Hemodynamic Performance and Blood Damage of the Intra-Aortic Pumps: A CFD-Based Investigation

This study utilizes CFD simulations with wall-modeled large eddy simulation to demonstrate that an impeller-driven intra-aortic pump outperforms single and triplet pump designs by achieving superior pressure head, hydraulic efficiency, and hemocompatibility, as evidenced by lower NIH values and a Hemolytic Number consistently below 1.

The Gist

Imagine your heart is the engine of a car, and your blood vessels are the highways. Sometimes, that engine gets weak (heart failure) and can't push enough blood to the rest of the body, especially to the kidneys. To help, doctors use Intra-Aortic Pumps. Think of these as tiny, high-tech "windmills" or "propellers" that get inserted into the main highway (the aorta) to give the blood an extra push, helping the heart rest and the kidneys get the water they need.

This paper is like a virtual race track where engineers built three different types of these "windmills" inside a computer to see which one works best without breaking the blood cells.

Here is the breakdown of their investigation using simple analogies:

1. The Three Contenders

The researchers didn't just test one design; they tested three different "racing cars":

• The Single Propeller (The Solo Racer): A single, fast-spinning propeller. It's like a lone cyclist trying to push a heavy cart.
• The Triplet Propeller (The Relay Team): Three smaller propellers lined up one after another. It's like three cyclists taking turns pushing the cart, hoping the work is shared.
• The Impeller (The Turbo Fan): A different design that looks more like a fan blade inside a tube. It's designed to pull and push blood more smoothly, like a high-performance jet engine intake.

2. The Virtual Test Drive (CFD)

Instead of building physical pumps and testing them on animals (which is expensive and risky), the team used Computational Fluid Dynamics (CFD).

• The Analogy: Imagine a video game where you can simulate wind, water, and traffic. They created a digital version of the human aorta (the main artery) and dropped these three pumps into it.
• The "Blood": They didn't use real blood, but a digital fluid that acts exactly like blood (thick when moving slowly, thin when moving fast).
• The "Damage" Check: They watched to see if the pumps were too rough. If a pump spins too violently, it's like a blender chopping up the blood cells (a process called hemolysis). The goal is to push the blood hard without shredding it.

3. The Results: Who Won the Race?

The Speed and Power (Hydraulic Performance)

• The Single & Triplet Pumps: These struggled a bit. When the blood flow was low, they actually started creating a "traffic jam" behind them. The blood would get pushed out the back, swirl around, and get sucked back in. This is called recirculation. It's like a fan blowing air into a corner where it just spins in circles instead of moving forward. They were also not very efficient, wasting a lot of energy.
• The Impeller Pump: This was the clear winner. It generated much more pressure (pushing power) and moved blood much faster. It didn't create those messy traffic jams. It was like a sleek sports car compared to the others.

The Safety Score (Hemolysis)

• The Problem: Blood cells are fragile. If they get hit by too much "shear stress" (like being squeezed through a tiny, fast-moving gap) or if they stay in a bad spot too long, they break.
• The Single Pump: It was the roughest. It created high-speed zones that shredded cells and had long "waiting lines" (recirculation) where cells got stuck and damaged.
• The Triplet Pump: Better than the single one, but still had some rough spots.
• The Impeller Pump: The safest. It moved blood so smoothly that the cells barely felt the stress. It had the shortest "exposure time" to danger.

4. The New Scorecard: The "Hemolytic Number" (HN)

The researchers realized that comparing these pumps was like comparing a bicycle to a truck using the same ruler. It didn't make sense because they are so different.

So, they invented a new metric called the Hemolytic Number (HN).

• The Analogy: Think of this as a "Safety-to-Work Ratio."
  • A high score means the pump is doing a lot of work but causing a lot of damage (Bad).
  • A low score means the pump is doing a lot of work with very little damage (Good).
• The Result: The Impeller Pump had the lowest score (below 1), meaning it is the safest and most efficient design. The other two had much higher scores, meaning they were "rougher" on the blood.

5. The Big Takeaway

The study concludes that while all these pumps try to help failing hearts, the Impeller-driven design is currently the best candidate. It pushes blood harder, moves more of it, and—most importantly—treats the blood cells with the most care.

In short: If you were building a tiny pump to save a heart, you wouldn't want the one that acts like a blender (the single pump). You'd want the one that acts like a smooth, powerful fan (the impeller pump). This research gives engineers the blueprint to build better, safer life-saving devices for the future.

Technical Summary

1. Problem Statement

Cardiovascular diseases and heart failure necessitate advanced Mechanical Circulatory Support (MCS) devices. Intra-aortic blood pumps (IABPs) offer a less invasive alternative to traditional Ventricular Assist Devices (VADs) by providing circulatory support without crossing the aortic valve. However, existing IABP designs (e.g., Aortix, ModulHeart, Second Heart Assist) vary significantly in architecture (single axial, multi-pump series, impeller-driven) and lack a systematic, side-by-side comparison under a unified numerical framework.

Key challenges addressed include:

• Hemodynamic Efficiency: Determining which design maximizes flow augmentation and pressure rise while minimizing energy loss.
• Hemocompatibility: Evaluating the risk of hemolysis (red blood cell damage) and thrombosis caused by high shear stress and flow recirculation.
• Lack of Standardized Metrics: Existing metrics like the Normalized Index of Hemolysis (NIH) are not dimensionless, making it difficult to compare pumps of different sizes and operating speeds directly.

2. Methodology

The study utilized Computational Fluid Dynamics (CFD) to simulate and compare three distinct intra-aortic pump geometries within a simplified 22 mm diameter pipe (representing the descending aorta).

