Robert F. Salamonsen

Noninvasive Activity-based Control of an Implantable Rotary Blood Pump: Comparative Software Simulation Study

A control algorithm for an implantable centrifugal rotary blood pump (RBP) based on a noninvasive indicator of the implant recipients activity level has been proposed and evaluated in a software simulation environment. An activity level index (ALI)-derived from a noninvasive estimate of heart rate and the output of a triaxial accelerometer-forms the noninvasive indicator of metabolic energy expenditure. Pump speed is then varied linearly according to the ALI within a defined range. This ALI-based control module operates within a hierarchical multiobjective framework, which imposes several constraints on the operating region, such as minimum flow and minimum speed amplitude thresholds. Three class IV heart failure (HF) cases of varying severity were simulated under rest and exercise conditions, and a comparison with other popular RBP control strategies was performed. Pump flow increases of 2.54, 1.94, and 1.15 L/min were achieved for the three HF cases, from rest to exercise. Compared with constant speed control, this represents a relative flow change of 30.3, 19.8, and -15.4%, respectively. Simulations of the proposed control algorithm exhibited the effective intervention of each constraint, resulting in an improved flow response and the maintenance of a safe operating condition, compared with other control modes.

Recovery From Anthracycline Cardiomyopathy After Long-term Support With a Continuous Flow Left Ventricular Assist Device

We report the clinical course of a 16-year-old girl in remission from non-Hodgkins lymphoma who presented in cardiogenic shock due to a severe anthracycline cardiomyopathy. The patient was initially stabilized using central extracorporeal membrane oxygenation support, followed by conversion to a left ventricular assist device. Unexpected evidence of cardiac recovery 9 months after implant enabled device weaning during a 3-month period, culminating in successful device explantation 1 year after implant. The patient survives 18 months after explant in New York Heart Association class I, on conventional heart failure medical management and metabolic therapy.

Numerical Optimization Studies of Cardiovascular-Rotary Blood Pump Interaction

A heart-pump interaction model has been developed based on animal experimental measurements obtained with a rotary blood pump in situ. Five canine experiments were performed to investigate the interaction between the cardiovascular system and the implantable rotary blood pump over a wide range of operating conditions, including variations in cardiac contractility and heart rate, systemic vascular resistance (SVR), and total blood volume (V(total) ). It was observed in our experiments that SVR decreased with increasing mean pump speed under the healthy condition, but was relatively constant during the speed ramp study under reduced cardiac contractility conditions. Furthermore, we also found a significant increase in pulmonary vascular resistance with increasing mean pump speed and decreasing total blood volume, despite a relatively constant SVR. Least squares parameter estimation methods were utilized to fit a subset of model parameters in order to achieve better agreement with the experimental data and to evaluate the robustness and validity of the model under various operating conditions. The fitted model produced reasonable agreement with the experimental measurements, both in terms of mean values and steady-state waveforms. In addition, all the optimized parameters were within physiological limits.

A Dynamic Lumped Parameter Model of the Left Ventricular Assisted Circulation

A lumped parameter model of the cardiovascular system (CVS) and its interaction with an implantable rotary blood pump (iRBP) is presented. The CVS model consists of the heart, the systemic and the pulmonary circulations. The pump model is made up of three differential equations, i.e. the motor equation, the torque equation and the hydraulic equation. Qualitative comparison with data from ex vivo porcine experiments suggests that the model is able to simulate different physiologically significant pumping states with varying pump speed set points. The combined CVS- iRBP model is suitable for use as a tool for investigating changes in the circulatory system parameters in the presence of the pump, and for testing control algorithms.

