mm) and a high gradient (130 to 160 mm Hg), increase in area was as great as 122% within a distance of only 5 mm. Thus, proximal jet width and cross-sectional area were directly related to the orifice size, which could be a valuable parameter for the evaluation of valvular regurgitation. Measurements of jet width by color M-mode seemed to be most accurate but are limited by the fact that in vivo valvular defects may be irregular rather than of circular shape.(ABSTRACT TRUNCATED AT 400 WORDS)
Aortic Valve Insufficiency↗
Sustained heart failure induced by repeated microsphere injections for left ventricular assist device testing.
A subacute to chronic heart failure model was established to test left ventricular assist device (LVAD) systems. Five calves were anesthetized and instrumented. Heart failure was induced by daily injection of polystyrene microspheres (37-70 microns) into a left coronary catheter. The last three animals had an atrio-aortic pulsatile LVAD implanted. Cardiac output (CO) decreased from 12 to 5.5 L/min, and SV from 120 to 60 ml, whereas left atrial pressure (LAP) increased from 12 to 25 mmHg within 10 to 20 days in the two animals without LVAD support. Both animals stopped eating and drinking. The LVAD experiments were terminated after 5, 10, and 11 days. Cardiac function was determined during daily pump-off tests. After repeated microsphere injections, LAP increased to 30 and 50 mmHg, and CO decreased to 4 and 6 L/min without pump support. Daily stepwise microsphere injections induce a sclerotic cardiomyopathy in a safe and reliable manner.
Animals↗
Atrial natriuretic factor production and secretion during clinical total artificial heart-bridge-to-transplantation.
In 5 total artificial heart (TAH)-bridge-to-transplant (BTT) patients production and secretion of atrial natriuretic factor (ANF)1 was studied before, during, and after TAH implantation. Bridging periods lasted between 9 and 28 days. Atrial biopsies were taken during implantation, and after TAH explantation for histologic and histochemical investigations to evaluate differences in morphology and ANF-content of the specific atrial granules. Plasma concentrations of ANF (pANF), aldosterone (PAC) and renin (PRC) were measured daily, as were hemodynamic parameters. In the preoperative state, pANF was always markedly elevated, while during TAH bridging, pANF remained moderately elevated with fluctuations. A positive correlation between pANF levels and right atrial pressure (RAP) was seen in all patients (p less than 0.05). Slight correlation was also observed between pANF and left atrial pressure (LAP), but no correlation was seen between pANF and systemic blood pressure, and no consistent pattern was seen in the plasma concentrations of either renin or aldosterone. After heart transplantation (HTX), which was performed in 3 patients, pANF levels were significantly higher than during TAH, and continued to show a positive correlation with RAP. From our data, we conclude that ANF production sites and secretory mechanisms remain intact during TAH-bridging, although upon implantation of a TAH, the remaining atria are deprived of all coronary blood supply and most autonomic innervation.
Adult↗
Effect of stationary guiding vanes on improvement of the washout behind the rotor in centrifugal blood pumps.
In centrifugal pumps, there always exists an area of stagnation between the rear of the rotor and the rear housing wall that promotes thrombus formation around the axle. Some current devices overcome the problem by using holes in the rotor plane, leading to increased hydrodynamic losses and shear stress. In this study, a simple apparatus was developed to overcome this problem. Guiding vanes were fixed to the rear housing wall. These vanes decrease the tangential velocity of the fluid and thus the centrifugal force, leading to an increased secondary flow toward the axle. The effect of such vanes was studied in videographic and ultrasound studies. An increase of washout and mixing between the flow layers could be demonstrated (stay time < 200 msec versus several seconds without vanes). In the first animal experiment using nonoptimized vanes, there was no thrombus at the back plane or the seal, and only a small thrombus at the transition between axle and rotor. Hemolysis was slightly elevated (3.2 mg/dl versus 2.5 mg/dl in control experiments). In conclusion, it is highly likely that this simple system will improve the flow characteristics in centrifugal pumps.
Animals↗
Compliance and formation of distal anastomotic intimal hyperplasia in Dacron mesh tube constricted veins used as arterial bypass grafts.
