Parameter Identification of Cardiovascular System Model Used for Left Ventricular Assist Device Algorithms.

Advancement of implanted left ventricular assist device (LVAD) technology includes modern sensing and control methods to enable online diagnostics and monitoring of patients using on-board sensors. These methods often rely on a cardiovascular system (CVS) model, the parameters of which must be identified for the specific patient. Some of these, such as the systemic vascular resistance (SVR), can be estimated online while others must be identified separately. This paper describes a three-staged approach for designing a parameter identification algorithm (PIA) for this problem. The approach is demonstrated using a two-element Windkessel model of the systemic circulation (SC) with a time-varying elastance for the left ventricle (LV). A parameter identifiability stage is followed by identification using an unscented Kalman filter (UKF), which uses measurements of LV pressure (Plv), aortic pressure (Pao), aortic flow (Qa), and known input measurement of LVAD flowrate (Qvad). Both simulation and experimental data from animal experiments were used to evaluate the presented methods. By bounding the initial guess for left ventricular volume, the identified CVS model is able to reproduce signals of Plv, Pao, and Qa within a normalized root mean squared error (nRMSE) of 5.1%, 19%, and 11%, respectively, during simulations. Experimentally, the identified model is able to estimate SVR with an accuracy of 3.4% compared with values from invasive measurements. Diagnostics and physiological control algorithms on-board modern LVADs could use CVS models other than those shown here, and the presented approach is easily adaptable to them. The methods also demonstrate how to test the robustness and accuracy of the identification algorithm.

Hybrid Mock Circulatory Loop Simulation of Extreme Cardiac Events.

OBJECTIVE: This paper presents preliminary methods of incorporating the pathological conditions of cardiac arrhythmias and valvular stenosis in hybrid mock circulation loop (hMCL) operation for the enhanced verification and validation of mechanical circulatory support devices such as VADs. METHODS: The MGH/MF Waveform datasets from PhysioNet database (including both nominal and clinically diagnosed arrhythmic ECG measurements) as well as cardiovascular system model updates are used to recreate arrhythmic events and valvular stenosis in vitro. RESULTS: Preliminary results show the hMCL can recreate each tested cardiac event within 2% and 4% mean error for reference pressure tracking in the aortic and left ventricular pressure chambers, respectively. Further, frequency spectrum analysis comparisons using the magnitude-squared coherence analysis shows close alignment between measured arrhythmic and hMCL realized pressure frequency content. CONCLUSION: The generation of cardiac arrhythmias and valvular stenosis around a VAD via both model and acute measurement based methods was achieved. SIGNIFICANCE: Pathological conditions such as cardiac arrhythmias and valvular stenosis are limited in documentation despite the large percentage of patients who experience these events. This paper provides a means to begin incorporating these events into hardware-in-the-loop mock circulatory systems for next generation VAD validation and verification.

Preload Sensitivity with TORVAD Counterpulse Support Prevents Suction and Overpumping.

PURPOSE: This study compares preload sensitivity of continuous flow (CF) VAD support to counterpulsation using the Windmill toroidal VAD (TORVAD). The TORVAD is a two-piston rotary pump that ejects 30 mL in early diastole, which increases cardiac output while preserving aortic valve flow. METHODS: Preload sensitivity was compared for CF vs. TORVAD counterpulse support using two lumped parameter models of the cardiovascular system: (1) an open-loop model of the systemic circulation was used to obtain ventricular function curves by isolating the systemic circulation and prescribing preload and afterload boundary conditions, and (2) a closed-loop model was used to test the physiological response to changes in pulmonary vascular resistance, systemic vascular resistance, heart rate, inotropic state, and blood volume. In the open-loop model, ventricular function curves (cardiac output vs left ventricular preload) are used to assess preload sensitivity. In the closed-loop model, left ventricular end systolic volume is used to assess the risk of left ventricular suction. RESULTS: At low preloads of 5 mmHg, CF support overpumps the circulation compared to TORVAD counterpulse support (cardiac output of 3.3 L/min for the healthy heart, 4.7 with CF support, and 3.5 with TORVAD counterpulse support) and has much less sensitivity than counterpulse support (0.342 L/min/mmHg for the healthy heart, 0.092 with CF support, and 0.306 with TORVAD counterpulse support). In the closed-loop model, when PVR is increased beyond 0.035 mmHg s/mL, CF support overpumps the circulation and causes ventricular suction events, but TORVAD counterpulse support maintains sufficient ventricular volume and does not cause suction. CONCLUSIONS: Counterpulse support with the TORVAD preserves aortic valve flow and provides physiological sensitivity across all preload conditions. This should prevent overpumping and minimize the risk of suction.

Scaling the Low-Shear Pulsatile TORVAD for Pediatric Heart Failure.

