Douglas C. Thomas

A sensorless approach to control of a turbodynamic left ventricular assist system

A fuzzy logic controller for a rotary, turbodynamic left ventricular assist system was developed to optimize the delivery of blood flow without inducing suction in the ventricle. The controller is based on the pulsatility in blood flow through the pump and assumes that the natural heart is still able to produce some pumping action. To avoid the use of flow transducers, which are not reliable for long term use, the controller estimates flow using a model of the assist device. The controller was tested in computer simulation, a mock circulatory system, and in animal experiments. Simulation studies suggest that the fuzzy logic controller is more robust to parameter changes than a traditional proportional-integral controller. Experimental results in animals showed that the controller is able to provide satisfactory flows at adequate perfusion pressures while avoiding suction in the left ventricle.

Modeling and identification of an axial flow blood pump

This paper presents a model for identification of an axial pump. It aims to provide flow and pressure difference estimates of the axial pump as well as the parameters of the pump characteristics without using flow and pressure sensors.

Induction of ventricular collapse by an axial flow blood pump.

An important consideration for clinical application of rotary blood pump based ventricular assist is the avoidance of ventricular collapse due to excessive operating speed. Because healthy animals do not typically demonstrate this phenomenon, it is difficult to evaluate control algorithms for avoiding suction in vivo. An acute hemodynamic study was thus conducted to determine the conditions under which suction could be induced. A 70 kg calf was implanted with an axial flow assist device (Nimbus/UoP IVAS; Nimbus Inc., Rancho Cordova, CA) cannulated from the left ventricular apex to ascending aorta. On initiation of pump operation, several vasoactive interventions were performed to alter preload, afterload, and contractility of the left ventricle. Initially, dobutamine increased contractility and heart rate ([HR] = 139; baseline = 70), but ventricular collapse was not achievable, even at the maximal pump speed of 15,000 rpm. Norepinephrine decreased HR (HR = 60), increased contractility, and increased systemic vascular resistance ([SVR] = 24; baseline = 15), resulting in ventricular collapse at a pump speed of 14,000 rpm. Isoproterenol (beta agonist) increased HR (HR = 103) and decreased SVR (SVR = 12), but ventricular collapse was not achieved. Inferior vena cava occlusion reduced preload, and ventricular collapse was achieved at speeds as low as 11,000 rpm. Esmolol (beta1 antagonist) decreased HR (HR = 55) and contractility, and ventricular collapse was achieved at 11,500 rpm. Episodes of ventricular collapse were characterized initially by the pump output exceeding the venous return and the aortic valve remaining closed throughout the cardiac cycle. If continued, the mitral valve would remain open throughout the cardiac cycle. Using these unique states of the mitral and aortic valves, the onset of ventricular collapse could reliably be identified. It is hoped that the ability to detect the onset of ventricular collapse, rather than the event itself, will assist in the development and the evaluation of control algorithms for rotary ventricular assist devices.

Continued development of the Nimbus/University of Pittsburgh (UOP) axial flow left ventricular assist system.

Nimbus and the University of Pittsburgh (UOP) have continued the development of a totally implanted axial flow blood pump under the National Institutes of Health (NIH) Innovative Ventricular Assist System (IVAS) program. This 62 cc device has an overall length of 84 mm and an outer diameter of 34.5 mm. The inner diameter of the blood pump is 12 mm. It is being designed to be a totally implanted permanent device. A key achievement during the past year was the completion of the Model 2 pump design. Ten of these pumps have been fabricated and are being used to conduct in vitro and in vivo experiments to evaluate the performance of different materials and hydraulic components. Efforts for optimizing the closed loop speed control have continued using mathematical modeling, computer simulations, and in vitro and in vivo testing. New hydraulic blade designs have been tested using computational fluid dynamics (CFD) and flow visualization. A second generation motor was designed with improved efficiency. To support the new motor, a new motor controller fabricated as a surface mount PC board has been completed. The program is now operating under a formal QA system.

Progress in Cleveland Clinic-Nimbus total artificial heart development

A totally implantable, Cleveland Clinic-Nimbus total artificial heart (TAH) uses electrohydraulic energy conversion and an automatic left master-alternate mode control scheme, with a filling sensitivity of 1.0 l/min/mmHg and a maximum output of 9.5 l/min. The TAHs were tested in 12 calves for 1-120 days with normal major organ and blood cell function. Post-operative suppression of platelet aggregation recovered by the second post-operative week. The gelatin-coated pump surface generally was clean without any anticoagulants and free from infection. Embolism, which occurred in two cases, was caused by complications attributable to fungal infection in a Dacron graft and by thrombus formed around a jugular vein catheter. A system with a hybridized microcircuit controller in the interventricular space has been tested successfully in the three most recent cases, with a peak device surface temperature elevation of 6.5 degrees C. Heat effects were confined to the tissues immediately adjacent to the hottest spots. The carbon fiber-reinforced epoxy housing and 60 ml butyl rubber compliance chamber showed good tissue compatibility with a thin, fibrous tissue capsule. The transcutaneous energy transmission system and the internal battery functioned well as designed in the most recent animal implant.

