About this note
A plain-language summary of the 1 publication listed under this topic on Academic Publications (Topic-wise). Full citations are under References.
Why this matters
Some children are born with only one working pumping chamber in the heart, a condition called univentricular (single ventricle) heart disease. They undergo Fontan surgery, which separates the circulation to the body from the circulation to the lungs. After the operation, the single ventricle pumps oxygenated blood to the body, while the venous blood coming back from the body flows passively into the lungs through a junction called the total cavopulmonary connection (TCPC). Here the two large veins, the superior and inferior vena cava, are joined directly to the pulmonary arteries (Dhayananth and Narasimhan, 2018).
Because no second pump pushes blood through the lungs, every bit of pressure lost at this junction is extra work for the lone ventricle. Surgeons and engineers have therefore tried many junction shapes (offsets between the two veins, flared or curved connections, Y-shaped grafts) to reduce the energy loss. Much of that loss comes from recirculation zones, regions where blood swirls back on itself as it abruptly changes direction at the junction.
What we did
With K. Dhayananth, we borrowed an idea from our heat exchanger research: placing a porous medium at the right location in a conduit can lower the overall pressure drop. Could a porous insert in the pulmonary arteries do the same for the Fontan junction (Dhayananth and Narasimhan, 2018)?
We built a three-dimensional computer model of a TCPC with a one-diameter offset between the two veins, based on imaging data of an 8-year-old Fontan patient used in an earlier published study, and placed a short porous plug in each pulmonary artery. Blood flow was treated as steady, laminar and Newtonian. The porous region was described by volume-averaged equations that include the viscous drag and form drag exerted by the pores. Two properties characterise the insert: porosity, the fraction of its volume that is open, and permeability, a measure of how easily fluid passes through a porous material, expressed here as a dimensionless Darcy number. After validating the model without an insert against published results, we varied the insert’s thickness, position, porosity and permeability, together with the vessel diameter, the total cardiac output and the split of flow between the left and right lungs.
What we found
A well-chosen insert lowers the energy loss because it curtails the recirculation zones across the junction. A thin insert, half a vessel diameter thick and placed at the best of the positions studied (a few diameters downstream of the junction), gave up to 21% lower energy loss. Inserts of porosity 0.7 cut the loss by 27% on average over the range of flow splits studied, and a highly permeable insert of porosity 0.6 reached a 40% reduction at a 40:60 flow split.
The benefit is not automatic. If the insert is too thick or too tight (low permeability), the pressure drop it creates exceeds what it saves. This led us to a simple selection rule: an insert is worthwhile only when its own pressure drop, estimated from the Hazen-Dupuit-Darcy model of flow through porous media, is no larger than the pressure drop caused by recirculation in the junction without it. Highly permeable inserts satisfy this rule most easily. The insert also helped more under simulated exercise (higher cardiac output) than in the low-output diseased state. The study assumed steady flow; the pulsatile flow caused by breathing and exercise is left for future work.
Related
- Heat Transfer Enhancement using Porous Media: heat exchanger studies in which porous inserts reduce pressure drop.
- Bio-Fluid Dynamics Concepts: the porous medium ideas behind the model.
References
- Dhayananth, K. and Narasimhan, A., (2018) “Evaluation of hemodynamic performance of total cavopulmonary connection (TCPC) with porous inserts,” Int J Numer Meth Biomed Engng. 34(4) :e2937. [DOI: https://doi.org/10.1002/cnm.2937 | Abstract ]