In vitro controllability of the MagScrew total artificial heart system

The purpose of this study was to evaluate the in vitro responses to preload and afterload of our total artificial heart (TAH), the MagScrew TAH. The TAH consists of two blood pumps and a control logic, developed at the Cleveland Clinic, OH, and the MagScrew actuator and its electronic control system, developed by Foster-Miller Technologies, Inc., Albany, NY. Tests were performed on a mock circulatory loop, using water as a test fluid. Preload sensitivity of the Mag-Screw TAH demonstrated a Frank-Starling response to preload in automatic mode. A peak flow of 10 L/min was obtained, with a left atrial pressure of 13 mm Hg. The relationship between right atrial pressure and left atrial pressure was well balanced when tested with a left bronchial shunt flow of 5% and a range of pulmonary artery and aortic pressures. With respect to afterload response, the left pump showed a relatively low sensitivity, which allowed the pump to maintain perfusion over a wide range of aortic pressures. The right pump, on the other hand, was much more sensitive to pulmonary artery pressure, which provided a measure of protection against pulmonary congestion. The very effective physiologic response of the MagScrew TAH is believed to result from employment of a left master, alternating ejection control logic, high inherent sensitivity of the blood pumps to atrial pressure, a lower effective stroke volume for the right pump, and a scaling of right side motor ejection voltage to 80% of that used for the left side ejection.     

Ovalis TAH: development and in vitro testing of a new electromechanical energy converter for a total artificial heart

A new electromechanical energy converting system has been developed to yield an efficient and durable orthotopic total artificial heart (TAH). The energy converter we developed transforms the unidirectional rotational motion of the motor into a longitudinal forward-reverse movement of an internal geared oval, linked directly to pusher plates on both sides. To ensure a permanent positive connection between the drive gear and the internally geared wheel, a ball bearing runs inside an oval shaped guide track. Motor, gear unit, and conical pusher plates are seated between alternately ejecting and filling ventricles. The unidirectional motion of the brushless DC motor affords easier motor control, reduces energy demand, and ensures longer life of the motor when compared with a bidirectional motion system. In vitro testing has been performed on a mock circulation loop. The overall system efficiency of the TAH Ovalis was 27-39% (mean, 36%) for the pump output range of 2-7 L/min. The maximum output of 7 L/min can be obtained with a pump rate of 130 min(-1) and an afterload pressure of 140 mm Hg. For an average sized human with a mean cardiac output of 6 L/min at a mean aortic pressure of 120 mm Hg, 5 watts of input power would be required. The size of the prototype is 560 cm3, the weight is 950 g. Our first in vitro studies demonstrated the excellent efficiency and pump performance of this new electromechanical energy converter. The results prove the feasibility of this new concept's use as an energy converter for a total artificial heart.     

Estimation of the following cardiac output using sympathetic tone and hemodynamics for the control of a total artificial heart.

A sympathetic neurogram is potentially useful for the development of a real time total artificial heart (TAH) control system. We used sympathetic tone and hemodynamic derivatives to estimate the following cardiac output in acute animal experiments using adult mongrel dogs. Moving averages of the mean left atrial pressure and mean aortic pressure were used as parameters of the preload and afterload, respectively. Renal sympathetic nerve activity (RSNA) was employed as a parameter of sympathetic tone. Equations for the following cardiac output were calculated using multiple linear regression analysis of the time series data. A significant correlation was observed between the estimated and following measured cardiac output. These results suggest the potential usefulness of the sympathetic neurogram for the real time TAH automatic control system.     

Durability testing of a completely implantable electric total artificial heart

In vitro durability testing was conducted on the Penn State/3M electric total artificial heart (ETAH) to determine device durability and to evaluate device failures. A specialized mock circulatory loop was developed for this testing. Customized software continuously acquired data during the test period, and failures were analyzed using FMEA (failure modes and effects analysis) and FMECA (failure modes, effects, and criticality analysis) principles. Redesigns were implemented when appropriate. Reliability growth principles were then applied to calculate the 1 and 2 year reliability. The 1 and 2 year reliability of the Penn State/3M ETAH was shown to be 96.1% and 59.9%, respectively, at 80% confidence.     

Cardiac output requirements and maximum dimensions for a neonate total artificial heart.

