Hemolytic Footprint of Rotodynamic Blood Pumps

Will blood-informed design signal the fourth generation of cardiac assist devices?

Mechanical circulatory support devices have profoundly transformed the management of severe cardiothoracic disorders. While heart transplantation is the gold standard therapy for end-stage heart disease, long-term mechanical support devices are a viable alternative for those ineligible and/or those awaiting organ availability. Major technological advancements were made over first 5 decades of development, resulting in improved durability and survival with reduced adverse events. However, gains have tapered recently for various complications (e.g., internal bleeding, multisystem organ failure), which collectively represent a significant proportion of disability and/or mortality. Further, in light of mature ventricular assist devices failing during clinical trials or even after clinical approval (class I withdrawals), it is timely to consider: Are our preclinical assessment protocols vital in the design and development of mechanical circulatory support devices, providing a realistic and reliable profile of future clinical performance? This commentary explores this question and analyses development pathways through the lens of the various disciplines involved in the preclinical assessment of mechanical circulatory support technologies: Limitations in approaches to benchtop blood testing, computational design and simulation, and animal testing are discussed as likely contributors to some of the common hemocompatibility-related adverse events (HRAEs). While it is acknowledged that some shortcomings are pragmatic in nature, possible solutions are presented that will only be realized through truly transdisciplinary and open approaches that challenge the current nature of medical device development. We suggest that these can and must be overcome to diminish HRAEs and will potentially demarcate the fourth generation of cardiac assist devices.

Toward an Adjustable Blood Pump for Wide-Range Operation: In-Vitro Results of Performance Curve and Hydraulic Efficiency

Rotary blood pumps in Extracorporeal Life Support (ECLS) applications are optimized for a specific design point. However, in clinical practice, these pumps are usually applied over a wide range of operation points. Studies have shown that a deviation from the design point in a rotary blood pump leads to an unexpected rise of hemolysis with corresponding clinical complications. Adjustable pumps that can adapt geometric parameters to the respective operation point are commonly used in other industrial branches, but yet not applied in blood pumps. We present a novel mechanism to adjust the impeller geometry of a centrifugal blood pump during operation together with in-vitro data of its hydraulic performance and efficiency. Three-dimensionalprinted prototypes of the adjustable impeller and a rigid impeller were manufactured and hydraulic performance and efficiency measured (n = 3). In a flow range of 1.5-9.5 L/min, the adjustable pump increased pump performance up to 47% and hydraulic efficiency by an average of 7.3 percentage points compared with a fixed setting. The adjustable pump allows customization of the pump's behavior (steepness of performance curve) according to individual needs. Furthermore, the hydraulic efficiency of the pump could be maintained at a high level throughout the complete flow range.

Multiobjective Optimization of Rotodynamic Blood Pumps: The Use Case of a Cavopulmonary Assist Device

Comprehensive optimization of rotodynamic blood pumps (RBPs) requires the consideration of three partially conflicting objectives: size, hemocompatibility, and motor efficiency. Optimizing these individual objectives independently, the potential of multiobjective optimizations often remains untapped. This study aimed at the multiobjective optimization of an RBP for cavopulmonary support accounting for all three objectives simultaneously. Hydraulic and electromagnetic design spaces were characterized using computational fluid dynamics and computational electromagnetics, respectively. Design variables included secondary flow gap widths, impeller diameters, and stator heights. The size objective encompassed the RBP widths and heights, the hemocompatibility objective was a weighted composite measure of well-established metrics, and the motor objective was determined by motor losses. Multiobjective optimization was performed through Pareto analysis. 81 designs were considered, and 21 Pareto-optimal designs were identified. The Pareto analysis indicated that hemocompatibility performance could be improved by 72.4% with a concomitant 1.5% reduction in the baseline pump volume. This, however, entailed an increase in motor losses by 0.2 W, while still meeting design requirements, with maximum local temperature rises remaining below 0.4 K. The multiobjective optimization led to a Pareto front, demonstrating the feasibility to improve hemocompatibility at reduced pump volume, however, at the cost of a diminished yet still acceptable motor performance.