Effect of Parameter Variations on the Hemodynamic Response Under Rotary Blood Pump Assistance

Numerical models, able to simulate the response of the human cardiovascular system (CVS) in the presence of an implantable rotary blood pump (IRBP), have been widely used as a predictive tool to investigate the interaction between the CVS and the IRBP under various operating conditions. The present study investigates the effect of alterations in the model parameter values, that is, cardiac contractility, systemic vascular resistance, and total blood volume on the efficiency of rotary pump assistance, using an optimized dynamic heart-pump interaction model previously developed in our laboratory based on animal experimental measurements obtained from five canines. The effect of mean pump speed and the circulatory perturbations on left and right ventricular pressure volume loops, mean aortic pressure, mean cardiac output, pump assistance ratio, and pump flow pulsatility from both the greyhound experiments and model simulations are demonstrated. Furthermore, the applicability of some of the previously proposed control parameters, that is, pulsatility index (PI), gradient of PI with respect to pump speed, pump differential pressure, and aortic pressure are discussed based on our observations from experimental and simulation results. It was found that previously proposed control strategies were not able to perform well under highly varying circulatory conditions. Among these, control algorithms which rely on the left ventricular filling pressure appear to be the most robust as they emulate the Frank-Starling mechanism of the heart.

Hemodynamic Response to Exercise and Head-Up Tilt of Patients Implanted With a Rotary Blood Pump: A Computational Modeling Study

The present study investigates the response of implantable rotary blood pump (IRBP)-assisted patients to exercise and head-up tilt (HUT), as well as the effect of alterations in the model parameter values on this response, using validated numerical models. Furthermore, we comparatively evaluate the performance of a number of previously proposed physiologically responsive controllers, including constant speed, constant flow pulsatility index (PI), constant average pressure difference between the aorta and the left atrium, constant average differential pump pressure, constant ratio between mean pump flow and pump flow pulsatility (ratioP I or linear Starling-like control), as well as constant left atrial pressure ( P l a ¯ ) control, with regard to their ability to increase cardiac output during exercise while maintaining circulatory stability upon HUT. Although native cardiac output increases automatically during exercise, increasing pump speed was able to further improve total cardiac output and reduce elevated filling pressures. At the same time, reduced venous return associated with upright posture was not shown to induce left ventricular (LV) suction. Although P l a ¯ control outperformed other control modes in its ability to increase cardiac output during exercise, it caused a fall in the mean arterial pressure upon HUT, which may cause postural hypotension or patient discomfort. To the contrary, maintaining constant average pressure difference between the aorta and the left atrium demonstrated superior performance in both exercise and HUT scenarios. Due to their strong dependence on the pump operating point, PI and ratioPI control performed poorly during exercise and HUT. Our simulation results also highlighted the importance of the baroreflex mechanism in determining the response of the IRBP-assisted patients to exercise and postural changes, where desensitized reflex response attenuated the percentage increase in cardiac output during exercise and substantially reduced the arterial pressure upon HUT.

Non-invasive measurements based model predictive control of pulsatile flow in an implantable rotary blood pump for heart failure patients

In this paper, firstly, we propose a linear time-variant (LTV) model to estimate the mean pulsatile flow (Qp) in an implantable rotary blood pump (RBP). Non-invasive measurement of mean pulse-width modulation (PWM) signal acquired from the pump controller was used as an input to estimate the mean pulsatility index of pump rotational speed (PIω) with this subsequently used to estimate the mean Qp. Secondly, the proposed LTV model was used to develop a controller to regulate and track the variations in the mean Qp and PIω. We used a model predictive control (MPC) approach to develop the controller where this allowed us to explicitly apply pre-defined practical constraints to control input PWM as well as the output and the states of the system including; Qp and PIω. The model and the controller were tested using a parameter-optimized model of the cardiovascular system-rotary blood pump under wide ranges of speed ramp experiments carried out under different operating conditions such as variations in afterload, preload and heart contractility.

