In Vivo Evaluation of a Physiologic Control System for Rotary Blood Pumps Based on the Left Ventricular Pressure-Volume Loop.

Current generation continuous flow assist devices to operate at a fixed speed, which limits preload response and exercise capacity in left ventricular assist device (LVAD) patients. A feedback control system was developed to automatically adjust pump speed based on direct measurements of ventricular loading using a custom cannula tip with an integrated pressure sensor and volume-sensing conductance electrodes. The input to the control system is the integral of the left ventricular (LV) pressure versus conductance loop (PGA) over each cardiac cycle. The feedback control system adjusts pump speed based on the difference between the measured PGA and the desired PGA. The control system and cannula tip were tested in acute ovine studies (n = 5) using the HeartMate II LVAD. The preload response of the control system was evaluated by partially occluding and releasing the inferior vena cava using a vessel loop snare. The cannula tip was integrated onto a custom centrifugal flow LVAD and tested in a 14-day bovine study. The control system adjusted pump support to maintain constant ventricular loading: pump speed increased (decreased) following an increase (decrease) in preload. This study demonstrated in vivo the Starling-like response of an automatic pump control system based on direct measurements of LV loading.

Miniaturized Fontan Circulation Assist Device: Chronic In Vivo Evaluation.

We have miniaturized and optimized our implantable rotary blood pump developed to provide long-term mechanical right heart support for patients who have failing Fontan circulation. The objective of this study was to evaluate the miniaturized Fontan circulation assist device (mini-FCAD) during 30-day sheep studies (n = 5). A complete right heart bypass was performed and all return flow was supported by the pump. Postoperatively, unfractionated heparin was given to maintain thromboelastography R times of 2× normal. The first two studies were terminated on day 0 and day 4 due to complications. In the final three studies, the animals remained healthy and were electively terminated at 30 ± 2 days. Pump flow was between 5 and 7 lpm, left atrial pressure remained normal, and inlet pressures were between 3 and 18 mm Hg with no incidents of suction. There was no evidence of hemolysis, end organ or pulmonary dysfunction, thromboembolic events, nor thermal damage to the surrounding tissue. Explanted devices from two studies were free of thrombi and in the third study there were unattached thrombi on the SVC inlet of the rotor. The mini-FCAD was successfully tested in vivo as a right heart replacement device demonstrating adequate circulatory support and normal physiologic pulmonary and venous pressures.

Cannula Tip With Integrated Volume Sensor for Rotary Blood Pump Control: Early-Stage Development.

The lack of direct measurement of left ventricular unloading is a significant impediment to the development of an automatic speed control system for continuous-flow left ventricular assist devices (cf-LVADs). We have developed an inlet cannula tip for cf-LVADs with integrated electrodes for volume sensing based on conductance. Four platinum-iridium ring electrodes were installed into grooves on a cannula body constructed from polyetheretherketone (PEEK). A sinusoidal current excitation waveform (250 µA pk-pk, 50 kHz) was applied across one pair of electrodes, and the conductance-dependent voltage was sensed across the second pair of electrodes. The conductance catheter was tested in an acute ovine model (n = 3) in conjunction with the HeartMate II rotary blood pump to provide circulatory support and unload the ventricle. Echocardiography was used to measure ventricular size during pump support for verification for the conductance measurements. The conductance measurements correlated linearly with the echocardiography dimension measurements more than the full range of pump support from minimum support to suction. This cannula tip will enable the development of automatic control systems to optimize pump support based on a real-time measurement of ventricular size.

Chronic In Vivo Test of a Right Heart Replacement Blood Pump for Failed Fontan Circulation.

An implantable rotary blood pump was developed to provide long-term mechanical right heart support for patients who have failing Fontan circulation. The objective of this study was to evaluate the pump in vivo in a 30 day sheep study. Pump speed was set at 3,900 rpm for the duration of the study, and pump power was between 4.3 and 4.6 W. The pump inlet pressures for the superior vena cava (SVC) and inferior vena cava (IVC) were 14 ± 15 and 11 ± 15 mm Hg, respectively, over the duration of the study. Hematocrit remained stable at 30% ± 4%. Partial thromboplastin time (PTT) steadily increased from 30 s preoperatively to a high of 59 s on postoperative day 20, while prothrombin time (PT) remained at 20 ± 2 s for the duration of the study. The implantation and postoperative recovery were successful, and the animal demonstrated normal physiologic pulmonary and venous pressures and cardiac output. On pump inspection, the IVC and SVC inlets were completely clear of any deposits, but there were small thrombi (approximately 0.5 mm diameter) between each of the three rotor blades and along 20% of the parting line of the two volute halves. A complete right heart bypass was performed, postoperative recovery was successful, and the pump demonstrated adequate circulatory support and normal physiologic pulmonary and venous pressures. This study was the first successful test of a right heart replacement device in a chronic animal study.

Rotary blood pump control using integrated inlet pressure sensor.

Due to improved reliability and reduced risk of thromboembolic events, continuous flow left ventricular assist devices are being used more commonly as a long term treatment for end-stage heart failure. As more and more patients with these devices are leaving the hospital, a reliable control system is needed that can adjust pump support in response to changes in physiologic demand. An inlet pressure sensor has been developed that can be integrated with existing assist devices. A control system has been designed to adjust pump speed based on peak-to-peak changes in inlet pressure. The inlet pressure sensor and control system have been tested with the HeartMate II axial flow blood pump using a mock circulatory loop and an active left ventricle model. The closed loop control system increased total systemic flow and reduced ventricular load following a change in preload as compared to fixed speed control. The increase in systemic flow occurred under all operating conditions, and maximum unloading occurred in the case of reduced ventricular contractility.

Development of an inlet pressure sensor for control in a left ventricular assist device.

A Tesla type continuous flow left ventricular assist device (VAD) has been designed by Penn State and Advanced Bionics, Inc. (ABI). When a continuous flow device is used, care must be taken to limit low pressures in the ventricle, which can produce an obstruction to the inlet cannula or trigger arrhythmias. Design of an inexpensive, semiconductor strain gauge inlet pressure sensor to detect suction has been completed. The research and design analysis included finite element modeling of the sensing region. Sensitivity, step-response, temperature dependence, and hysteresis tests have been performed on prototype units. All sensors were able to withstand the maximum expected strain of 82 microm/in at 500 mm Hg internal pressure. Average sensitivity was 0.52 +/- 0.24 microV/mm Hg with 0.5 V excitation (n = 5 units). Step-response time for a 0- to 90-mm Hg step change averaged 22 msec. Hysteresis was measured by applying and holding 75 mm Hg internal pressure for 4 hours, followed by a zero pressure measurement, and ranged from -15 to 4.1 mm Hg (n = 3 units). Offset drift varied between 180 and -140 mm Hg over a 4-week period (n = 2 units). Span temperature sensitivity ranged from 18 to -21 muV/ degrees C (n = 5 units). Gain temperature sensitivity ranged from -7.4 to 4.9 muV/ degrees C (n = 5 units). With the inherent drift, it is currently not possible to use the transducer to measure actual pressures, but it can easily be used to measure pressure changes throughout the cardiac cycle. This signal can then be used in the control system to avoid ventricular suction events.