Dilated and varicose veins constricted with a Dacron mesh tube were successfully used as arterial bypass grafts to avoid nonautogenous vascular prostheses. Mesh constriction has also been used to adapt the venous graft lumen to the diameters of grafted arteries. The influence of the external mesh on the wall elasticity of such venous grafts and the reactions of the host artery were not investigated. Elastic properties of mesh constricted autologous veins used as arterial grafts in femoropopliteal reconstructions, as well as consecutive formation of distal anastomotic intimal hyperplasia (DAIH), were investigated in this experiment. Twenty-four autologous venous grafts were implanted in 12 sheep. Grafts were left natural (Groups 1 and 3) or were constricted with an external Dacron mesh (Groups 2 and 4); their diameters were left unchanged (Groups 1 and 2) or were matched to the diameter of the host artery (Groups 3 and 4). Wall elasticity of the graft, distal anastomosis, and distal artery were measured by locally applied crystal transducers during surgery and follow-up. Formation and localization of DAIH was evaluated histomorphologically after a median of 8.3 months. Graft wall elasticity was found to be lower (54.6 vs. 147.9, P = 0.006) and overall DAIH was found to be higher in mesh tube grafts (49.42 vs. 20.8 microns, P = 0.001, Mann-Whitney U-test). No differences in elasticity and DAIH formation were observed between grafts with adapted and large diameters. Constriction of venous grafts by a Dacron mesh tube reduces graft wall elasticity and promotes formation of DAIH. To avoid such an increased mismatch in compliance while making use of the advantages of this method, the external mesh tube must not be brought close to the distal anastomotic area itself.
Anastomosis, Surgical↗
Recent advances in the gyro centrifugal ventricular assist device.
The gyro pump was developed as an intermediate-term assist pump (C1E3) as well as a long-term centrifugal ventricular assist device (VAD). The antithrombogenic design concept of this pump was confirmed throughout three 1 month ex vivo studies. The normalized index of hemolysis (NIH) of this gyro C1E3 model was lower than that of the BP-80. In the next step, a miniaturized centrifugal blood pump (The Gyro permanently implantable model PI-601) has been developed for use as a permanently implantable device after design optimization. A special motor design of the magnet circuit was utilized in this system in collaboration with the University of Vienna. The priming volume of this pump is 20 ml. The overall size of the pump actuator package is 53 mm in height, 65 mm in diameter, 145 ml of displacement volume, and 305 g in weight. This pump can provide 5 L/min against 120 mm Hg total pressure head at 2,000 rpm. The NIH value of this pump was comparable to that of the BP-80. The gyro PI-601 model is suitable for a VAD. The expected life from the endurance study is approximately 8 years. The evolution from C1E3 to the PI-601 converts this pump to a totally implantable centrifugal pump. Recent technologic advances in continuous flow devices are likely to realize a miniaturized and economical totally implantable VAD.
Animals↗
Current progress in the development of a totally implantable Gyro centrifugal artificial heart.
A totally implantable centrifugal artificial heart has been developed using a miniaturized pivot bearing supported centrifugal pump (Gyro PI pump). The authors report current progress in its development. The Gyro PI-601 has a priming volume of 20 ml, weighs 100 g, has a height of 60 mm, and has a diameter of 65 mm. This pump can provide 8 L/min against 150 mmHg at 2,250 rpm. It is driven by an miniaturized DC brushless motor with the coils fixed in a plastic mold that is waterproof and made of titanium (weight, 204 g; height, 18 mm; diameter, 65 mm). In this centrifugal artificial heart, two Gyro PI pumps are implanted independently to replace cardiac function without resecting the native heart. Its anatomic and surgical feasibility were confirmed experimentally. The Gyro PI-601 was implanted as a right or left ventricular assist device in the preperitoneal space of five calves. All five tests proceeded without any thromboembolic symptoms. One of five tests was extended more than 1 month to confirm the long-term feasibility of the Gyro PI-601 pump system. Based on the satisfactory results of the in vivo tests, the material conversion of the Gyro PI from polycarbonate to titanium alloy (Ti-6A1-4V) was undertaken to improve its biocompatibility for long-term implantation.
Alloys↗
Long-term in vivo left ventricular assist device study with a titanium centrifugal pump.
A totally implantable centrifugal artificial heart has been developed. The plastic prototype, Gyro PI 601, passed 2 day hemodynamic tests as a functional total artificial heart, 2 week screening tests for antithrombogenicity, and 1 month system feasibility. Based on these results, a metallic prototype, Gyro PI 702, was subjected to in vivo left ventricular assist device (LVAD) studies. The pump system employed the Gyro PI 702, which has the same inner dimensions and the same characteristics as the Gyro PI 601, including an eccentric inlet port, a double pivot bearing system, and a magnet coupling system. The PI 702 is driven with the Vienna DC brushless motor actuator. For the in vivo LVAD study, the pump actuator package was implanted in the preperitoneal space in two calves, from the left ventricular apex to the descending aorta. Case 1 achieved greater than 9 month survival without any complications, at an average flow rate of 6.6 L/min with 10.2 W input power. Case 2 was killed early due to the excessive growth of the calf, which caused functional obstruction of the inlet port. There was no blood clot inside the pump. During these periods, neither case exhibited any physiologic abnormalities. The PI 702 pump gives excellent results as a long-term implantable LVAD.