An Atraumatic Mock Loop for Realistic Hemocompatibility Assessment of Blood Pumps

OBJECTIVE

Conventional mock circulatory loops (MCLs) cannot replicate realistic hemodynamic conditions without inducing blood trauma. This constrains in-vitro hemocompatibility examinations of blood pumps to static test loops that do not mimic clinical scenarios. This study aimed at developing an atraumatic MCL based on a hardware-in-the-loop concept (H-MCL) for realistic hemocompatibility assessment.

METHODS

The H-MCL was designed for 450 ± 50 ml of blood with the polycarbonate reservoirs, the silicone/polyvinyl-chloride tubing, and the blood pump under investigation as the sole blood-contacting components. To account for inherent coupling effects a decoupling pressure control was derived by feedback linearization, whereas the level control was addressed by an optimization task to overcome periodic loss of controllability. The HeartMate 3 was showcased to evaluate the H-MCL's accuracy at typical hemodynamic conditions. To verify the atraumatic properties of the H-MCL, hemolysis (bovine blood, n = 6) was evaluated using the H-MCL in both inactive (static) and active (minor pulsatility) mode, and compared to results achieved in conventional loops.

RESULTS

Typical hemodynamic scenarios were replicated with marginal coupling effects and root mean square error (RMSE) below 1.74 ± 1.37 mmHg while the fluid level remained within ±4% of its target value. The normalized indices of hemolysis (NIH) for the inactive H-MCL showed no significant differences to conventional loops ( ∆NIH = -1.6 mg/100 L). Further, no significant difference was evident between the active and inactive mode in the H-MCL ( ∆NIH = +0.3 mg/100 L).

CONCLUSION AND SIGNIFICANCE

Collectively, these findings indicated the H-MCL's potential for in-vitro hemocompatibility assessment of blood pumps within realistic hemodynamic conditions, eliminating inherent setup-related risks for blood trauma.

Experimental validation of the power law hemolysis model using a Couette shearing device

BACKGROUND

The study of blood trauma, such as hemolysis in blood-carrying devices, is crucial due to the high incidence of adverse events like alteration of blood function, bleeding, and multi-organ failure. The extent of flow-induced hemolysis, predominantly influenced by stress duration and intensity, is described by established model parameters based on the power law approach. In recent years, various parameters were determined using different Couette shearing devices and donor species. However, they have not been validated due to limited experimental data.

METHODS

This study provides hemolysis measurements in a Couette shearing device and evaluates the suitability of different power law parameters. The revised Couette shearing device generates well-defined dynamic stress loads that are repeatedly applied to blood samples at a defined temperature. Human blood samples with an adjusted hematocrit of 30%, were tested with varying repetitions (20 to 80 times). The half-sinusoidal stress loads had amplitudes of 73 to 140 Pa and exposure times of 24 msec per repetition. The parameters of five common power law hemolysis approaches were then compared with the experimental data.

RESULTS

The prediction with the power law model parameters C = 3.458 × 10-6, α = 0.2777 and β = 2.0639 showed a good agreement with the experimental results.

CONCLUSION

The effect of multiple short-time stresses on hemolysis was investigated to validate the power law hemolysis model with the Couette shearing device of this study.