Chronic animal health assessment during axial ventricular assistance: importance of hemorheologic parameters.

Chronic testing of the Nimbus/UOP Axial Flow Pump was performed on 22 calves for periods of implantation ranging from 27 to 226 days (average, 74 days). The following parameters were measured: plasma free hemoglobin, blood and plasma viscosity, erythrocyte deformability and mechanical fragility, oxygen delivery index (ODI), blood cell counts, hematocrit, hemoglobin, blood urea nitrogen, creatinine, bilirubin, total protein, fibrinogen, and plasma osmolality. Most of the above parameters were stable during the full course of support. Compared with baseline, statistically significant differences during the entire period of implantation were only found in: hematocrit (p<0.001), hemoglobin (p<0.005), red blood cell (RBC) count (p<0.001), and whole blood viscosity (p<0.01). Plasma viscosity and ODI were mostly stable during the period of implantation. In some animals, an acute increase in fibrinogen concentration, plasma and blood viscosity, and a decrease in ODI were found to be early signs of the onset of infection. A small (10%) decrease in deformability of RBCs was found during the first 2 weeks after implantation. This alteration in RBC deformability was highly correlated (r = 0.793) with changes in total plasma protein concentration that fell more than 15% (p<0.001) during the same period. Mechanical fragility of RBCs was found to be slightly increased after implantation. Plasma free hemoglobin remained close to baseline level (p>0.2). After the first 2 weeks of the postoperative period, pump performing parameters for all animals were consistent and stable. In general, the Nimbus/UOP Axial Flow Pump demonstrated basic reliability and biocompatibility and did not produce significant alterations in the mechanical properties of blood or animal health status. The pump provided adequate hemodynamics and was well tolerated by the experimental animal for periods as long as 7.5 months. Monitoring rheologic parameters of blood is very helpful for evaluation of health during heart-assist device application.

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.

Development and initial testing of a pediatric centrifugal blood pump

BACKGROUND We are developing a miniaturized centrifugal blood pump for use as a temporary cardiac assist device in neonatal and pediatric sized patients. This pump has a very low priming volume of 13 mL. A small motor stator has also been designed, which resulted in a device that can be placed very close to the patient, thereby minimizing overall circuit volume. METHODS Testing to date has included in vitro hemodynamic performance, in vitro hemolysis generation, and in vivo evaluation in 5 lambs weighing 5.5 to 21 kg. Two lambs underwent peripheral cannulation from external jugular vein to carotid artery, whereas 3 others were cannulated from left atrium to carotid artery. RESULTS In vitro data demonstrated pump capacity spanning 0.3 to 3.0 L/min and very low hemolysis generation at these conditions. In vivo, the pump functioned satisfactorily for periods up to 148 hours, and the bypass appeared to be well tolerated by the animals. Plasma free hemoglobin levels remained less than 25 mg/dL during all animal experiments. All devices were thrombus-free at explantation. CONCLUSIONS We conclude that this device has merit as an alternative to current oversized systems used for neonatal and pediatric cardiac assistance. In addition, a chronic neonatal lamb model in which to evaluate pediatric circulatory assist devices has been developed successfully.

Initial in vivo tests of an electrohydraulic actuated total artificial heart

The authors are involved in developing a total artificial heart (TAH) for permanent human use. This device was designed to fit human anatomy, and it has housings made of carbon fiber-epoxy composite and titanium. Tissue valves and protein coating of blood contacting surfaces minimize the need for anticoagulants. A continuously reciprocating electrohydraulic actuator is packaged between two alternately ejecting and passively filling ventricles. The control system varies the pump rate to maintain average left ventricular filling at 90%. This TAH in vivo successively progressed through 1, 5, 9, and 45 day implants in calves of 84, 94, 82, and 82 kg preoperative body weights. The operating modes include automatic and fixed rate. The chronic and acute effects of varying the right pump displaced stroke volume indicated the need for it to be limited to 85% of that of the left for stable hemodynamics at maximum flow. The pump exhibited afterload insensitive and preload sensitive performance. Pump output ranged from 4.0-9.5 L/min at left atrial pressures of 7-16 mmHg at pump rates of 80-160 beats/min in these four experiments. These data suggest that this device will meet clinical hemodynamic requirements; it has the potential for total implantable cardiac replacement.