This article provides an overview of the design challenges associated with scaling the low-shear pulsatile TORVAD ventricular assist device (VAD) for treating pediatric heart failure. A cardiovascular system model was used to determine that a 15 ml stroke volume device with a maximum flow rate of 4 L/min can provide full support to pediatric patients with body surface areas between 0.6 and 1.5 m. Low-shear stress in the blood is preserved as the device is scaled down and remains at least two orders of magnitude less than continuous flow VADs. A new magnetic linkage coupling the rotor and piston has been optimized using a finite element model (FEM) resulting in increased heat transfer to the blood while reducing the overall size of TORVAD. Motor FEM has also been used to reduce motor size and improve motor efficiency and heat transfer. FEM analysis predicts no more than 1°C temperature rise on any blood or tissue contacting surface of the device. The iterative computational approach established provides a methodology for developing a TORVAD platform technology with various device sizes for supporting the circulation of infants to adults.

Preservation of native aortic valve flow and full hemodynamic support with the TORVAD using a computational model of the cardiovascular system.

This article describes the stroke volume selection and operational design for the toroidal ventricular assist device (TORVAD), a synchronous, positive-displacement ventricular assist device (VAD). A lumped parameter model was used to simulate hemodynamics with the TORVAD compared with those under continuous-flow VAD support. Results from the simulation demonstrated that a TORVAD with a 30 ml stroke volume ejecting with an early diastolic counterpulse provides comparable systemic support to the HeartMate II (HMII) (cardiac output 5.7 L/min up from 3.1 L/min in simulated heart failure). By taking the advantage of synchronous pulsatility, the TORVAD delivers full hemodynamic support with nearly half the VAD flow rate (2.7 L/min compared with 5.3 L/min for the HMII) by allowing the left ventricle to eject during systole and thus preserving native aortic valve flow (3.0 L/min compared with 0.4 L/min for the HMII, down from 3.1 L/min at baseline). The TORVAD also preserves pulse pressure (26.7 mm Hg compared with 12.8 mm Hg for the HMII, down from 29.1 mm Hg at baseline). Preservation of aortic valve flow with synchronous pulsatile support could reduce the high incidence of aortic insufficiency and valve cusp fusion reported in patients supported with continuous-flow VADs.

Verification of a computational cardiovascular system model comparing the hemodynamics of a continuous flow to a synchronous valveless pulsatile flow left ventricular assist device.

The purpose of this investigation is to use a computational model to compare a synchronized valveless pulsatile left ventricular assist device with continuous flow left ventricular assist devices at the same level of device flow, and to verify the model with in vivo porcine data. A dynamic system model of the human cardiovascular system was developed to simulate the support of a healthy or failing native heart from a continuous flow left ventricular assist device or a synchronous pulsatile valveless dual-piston positive displacement pump. These results were compared with measurements made during in vivo porcine experiments. Results from the simulation model and from the in vivo counterpart show that the pulsatile pump provides higher cardiac output, left ventricular unloading, cardiac pulsatility, and aortic valve flow as compared with the continuous flow model at the same level of support. The dynamic system model developed for this investigation can effectively simulate human cardiovascular support by a synchronous pulsatile or continuous flow ventricular assist device.

Improved left ventricular unloading and circulatory support with synchronized pulsatile left ventricular assistance compared with continuous-flow left ventricular assistance in an acute porcine left ventricular failure model.

OBJECTIVE: Controversy exists regarding the optimal pumping method for left ventricular assist devices. The purpose of this investigation was to test the hypothesis that pulsatile left ventricular assist synchronized to the cardiac cycle provides superior left ventricular unloading and circulatory support compared with continuous-flow left ventricular assist devices at the same level of ventricular assist device flow. METHODS: Seven male pigs were used to evaluate left ventricular assist device function using the TORVAD synchronized pulsatile-flow pump (Windmill Cardiovascular Systems, Inc, Austin, Tex) compared with the Bio-Medicus BPX-80 continuous-flow centrifugal pump (Medtronic, Inc, Minneapolis, Minn). Experiments were carried out under general anesthesia, and animals were instrumented via a median sternotomy. Hemodynamic measurements were obtained in the control state and with left ventricular assistance using the TORVAD and BPX-80 individually. Left ventricular failure was induced with suture ligation of the mid-left anterior descending coronary artery, and hemodynamic measurements were repeated. RESULTS: During left ventricular assist device support, mean aortic pressure and total cardiac output were higher and left atrial pressure was lower with pulsatile compared with continuous flow at the same ventricular assist device flow rate. During ischemic left ventricular failure, pulsatile left ventricular support resulted in higher total cardiac output (5.58 ± 1.58 vs 5.12 ± 1.19, P < .05), higher mean aortic pressure (67.8 ± 14 vs 60.2 ± 10, P < .05), and lower left atrial pressure (11.5 ± 3.5 vs 13.9 ± 6.0, P < .05) compared with continuous flow at the same left ventricular assist device flow rate. CONCLUSION: Synchronized, pulsatile left ventricular assistance produces superior left ventricular unloading and circulatory support compared with continuous-flow left ventricular assist at the same flow rates.