K16 - A Computational “Digital Twin” of a Benchtop Cardiovascular Simulator for Device Evaluation

Introduction: : Ventricular assist devices (VADs) are implantable heart pumps for treating end-stage heart failure patients. VAD technology continues to advance rapidly, posing a challenge for regulatory reviewers in the United States and abroad. While benchtop and animal testing are accepted as evidence for safety and efficacy of cardiac devices, it is difficult to assess the reliability and clinical relevance of these tests. Future VAD innovations require tools for evaluating their potential impact on patient physiology and safety, including expanded use of mock circulatory loop (MCL) and computational models. The SDSU-CS is a MCL that has been used extensively for VAD research but is a custom-built system. The objective of the current study is to develop a computational analog of the SDSU-CS and expand the use of this tool to aid clinicians, engineers, and regulatory reviewers worldwide.

Materials and

Methods: : The LPM is the foundation of a digital twin of the SDSU-CS. The circuit model replicates the physical setup as shown in Figure 1. The mathematical approach employs a conventional time-varying elastance model of the heart [1] but with two capacitors that represent the silicone heart and tank compliance, respectively. The VAD is coupled to the system with the pressure-flow relationship. The model equations were developed for three biomechanical states: (D) mitral valve open, aortic valve closed, i.e. diastolic filling; (S) mitral valve closed, aortic valve open, i.e. systolic ejection; (IV) mitral and aortic valves closed, i.e. isovolumic conditions. The system of 11 equations with 11 unknowns reduced to 7 equations (6 ODEs and 1 algebraic) with 7 unknowns for D. The resulting differential-algebraic system of equations were solved for a simple case of VAD speed change as shown in Figure 2.

Results, Conclusions, and Discussions:: The transient behavior measured during a speed change of the VAD (Figure 2) was characterized by a rapid (~40 ms) change in all measured variables followed by a slower 1.5 second equilibrium phase to reach the new steady state condition (right panel). The LPM was able to reproduce these distinct phases with the second capacitor, demonstrating the physical mechanisms underlying the two phases of transient behavior with a novel pressure-induced volume change approach. During a step change in VAD speed, the pump creates an instantaneous pressure change throughout the entire system which is possible due to the compliant elements in the system (i.e., silicon heart and the compliances of the aortic, systemic, LA and tank ) corresponding to the first rapid phase. A redistribution of the pressure-induced volume changes follows as fluid flows to achieve a new equilibrium. The LPM predicts that this slower transient phase corresponds to a redistribution of the fixed fluid volume in the system resulting from the hydraulic pressure change induced by the change in HM2 speed. In conclusion, the novel expansion of the Pawar circuit model with two capacitators enabled the delineation of distinct pressure measurements between the tank pressure (TP) and left ventricular pressure (LVP), elucidating the role of the silicone heart as an "accumulator" in MCL. The solution of a simple VAD speed change required recreating the physical behavior of the MCL components in the LPM. Further development of the LPM will add the native heartbeat created by a piston pump in the SDSU-CS.

Acknowledgements and/or References (Optional):: The authors gratefully acknowledge funding support from the National Science Foundation.
[1] S.R. Pawar, E.S. Rapp, J.R. Gohean and R.G. Longoria, Parameter Identification of Cardiovascular System Model Used for Left Ventricular Assist Device Algorithms, J. Eng. Sci. Med. Diagnostics Ther. 5 (2022), pp. 1–8.