Pulsatility and Compliance — More Than Just Flow Rates
Why Simulating the Beating Heart Takes More Than a Pump
In the last two articles, we explained:
Why the pressure shown on a pump differs from what’s measured inside a vascular model (Link)
How Bernoulli’s principle, the continuity equation, and Poiseuille’s law explain velocity changes and pressure drops across tubing and vessels (Link)
This week, we’re shifting focus to something just as critical — yet often misunderstood in simulation systems: pulsatility and vessel compliance.
When replicating blood flow, it’s not enough to set a pump to a constant flow rate or target pressure. To simulate real cardiovascular conditions, your system must also reproduce the pulsatile nature of the heartbeat and the compliant behavior of the vessel walls. Together, these two factors shape everything from pressure waveforms to flow direction — and even the feel of the model during training.
What Is Pulsatile Flow?
In the human body, blood flow isn’t constant — it’s rhythmic, driven by the cyclical contraction and relaxation of the heart. This creates:
Systolic pressure peaks (during heart contraction)
Diastolic pressure valleys (between beats)
A pulse pressure waveform that travels through the arteries
In your simulation setup, a pulsatile pump is designed to mimic this behavior — by generating waves of pressure and flow at realistic frequencies (e.g., 60–100 beats per minute).
But pulsatility isn't just a visual or timing effect. It has deep consequences for:
Pressure gradients along the model
Wall expansion and recoil
Shear forces experienced by the fluid and model walls
What Is Compliance, and Why Does It Matter?
Compliance refers to how much a vessel expands in response to pressure. In real arteries — especially the aorta — walls stretch with each pulse, storing some of the energy and helping smooth out blood flow.
In your vascular model, this means:
If the material is compliant (like silicone or TPU), it expands during systole and recoils during diastole
If the model is too rigid, you lose this effect — and the pressure wave becomes artificially sharp or unstable
Result: Compliance helps dampen pulsatile spikes and maintain a more realistic flow curve.
This is especially important when simulating:
Aortic aneurysms
Valve disorders
Peripheral circulation
Pediatric and geriatric vessel behavior
How Pulsatility and Compliance Interact
Here’s where it gets interesting: pulsatility and compliance don’t act separately — they amplify or buffer each other.
For example:
If you increase stroke volume or pulse rate, compliant models will expand more, which may reduce peak pressure
In a rigid model, the same pulse settings may create higher pressure peaks, because there's no energy absorption by the walls
This means:
You can’t just look at flow rate alone to assess realism
You need to match the material properties of your model to the intended pressure and waveform characteristics
Material Property Comparison: Simulation Materials vs. Human Vascular Tissue
Practical Insight: Why This Matters for Users
Many users notice that the flow rate looks fine, but:
The pressure waveform is too sharp or flat
The model “balloons” unnaturally
Devices don’t respond the way they do in patients
Often, this comes down to a mismatch in pulse waveform settings or model compliance.
To improve realism:
Use a compliant vascular model if you want to simulate real pressure propagation and vessel wall behavior
Tune your pump’s pulse amplitude, rate, and stroke volume — not just target pressure
Observe the waveform using pressure sensors or imaging — not just the flow rate or volume per minute
Don’t Overestimate Stroke Volume — Less Can Be More
A common mistake in vascular simulation is setting the pump to mimic a physiological stroke volume of 55–75 mL — the typical range of a healthy human heart. This makes intuitive sense at first… but often leads to unrealistic results.
In practice, most simulation setups only include the aorta and a small segment of the vascular tree — often just the region of interest (ROI) for a procedure or training task. You're not replicating the entire systemic circulation with its distributed resistance, capacitance, and venous return.
Pumping 70 mL per stroke into such a limited model can result in:
Unrealistically high flow velocities in the ROI
Elevated pressure spikes due to the lack of distributed compliance
Turbulence and instability, especially in bifurcations or branch vessels
Misleading device behavior, since the hemodynamics no longer reflect realistic in vivo conditions
The graphic helps to determine the correct blood flow in the vascular simulation system.
Unless you're specifically simulating an aortic valve or full-body circulation, a reduced stroke volume — carefully scaled to the volume and compliance of your model — will provide more accurate, controllable, and physiologically appropriate flow conditions.
Tip for users: Start low (e.g., 15–30 mL per stroke), monitor pressure and velocity in your ROI, and increase only as needed to match physiological waveforms.
Takeaway
Pulsatile flow and vessel compliance are essential ingredients in realistic cardiovascular simulation. While flow rate and pressure are important, they don't tell the whole story.
By combining:
A well-calibrated pulsatile pump
A material-matched compliant vascular model
And a basic understanding of wave behavior
…you can achieve a simulation environment that truly mimics how the human circulatory system behaves — helping medical professionals train, test, and learn in a setting that reflects real-world physiology.
In next week’s article, we’ll look at calibrating pressure readings — and how to adjust pump sensor data to match actual values inside your model using real-world measurements and formulas.