Two equally important issues need to be addressed during the early stages of the design of an implantable total artificial heart (TAH): proper anatomical fit and cardiac output capacity. As part of a first-time feasibility study to develop a neonate-size TAH, two studies were conducted to establish useful anatomical and physiological standards. The first (Study A) was conducted to determine the maximum dimensions of a neonate-size TAH. Twelve preserved hearts from full-term neonates with the hypoplastic left heart syndrome were examined. A second study (Study B) was designed to determine the acceptable minimum stroke volume compatible with minimum neonate cardiac output requirements. This study was based on a combination of: a) reported cardiac output studies in healthy term neonates, and term neonates with heart failure, b) body weight range, and c) limiting factors of TAH technology, e.g., valvular regurgitation and leveling off of the maximum cardiac output value at a specific heart rate and filling pressure. The proposed neonatal standards for TAH technology are presented.     

轴流泵式全人工心脏的体外测试及对负荷反应特性

目的 在体外模拟循环台测试轴流泵式全人工心脏的基本负荷反应特性,为探索生理性控制方案提供基础。方法 轴流泵式全人工心脏样机采用2个轴流泵共同设置在刚性外壳中,直径65 mm,长度70 mm。于模拟循环台上串联连接组成全人工心脏的2个轴流泵,在外周动脉和肺动脉阻力不变的条件下观测前、后负荷变化对心脏输出量的影响。结果 在前负荷固定不变的条件下,增加后负荷时心脏输出流量逐步下降,增大泵转速可对抗后负荷对输出量的抑制,泵转速设定为右心泵8 500 r/min、左心泵11 000 r/min时,心脏输出压力为13.3 kPa（100 mmHg）和输出量6 L/min。当后负荷增大到26.7 kPa（200 mmHg）时心输出量下降为0 L/min。在后负荷固定不变的条件下,前负荷的增加不导致心脏输出量明显改变。设定左心泵转速为11 000 r/min、右心泵转速8 500 r/min时前负荷由0.27 kPa（2 mmHg）增加到1.87 kPa（14 mmHg）,流量基本维持在7 L/min。结论 轴流泵式全人工心脏对后负荷增加表现出明显的流量抑制趋势,此趋势可通过调节泵转速改善。轴流泵式全人工心脏对前负荷反应不明显,有别于自然心脏,其机制及调节意义尚待进一步研究。     

A design of an artificial control system of the cardiac output

Summary An automatic control system of the cardiac output of a dog was designed by a method where a cardiac vagus and the heart were involved in the feedback system. The cardiac output was measured at the ascending aorta by a sine wave electromagnetic flowmeter. The output voltage of the flowmeter was compared with a controllable reference voltage. The error voltage was amplified or integrated, and converted to pulses of the frequencies which was proportional to the input of the AD convertor. A peripheral cut end of a vagus was stimulated by an electric stimulator triggered by those pulses. Vagal stimuli decrease the cardiac output by their negative chronotropic and inotropic action. The error approximated zero when the feedback loop gain was more than about 10 or an integral control was used. Complete stability could not be achieved with a zero error and an unstable oscillation of the smoothed cardiac output signal with a period of several seconds was observed. However, the instability was slight and did not disturb its application. Effects of disturbances such as change of venous return and peripheral resistance were sufficiently reduced.     

In vivo studies of the MagScrew total artificial heart in calves

The purpose of this study was to evaluate the in vivo pump performance of our total artificial heart (TAH), the "MagScrew TAH." The TAH consists of a blood pump and control logic developed at the Cleveland Clinic and the MagScrew actuator and electronic control system developed by Foster-Miller Technologies, Inc. (Albany, NY). MagScrew TAH implantation was performed in two calves. Study durations were 50 and 5 days. The causes of termination were prosthetic valve endocarditis in one case and cable failure in the other. Mean left pump flow ranged from 8.0 to 9.7 L/min, with left atrial pressure of 3.0 to 16.0 mm Hg. Preload sensitivity of the MagScrew TAH demonstrated a Frank-Starling response to preload in automatic mode. The relationship between right and left atrial pressure was well balanced. Mean arterial pressure and mean pulmonary artery pressure were maintained within physiologic ranges over study duration. There were no signs of bleeding, hemolysis, or organ failure. The MagScrew TAH showed physiologic pump performance, and hemodynamics were well maintained without any organ failure. Further development testing will bring the MagScrew TAH to the point of preclinical readiness testing.     

