How can we test the effect of a device modification when systematically trying different configurations in a patient is not practical? Our recent study in Cardiology in the Young used a bench-top circulation model to investigate this question for microvascular plugs used as pulmonary flow restrictors.
The study examined how adding slits to the device’s polytetrafluoroethylene (PTFE) membrane changes the balance between pulmonary and systemic flow. Beyond comparing device configurations, it illustrates the value of in vitro experiments for testing hemodynamic hypotheses under controlled conditions.
Why pulmonary flow restriction matters
In a functionally single-ventricle circulation, pulmonary and systemic blood flow must be balanced. Excessive flow to the lungs can compromise delivery to the body. Modified microvascular plugs have been used as catheter-delivered pulmonary flow restrictors, but the relationship between membrane modification and the resulting flow restriction remains difficult to predict.
A practical question follows: how much does an additional slit change flow, and does the answer depend on the circulation’s starting condition?
What the bench-top model made possible
Comparing several device modifications while independently controlling the starting flow distribution is difficult in vivo. Repeatedly changing a device in a vulnerable patient solely for experimental comparison would also introduce procedural risk. A bench-top model provides a way to examine these questions without exposing a patient to those experimental changes.
We used a 2.5-times enlarged, CT-derived 3D-printed pulmonary artery model connected to a pulsatile pump, with two pulmonary branches and a systemic outlet. Adjustable outlet resistance established nominal baseline pulmonary-to-systemic flow ratios of 2:1, 3:1, and 4:1. Reynolds and Womersley numbers guided the scaling of the experimental conditions.
The circuit was tested without devices and with bilateral MVP-9Q devices whose membranes were intact or modified with one to four slits. Ultrasonic sensors measured flow in both pulmonary branches and the systemic limb. These measurements allowed us to assess redistribution between the two circulations, rather than examining pulmonary flow alone.
What changed as more slits were added?
At the nominal 2:1 baseline, both one- and two-slit configurations brought Qp:Qs close to 1:1 in the model. Adding a third or fourth slit increased Qp:Qs, indicating less effective restriction relative to the one- and two-slit configurations.
The initial flow condition mattered. At nominal baselines of 3:1 and 4:1, one or two slits did not restore a 1:1 balance. Even intact devices left substantial pulmonary flow. The same modification therefore did not produce the same final flow balance across conditions.
| Device configuration | Nominal baseline 2:1 | Nominal baseline 3:1 | Nominal baseline 4:1 |
|---|---|---|---|
| No device | 1.91 | 3.17 | 4.35 |
| Intact membrane | 1.13 | 1.62 | 2.24 |
| One slit | 0.99 | 1.65 | 2.51 |
| Two slits | 0.99 | 1.78 | 2.48 |
| Three slits | 1.18 | 2.01 | 2.84 |
| Four slits | 1.29 | 2.24 | 3.02 |
Qp is pulmonary flow and Qs is systemic flow. A ratio of 1 represents equal pulmonary and systemic flow. Column headings indicate the intended baseline conditions; the “No device” row gives the measured baseline ratios. These are experimental model results, not patient-specific treatment targets.
Why this study is important
It makes a difficult clinical question experimentally testable. The model allowed systematic comparison of device modifications across different starting flow conditions, helping distinguish the effect of the modification from the influence of the circuit.
It shows why slit count alone is insufficient. Flow changed nonlinearly, and the achieved balance depended on the baseline condition. A simple rule based only on the number of slits cannot capture this behavior.
It provides a basis for better device development. Controlled bench testing can help evaluate alternative opening geometries and identify limitations before further in vivo investigation. The findings support continued development of purpose-designed flow restrictors with more predictable performance.
What the model cannot tell us
This experiment evaluated hemodynamic behavior in vitro; it did not establish clinical safety, long-term efficacy, or an optimal modification for every patient. Water was used instead of blood, the model and tubing did not reproduce neonatal vessel compliance, and the pulmonary and systemic outlets shared the same pressure condition.
Flow through the membrane or around the device may have affected the measurements. The model also did not capture platelet aggregation, thrombus or fibrin deposition, or changes over time after implantation. The authors therefore emphasize the relative effects of device modification rather than treating the absolute ratios as clinical predictions.
The broader value of in vitro testing
The contribution of this work is a controlled experimental framework for examining device behavior that is difficult to isolate in a living circulation. It helps define both the possibilities and the limits of a proposed approach, while identifying the questions that subsequent physiological and clinical studies need to address.
Future work could extend this framework to other modification strategies and incorporate 4D flow MRI or computational modeling to characterize the underlying flow patterns. These were proposed directions, not measurements performed in the present study.
Publication
Soszyn N, Park S, Vargas Acevedo C, Mejia E, Barker A, Zablah J, Morgan G. Two slits good, four slits bad? Assessing the hemodynamic impact of modifications to microvascular plugs in pulmonary flow restriction using a bench-top model. Cardiology in the Young. 2026. doi:10.1017/S1047951126123932.