Equipment Design↗
Development of an implantable small right ventricular assist device.
Currently, at least two permanent implantable left ventricular assist devices (LVADs) are used clinically. Unfortunately, there is no small implantable right ventricular assist device (RVAD) available, even though at least 25-30% of this patient population has right ventricular failure. If a small implantable RVAD were available, biventricular assist could support patients with right ventricular failure. A small atraumatic and antithrombogenic RVAD is being developed to meet this clinical need. This small centrifugal blood pump, the Gyro PI pump, is 6.5 cm in diameter and 4.6 cm in height and has three unique characteristics to prevent thrombus formation: (1) the double pivot bearing and magnetic coupling system enable this pump to be completely sealless; (2) the secondary vanes at the bottom of the impeller accelerate the blood flow and prevent blood stagnation; and (3) the eccentric inlet port enables the top female bearing to be embedded into the top housing and decrease blood cell trauma. The inflow conduit consists of a wire reinforced tube and a hat-shaped tip that is biolized with gelatin to create a thrombus resistant material. This conduit is directly implanted into the right ventricle, and the outflow conduit is anastomosed to the PA. The pump can be implanted inside the abdominal wall or in the thoracic cavity. Biocompatibility of this pump was proved in two calves by thrombus free implantation as an LVAD for 284 days and 200 days. Two RVAD implantations were conducted, aiming for 1-month system feasibility studies. During the month, the RVADs operated satisfactorily without any thromboembolic incident. No blood clots or abnormal findings were seen inside the pump, nor were there abnormal findings in the explanted lungs except for small areas of atelectasis. The pump flow was 3.02 +/- 0.38 L/min in calf 1 and 3.75 +/- 1.18 L/min in calf 2. The power requirement was 7.28 +/- 0.43W for calf 1 and 14.52 +/- 3.93W for calf 2. The PaO2 was 72.0 +/- 3.60 mm Hg (calf 1) and 72.0 +/- 7.63 mm Hg (calf 2); PaCO2 was 38.3 +/- 2.17 mm Hg (calf 1) and 34.1 +/- 1.95 mm Hg (calf 2); and SaO2 was 94.1 +/- 1.37% (calf1) and 95.0 +/- 1.95% (calf 2). Gas exchange via the lungs was maintained. These studies indicate that the Gyro PI pump is suitable as a single implantable RVAD, and is a feasible RVAD as a part of a BiVAD system in terms of pump performance and thrombus resistance.
Animals↗
Influence of centrifugal blood pumps on the elasticity of erythrocytes.
The influence of centrifugal blood pumps on the elasticity of erythrocytes was tested in an in vitro set-up. Fresh bovine blood was pumped by vaneless and impeller blood pumps with low and high hemolytic potential (1L priming volume, flow 5 L/min versus 150 mmHg, pumping time 6 hours). The elasticity of the red blood cells was measured by laser diffraction. Starting with an elasticity of 238 +/- 39, the overall change during 6 hours was 12 +/- 38 compared to a change of the control value of -13 +/- -58. Even a pump with very high hemolytic potential (Hemolysis index 21.9) did not cause relevant changes of elasticity. It is concluded that 1) elasticity is not a useful parameter to use in the evaluation of centrifugal pumps, and 2) the mechanical trauma caused by centrifugal pumps produces no relevant permanent alteration of the elastic properties of red blood cells, at least under in-vitro conditions.
Animals↗
Computer simulation of the circulatory system during support with a rotary blood pump.
In order to investigate the performance characteristics of nonpulsatile centrifugal pumps and their interaction with the pulsating heart, a computer simulation model was developed. A model of the pump, including the cannulae and the interference of the cannula and the left atrial wall, was added to a simulator of the circulation. It was implemented on a PC-AT using the analog simulating software AGO1000. An interval of 1 msec real-time per calculation step was evaluated to provide stable results. The calculation speed was approximately 100 msec per step. First simulation studies showed that both the positioning and resistance of the cannulae exert a major influence on system performance and equalized the different pressure-flow characteristics of vaneless and impeller pumps (difference of dp/dQ decreases from more than 6 times without cannulae to 130% with cannulae). In addition, the effects of suction in the atrial cannula, and changes of ventricular performance during changes in pumping power, could be properly simulated. The comparison with animal tests proved that the model is a suitable tool for testing control algorithms for centrifugal pumps.
Algorithms↗