Low-flow hypothermic crystalloid perfusion is superior to cold storage during prolonged heart preservation

BACKGROUNDnPreservation of donor hearts for transplantation has traditionally been performed with the use of static cold storage. We have developed and tested a novel gravity-powered system of cold crystalloid perfusion for prolonged donor heart preservation.nnnMETHODSnGreyhounds were anesthetized; their hearts were arrested with cold cardioplegic solution and excised. Hearts were allocated to 12 hours of perfusion preservation (nxa0= 6) or cold storage in ice (nxa0= 5). Non-preserved hearts (nxa0= 5) served as a normal reference group. Perfusion hearts were perfused (20 mL/min, 8-12°C) with a novel oxygenated nutrient-containing preservation solution. After preservation, the recovery of the hearts was assessed in a blood-perfused working heart rig over 2 hours in terms of function, blood lactate level, myocardial adenosine triphosphate, and histology.nnnRESULTSnAfter 2 hours of reperfusion, in comparison with cold storage hearts, perfused heart function curves showed superior recovery of cardiac output (Pxa0= .001), power (Pxa0= .001), and efficiency (0.046 ± 0.01 vs 0.004 ± 0.003 joules/mL O2, Pxa0= .034). Myocardial adenosine triphosphate content (mmol/mg protein) was reduced significantly from the normal level of 26.5 (15.9, 55.8) to 5.08 (0.50, 10.4) (Pxa0= .049) in cold storage hearts but not in perfused hearts. Over a period of 2 hours, lactate levels in the blood perfusate were significantly lower in the perfusion group than in the cold storage group (Pxa0< .05).nnnCONCLUSIONSnContinuous hypothermic crystalloid perfusion provides myocardial preservation superior to cold storage for long-term heart preservation, with potential applicability to marginal and donation after circulatory death hearts.

Impact of Left Ventricular Assist Device Speed Adjustment on Exercise Tolerance and Markers of Wall Stress

Introduction Left ventricular assist devices are crucial in rehabilitation of patients with end-stage heart failure. Whether cardiopulmonary function is enhanced with higher pump output is unknown. Methods 10 patients (aged 39 ± 16 years, mean ± SD) underwent monitored adjustment of pump speed to determine minimum safe low speed and maximum safe high speed at rest. Patients were then randomized to these speed settings and underwent three 6-minute walk tests (6MWT) and symptom-limited cardiopulmonary stress tests (CPX) on separate days. Results Pump speed settings (low, normal and high) resulted in significantly different resting pump flows of 4.43 ± 0.6, 5.03 ± 0.94, and 5.72 ± 1.2 l/min (P<.001). There was a significant enhancement of pump flows (greater at higher speed settings) with exercise (P<0.05). Increased pump speed was associated with a trend to increased 6MWT distance (P = .10); and CPX exercise time (p = .27). Maximum workload achieved and peak oxygen consumption were significantly different comparing low to high pump speed settings only (P<.05). N-terminal-pro-B-type natriuretic peptide release was significantly reduced at higher pump speed with exercise (P<.01). Conclusions We have found that alteration of pump speed setting resulted in significant variation in estimated pump flow. The high-speed setting was associated with lower natriuretic hormone release consistent with lower myocardial wall stress. This did not, however, improve exercise tolerance.

In vivo validation of pulsatile flow and differential pressure estimation models in a left ventricular assist device

Implantation of sensors to measure hemodynamic parameters such as pulsatile pump flow and differential pressure (head) in an implantable rotary pump (IRBP) requires regular in situ calibration due to measurement drift. In addition, risks associated with sensor failure and thrombus formation makes the long-term implantation in patients problematic. In our laboratory, two stable and novel dynamical models for non-invasive pulsatile flow and head estimation were proposed and tested in vitro using mock circulatory loop experiments with varying hematocrit (HCT). Noninvasive measurements of power and pump speed were used as inputs to the flow model while the estimated flow was used together with the pump rotational speed as inputs to the head estimation model. In this paper, we evaluated the performance of the proposed models using in vivo experimental data obtained from greyhound dogs (N=5). Linear regression analysis between estimated and measured pulsatile flows resulted in a highly significant correlation (R2 = 0.946) and mean absolute error (e) of 0.810 L/min, while for head, R2 = 0.951 and e = 10.13 mmHg were obtained.