In-Vitro Flow Validation of Third-Generation Ventricular Assist Devices: Feasibility and Challenges

Computational fluid dynamics (CFD) is a powerful tool for the in-silico evaluation of rotodynamic blood pumps (RBPs). Corresponding validation, however, is typically restricted to easily accessible, global flow quantities. This study showcased the HeartMate 3 (HM3) to identify feasibility and challenges of enhanced in-vitro validation in third-generation RBPs. To enable high-precision acquisition of impeller torques and grant access for optical flow measurements, the HM3 testbench geometry was geometrically modified. These modifications were reproduced in silico , and global flow computations validated along 15 operating conditions. The globally validated flow in the testbench geometry was compared with CFD-simulated flows in the original geometry to assess the impact of the necessary modifications on global and local hydraulic properties. Global hydraulic properties in the testbench geometry were successfully validated (pressure head: r = 0.999, root mean square error [RMSE] = 2.92 mmHg; torque: r = 0.996, RMSE = 0.134 mNm). In-silico comparison with the original geometry demonstrated good agreement ( r > 0.999, relative errors < 11.97%) of global hydraulic properties. Local hydraulic properties (errors up to 81.78%) and hemocopatibility predictions (deviations up to 21.03%), however, were substantially affected by the geometric modifications. Transferability of local flow measures derived on advanced in-vitro testbenches toward original pump designs is challenged by significant local effects associated with the necessary geometrical modifications.

Dissipated energy and efficiency as objective functions for the design of the NeoVAD rotary blood pump

Minimising haemolytic blood damage is an important objective when designing rotary blood pumps, however, calculating haemolysis can be computationally expensive and inaccurate. Efficiency and dissipated energy are much more easily calculable hydraulic parameters in the design and analysis of rotary blood pumps and although there is work to suggest that efficiency is not a good indicator of haemocompatibility, i.e. more efficient pumps do not necessarily cause less damage, there is recent speculation that dissipated energy can act as an easily calculable haemolysis analogue.This study shows that for design purposes, optimising for maximum efficiency and minimum dissipated energy are functionally the same as they are inherently and closely linked. Moreover a demonstration of rotary blood pump design has been completed using the NeoVAD paediatric left ventricular assist device optimising for both objective functions. The resulting designs appear similar in rotor blade shape and are similar in hydraulic performance.Clinical relevance- This reinforces the direct link between efficiency and dissipated energy when analysing rotary blood pumps at a given design operating point. This raises questions either of the claim that efficiency cannot be used as an easily calculable analogue for haemolysis or the validity of dissipated energy to act in this same manner.

Fluid-structure interaction modelling of a positive-displacement Total Artificial Heart

For those suffering from end-stage biventricular heart failure, and where a heart transplantation is not a viable option, a Total Artificial Heart (TAH) can be used as a bridge to transplant device. The Realheart TAH is a four-chamber artificial heart that uses a positive-displacement pumping technique mimicking the native heart to produce pulsatile flow governed by a pair of bileaflet mechanical heart valves. The aim of this work was to create a method for simulating haemodynamics in positive-displacement blood pumps, using computational fluid dynamics with fluid-structure interaction to eliminate the need for pre-existing in vitro valve motion data, and then use it to investigate the performance of the Realheart TAH across a range of operating conditions. The device was simulated in Ansys Fluent for five cycles at pumping rates of 60, 80, 100 and 120 bpm and at stroke lengths of 19, 21, 23 and 25 mm. The moving components of the device were discretised using an overset meshing approach, a novel blended weak-strong coupling algorithm was used between fluid and structural solvers, and a custom variable time stepping scheme was used to maximise computational efficiency and accuracy. A two-element Windkessel model approximated a physiological pressure response at the outlet. The transient outflow volume flow rate and pressure results were compared against in vitro experiments using a hybrid cardiovascular simulator and showed good agreement, with maximum root mean square errors of 15% and 5% for the flow rates and pressures respectively. Ventricular washout was simulated and showed an increase as cardiac output increased, with a maximum value of 89% after four cycles at 120 bpm 25 mm. Shear stress distribution over time was also measured, showing that no more than [Formula: see text]% of the total volume exceeded 150 Pa at a cardiac output of 7 L/min. This study showed this model to be both accurate and robust across a wide range of operating points, and will enable fast and effective future studies to be undertaken on current and future generations of the Realheart TAH.