Development and in vitro validation of a device for measuring non-shunt cardiac output by nitrous oxide throughflow

A system has been developed for measuring non-shunt cardiac output by the throughflow technique, using nitrous oxide in patients undergoing general anaesthesia. The throughflow measurement technique is a non-invasive method based on inert gas throughflow theory. In vitro validation of the measurement system was performed using a lung gas exchange simulator. The accuracy and precision of the throughflow measurement system was assessed by comparing measured and target values for five simulated values of non-shunt cardiac output, from 2.88 to 9.86 l min⁻¹. This showed an overall mean bias of −0.03l min⁻¹ (range −0.00 to −0.10 l min⁻¹), with a mean coefficient of variation of the difference of 1.39% (1.20–1.93%). These results indicate that the measurement system is suitable for monitoring the non-shunt cardiac output in patients undergoing general anaesthesia using nitrous oxide throughflow.     

Transient response of remnant atrial heart rate to step changes in total artificial heart output

In a total artificial heart (TAH) recipient, the remnant atrial rate reflects the control signal on cardiac function from the recipient's central nervous system via both the neural and the humoral pathways. Therefore, it can provide useful information for physiologic control of a TAH. We investigated step responses in the remnant atrial rate to step changes in TAH output in a calf implanted with a pneumatic TAH and characterized the dynamic property of the open-loop native cardiac output control system. The pumping rate was changed suddenly from 90 to 60 bpm, and in the next stage from 60 to 90 bpm. This process was repeated three times. Along with the step changes in TAH pumping rate, TAH output also changed at once in a similar fashion. The atrial rate altered immediately and converged after an overshoot. These changes were regarded as second-order delay responses, and the dynamic properties were estimated. When the pumping rate was decreased from 90 to 60 bpm and when it was increased from 60 to 90 bpm, the rise times were 70 ± 1 and 132 ± 38 s, the peak times were 127 ± 15 and 179 ± 15 s, the maximum overshoots were 30 ± 4% and 12 ± 7%, and the settling times were 232 ± 11 and 245 ± 10 s, respectively. We conclude that the obtained open-loop characteristics and dynamic properties can be useful indexes for physiologic control of TAH output, which reflects the properties of the native central nervous control system. Received: October 25, 2000 / Accepted: May 22, 2001     

Computerized hydrodynamic ″flow″ system for artificial heart valve testing

Scientific-Research Institute of Transplantation and Artificial Organs, Ministry of Health of the USSR, Moscow. Translated from Meditsinskaya Tekhnika, No. 6, pp. 30–32, November–December, 1990.     

Comparison of two impedance cardiographic techniques for measuring cardiac output

The purpose of the present study was to compare cardiac outputs obtained by both the Kubicek (MIC) and Sramek (NCCOM3) impedance cardiographic techniques with thermodilution (TD) in critically ill patients. The two impedance techniques were also compared in normal subjects. Seven healthy subjects and ten patients in the intensive care unit were enlisted in the study. Only those subjects with successful measurements by all three methods were used in the data analysis. Three measurements of cardiac output were made in each subject. In patients, there were no significant differences in cardiac outputs as measured by TD (6.61/min), MIC (6.3 1/min), NCCOM3 (6.4 1/min). MIC and NCCOM3 cardiac outputs were correlated and approximated the line of identify when compared to TD. In normals, however, the NCCOM3 overestimated the cardiac output (NCCOM3, 9.2 1/min; MIC, 6.2 1/min). Because of these inconsistent results, caution is urged when interpreting the values obtained by the NCCOM3. In contrast, the use of the MIC in both populations has been reaffirmed. Presented at The Symposium “Impedance Techniques in Biological Systems,” 72nd Annual Meeting FASEB, May 1–5, 1988, Las Vegas, Nevada. Supported by The Miami Valley Heart Chapter of The American Heart Association.     

