P. Litwak

An intraventricular axial flow blood pump integrated with a bearing purge system

The future development of implantable axial flow blood pumps must address two major issues: mechanically induced hemolysis and shaft seal reliability. The recent revisions to our miniature intraventricular axial flow left ventricular assist device (LVAD) were aimed particularly at addressing these concerns. To improve hemocompatibility, a new impeller has been designed according to the following criteria: 1) gradual pressure rise along the blade chord; 2) minimized local fluid acceleration to prevent cavitation; 3) minimum surface roughness; and 4) radius edges. Subsequent in vitro hemolysis tests conducted with bovine and ovine blood have demonstrated very low hemolysis (normalized index of hemolysis = 0.0051 +/- 0.0047 g/100 L) with this new impeller design. To address the need for a reliable seal, we have developed a purged seal system consisting of a miniature lip seal and ceramic pressure groove journal bearing that also acts as a purge pump. Several spiral grooves formed on the bearing surface provide viscous pumping of the purge fluid, generating more than 3,000 mmHg at 10,000 rpm. This purge flow flushes the lip seal and prevents blood backflow into the bearing. We have found this purge pump to offer several advantages because it is simple, compact, durable, does not require separate actuation, and offers a wide range of flow, depending upon the groove design. In vivo animal tests demonstrated the potential of the purged seal system.

Progress on development of the Nimbus-University of Pittsburgh axial flow left ventricular assist system

Nimbus Inc. (Rancho Cordova, CA) and the University of Pittsburgh have completed the second year of development of a totally implanted axial flow blood pump under the National Institutes of Health Innovative Ventricular Assist System Program. The focus this year has been on completing pump hydraulic development and addressing the development of the other key system components. Having demonstrated satisfactory pump hydraulic and biocompatibility performance, pump development has focused on design features that improve pump manufacturability. A controller featuring full redundancy has been designed and is in the breadboard test phase. Initial printed circuit layout of this circuit has shown it to be appropriately sized at 5 x 6 cm to be compatible with implantation. A completely implantable system requires the use of a transcutaneous energy transformer system (TETS) and a diagnostic telemetry system. The TETS power circuitry has been redesigned incorporating an improved, more reliable operating topography. A telemetry circuit is undergoing characterization testing. Closed loop speed control algorithms are being tested in vitro and in vivo with good success. Eleven in vivo tests were conducted with durations from 1 to 195 days. Endurance pumps have passed the 6 month interval with minimal bearing wear. All aspects of the program continue to function under formal quality assurance.

Effect of Perfluorochemical Emulsion on Hemorheology and Shear Induced Blood Trauma

Blood trauma has been recognized as one of the key problems associated with assisted circulation. Indeed, the main requirement for improved heart-assist devices is the reduction of blood cell damage. The extremely high shear forces and prolonged contact between blood and foreign surfaces can cause mechanical destruction of erythrocytes (hemolysis), activation of platelets, changes in mechanical properties of erythrocytes1 and thus reduction of oxygen delivery. Even low level of hemolysis, in turn, drastically increases RBC aggregation at low shear conditions2. Additionally, plasma free hemoglobin can have a toxic effect on the cardiovascular system, probably because of hemoglobin vasoactivity, mediated by its property to bind nitric oxide (NO), an endothelium-derived relaxing factor3. Alternatively, NO might be destroyed by O2 radicals formed in the presence of hemoglobin3.

DISTAL THORACIC AORTA HEMODYNAMICS DURING EXERCISE WITH CONTINUOUS FLOW LEFT VENTRICULAR ASSIST SYSTEM

OBJECTIVES Continuous flow left ventricular assist systems (LVAS) are being discussed as a destination therapy. LVAS patients will have expanded activity of daily life, including exercise. In this study, we analyzed the effects of exercise on blood flow in the distal thoracic aorta of LVAD implanted animals. METHODS Five calves with a continuous flow LVAS exercised on treadmill at two different pump flow rates (PFR), 60-80% (high PFR) and 25-30% (low PFR) of pulmonary artery flow rate. Pump, pulmonary artery and descending thoracic aorta flow waves were recorded before, during and after exercise. Systolic and diastolic flow volume in each cardiac cycle in pump and descending thoracic aorta flow was calculated. RESULTS (1) Average flow rates - Pulmonary artery and descending thoracic aorta flow rates increased with heart rate during exercise and there was no difference between groups. (2) Pump flow wave - Pump regurgitation increased temporally during exercise at both PFRs, but sustained incidences of regurgitation after exercise were only observed at low PFR. Systolic and diastolic pump flow volume decreased during exercise at both PFRs, but systolic volume increased and diastolic volume decreased significantly after exercise at low PFR. (3) Descending thoracic aorta flow wave - At high PFR, systolic volume of descending thoracic aorta increased but diastolic flow volume decreased during exercise. At low PFR, both systolic and diastolic volume of the descending thoracic aorta decreased during exercise, but systolic volume increased and diastolic volume decreased after exercise. Systolic volume of the descending thoracic aorta in low PFR was significantly greater and diastolic volume was less than those in high PFR during and after exercise. CONCLUSION Exercise temporarily increases pump regurgitation with continuous flow LVAS support. Average flow rate of the descending thoracic aorta was maintained by compensation from increased heart rate, although the diastolic flow of the descending thoracic aorta decreased after exercise at the lower pump flow rate. Further study will be needed to evaluate whether or not this flow decrease causes hemodynamic and/or an oxygen delivery mismatch to peripheral tissue.

Low Hemolytic Intraventricular Axial Flow Blood Pump Integrated with Totally Implantable Bearing Purge System

Future development of implantable axial flow blood pumps must address two major issues: mechanically induced hemolysis and shaft seal reliability. Recent revisions of the design of our miniature intraventricular axial flow left ventricular assist device (LVAD) were aimed particularly toward addressing these concerns. To improve hemocompatibility, a new impeller (13.5-mm diameter) has been designed according to the following criteria: (1) gradual pressure rise along the blade chord, (2) minimized local fluid acceleration to prevent cavitation, (3) minimum surface roughness, and (4) radiused edges. Subsequent in-vitro hemolysis tests conducted with bovine and ovine blood have demonstrated very low hemolysis (normalized index of hemolysis, 0.005 ± 0.002 g/1001) with this new impeller design. These studies were conducted for 4h at 37°C, with an impeller speed maintained between 10000 and 11 000 rpm, providing a flow rate of 4–51/min against a 90–100 mmHg afterload. To address the need for a reliable seal system, we have recently developed an implantable purge system consisting of a miniature lip seal and a ceramic pressure-groove journal bearing (7 × 10mm). Several spiral grooves formed on the bearing surface provide viscous pumping of the purge fluid, generating over 3000 mmHg at 10000 rpm. This purge flow flushes the lip seal and prevents blood back-flow into the bearing. We have found this purge pump to offer several advantages, since it is simple, compact, durable, and does not require separate actuation. These recent developments with our device provide additional promise towards realizing a totally implantable purged axial flow LVAD.