• Pump Configurations:
  1. Single Pump: An axial-flow design inspired by the Aortix™ (HeartMate II-like geometry), 6 mm diameter, rotating at 25,000 rpm.
  2. Triplet Pump: Three identical axial modules in series (inspired by ModulHeart™), each 4 mm diameter, rotating at 14,000 rpm.
  3. Impeller-Driven Pump: A design inspired by the Second Heart Assist device, featuring NACA 6509 airfoil blades, 22 mm outer diameter, rotating at 10,500 rpm.
• Numerical Approach:
  • Solver: ANSYS Fluent v22.1 using a pressure-based transient solver with the PISO algorithm.
  • Turbulence Model: Wall-Modeled Large Eddy Simulation (WMLES). This was chosen to resolve large-scale unsteady flow structures (recirculation, wakes) and transitional-to-turbulent regimes ($Re \approx 4 \times 10^3 - 2 \times 10^4$) without the prohibitive cost of fully wall-resolved LES.
  • Fluid Model: Blood was modeled as a non-Newtonian fluid using the Carreau viscosity model.
  • Grid Study: A Grid Convergence Index (GCI) analysis was performed to ensure mesh independence, resulting in meshes of 1.1–1.7 million cells with $30 < y^+ < 60$.
• Performance Metrics:
  • Hemodynamics: Pressure-flow curves, hydraulic efficiency ($\eta$), torque, and axial force.
  • Hemolysis: Scalar Shear Stress (SSS), exposure time ($t_{exp}$), Hemolysis Index (HI), and Normalized Index of Hemolysis (NIH).
  • New Metric: Introduction of the Hemolytic Number (HN), a dimensionless parameter derived via the Buckingham Pi theorem to standardize hemolysis comparison.

3. Key Contributions

1. Comparative Analysis: The first systematic CFD-based comparison of single, triplet, and impeller-driven intra-aortic pump architectures under a unified numerical framework.
2. Introduction of the Hemolytic Number (HN): A novel dimensionless parameter defined as:
   $$HN = C_E \cdot \frac{\tau \cdot t_{exp}}{\rho \cdot \omega \cdot D^2}$$
   This metric links shear stress, exposure time, and inertial loading, allowing for a standardized comparison of hemocompatibility across different pump sizes and speeds.
3. High-Fidelity Simulation: Application of WMLES to capture complex flow phenomena (rotor-stator interaction, tip leakage vortices, and wake dynamics) in intra-aortic devices, which are often under-predicted by RANS models.

4. Key Results

Hemodynamic Performance

• Pressure Rise: The impeller-driven pump significantly outperformed the others, generating a peak pressure head of ~800 Pa at 4 L/min. The single and triplet pumps generated much lower pressures (~170 Pa and ~50 Pa, respectively).
• Hydraulic Efficiency: The impeller pump achieved the highest efficiency (~6% at 14 L/min), compared to the single pump (2.7%) and triplet pump (2.2%). The study notes that intra-aortic pumps inherently have lower efficiency than VADs due to their non-occlusive, entrainment-based design.
• Recirculation: Both the single and triplet pumps exhibited significant flow recirculation (retrograde flow from outlet to inlet) at low flow rates (red-shaded regions in performance curves). This phenomenon was absent in the impeller-driven pump, which maintained stable flow across its operating range.

Hemolysis and Blood Damage

• Shear Stress & Exposure: The single pump exposed the highest percentage of fluid volume to high shear stresses (>150 Pa: 12.1%) and had the longest mean exposure time (0.0303 s). The impeller pump had the lowest high-shear exposure (0.21% >150 Pa) and the shortest exposure time (0.0072 s).
• NIH Values: The impeller pump demonstrated the lowest Normalized Index of Hemolysis (0.0035 g/100L), indicating superior hemocompatibility. The single pump had the highest NIH (~0.1434 g/100L).
• Hemolytic Number (HN): The impeller-driven pump consistently maintained an HN < 1 across all flow rates, whereas the single and triplet pumps had higher values. A lower HN indicates better hemocompatibility relative to the device's flow generation capability.

Flow Field Characteristics

• Single/Triplet Pumps: Showed complex, turbulent downstream structures and distinct retrograde flow zones at low inlet rates, increasing the risk of thrombosis due to prolonged residence time.
• Impeller Pump: Generated a more uniform velocity profile with a distinct near-wake and far-wake region, avoiding the severe recirculation seen in axial designs.

5. Significance and Conclusion

• Design Optimization: The study concludes that the impeller-driven design is superior for intra-aortic applications, offering a better balance of high pressure generation, flow capacity, and hemocompatibility compared to axial (single/triplet) designs.
• Clinical Implications: The findings suggest that impeller-driven pumps may reduce the risk of hemolysis and thrombosis, particularly at low flow rates where axial pumps suffer from recirculation. This supports the personalized selection of devices based on patient-specific hemodynamic needs.
• Methodological Advancement: The proposed Hemolytic Number (HN) provides a crucial tool for future device development, enabling engineers to compare disparate pump geometries on a single, dimensionless scale.
• Limitations: The study utilized idealized geometries (reverse-engineered from literature) and a rigid pipe model with steady inlet conditions. Future work should incorporate patient-specific anatomies, pulsatile flow, and experimental validation.

In summary, this research provides a robust computational framework for evaluating intra-aortic pumps, identifying the impeller-driven architecture as the most promising candidate for minimizing blood damage while maximizing hemodynamic support, and introduces a new standardized metric (HN) to guide future device optimization.