Closing behavior of the mechanical heart valve in a total artificial heart

Recently, cavitation on the surface of mechanical heart valves has been studied as a cause of fractures occurring in implanted mechanical heart valves. The cause of cavitation in mechanical heart valve was investigated in both 25-mm Björk–Shiley and 25-mm Medtronic Hall valves. The closing events of these valves in the mitral position were simulated in an electrohydraulic total artificial heart with a stroke volume of 85?ml. The tests were conducted under physiologic pressures at heart rates of 60, 70, 80, and 90 beats/min with cardiac outputs of 4.5, 5.5, 6.4, and 7.5?l/min, respectively. The disk closing behavior was measured by a laser displacement sensor. The closing behaviors were investigated under various atrial and aortic pressures. In both valves, the duration of closing decreased with an increase in the cardiac output. The greater the amount of atrial pressure, the shorter the closing duration of both valves. The maximum closing velocity of the Medtronic Hall monostrut valve ranged from 0.8 to 0.9?m/s, and that of the Björk–Shiley monostrut valve ranged from 0.73 to 0.78?m/s. In both valves, the maximum closing velocities were less than the reported cavitation thresholds. This suggests that there should be no possibility of occurrence of cavitation in an electrohydraulic total artificial heart with mechanical heart valve.     

Operating point control system for a continuous flow artificial heart: in vitro study

We proposed and developed a practical and effective servo control system for rotary blood pumps. A rotary blood pump for assisting the failing natural heart should be operated only in physiologically acceptable conditions. The operation of a rotary blood pump is based on the rotational speed of the impeller and pressure head. If the pump flow and the pressure head are set within an acceptable range, the driving condition is deemed normal condition, and this control system maintains the preset operating point by applying proportional and detective control (PD control). If the pump flow or pressure head is outside the acceptable range, the driving condition is determined to be abnormal condition, and this system operates the pump in a recovery fashion. If the driving condition is kept under abnormal conditions of sudden decrease of the flow, the condition is termed a suction condition. The controller releases the pump from the suction condition and later returns it to the normal condition. In this study, we evaluated these servo control modes of the centrifugal pump and confirmed whether the performance of this proposed operating point control system was practical.     

In vitro fatigue testing of prosthetic ligaments: a new concept.

A cruciate ligament fatigue/wear testing system TTF600 was developed at Ecole Poly-technique in order to reproduce a little better the loading conditions and environment in which they perform. This machine was designed to subject the ligaments to simultaneous tension, bending and torsion. The amplitude of each loading can be adjusted separately. This allows to better define the effect of each loading mode on the viability of a prosthetic device. Six devices can be tested simultaneously in a liquid environment at 37 degrees C. The loading conditions and the behaviour of the prosthesis are monitored by computer.     

Clinical use of the total artificial heart

We report here our first experience with the use of a total artificial heart in a human being. The heart was developed at the University of Utah, and the patient was a 61-year-old man with chronic congestive heart failure due to primary cardiomyopathy, who also had chronic obstructive pulmonary disease. Except for dysfunction of the prosthetic mitral valve, which required replacement of the left-heart prosthesis on the 13th postoperative day, the artificial heart functioned well for the entire postoperative course of 112 days. The mean blood pressure was 84 +/- 8 mm Hg, and cardiac output was generally maintained at 6.7 +/- 0.8 liters per minute for the right heart and 7.5 +/- 0.8 for the left, resulting in postoperative diuresis and relief of congestive failure. The postoperative course was complicated by recurrent pulmonary insufficiency, several episodes of acute renal failure, episodes of fever of unidentified cause (necessitating multiple courses of antibiotics), hemorrhagic complications of anticoagulation, and one generalized seizure of uncertain cause. On the 92nd postoperative day, the patient had diarrhea and vomiting, leading to aspiration pneumonia and sepsis. Death occurred on the 112th day, preceded by progressive renal failure and refractory hypotension, despite maintenance of cardiac output. Autopsy revealed extensive pseudomembranous colitis, acute tubular necrosis, peritoneal and pleural effusion, centrilobular emphysema, and chronic bronchitis with fibrosis and bronchiectasis. The artificial heart system was intact and uninvolved by thrombosis or infectious processes. This experience should encourage further clinical trials with the artificial heart, but we emphasize that the procedure is still highly experimental. Further experience, development, and discussion will be required before more general application of the device can be recommended.     

Portable transportable pneumatic drive for an artificial heart with an independent electrochemical energy source

Scientific-Research Institute of Transplantology and Artificial Organs, Ministry of Public Health, USSR. Translated from Meditsinskaya Technika, No. 4, pp. 37–40, July–August, 1990.