Microsurgery Blood Flow Modeling
Kate Elzinga, MD1; Breana Siljander, MD2; George Karniadakis, MS PhD3; Jessica Billig, MD2; Jeffrey N. Lawton, MD4; (1)Division of Plastic and Reconstructive Surgery, University of Michigan, Ann Arbor, MI, (2)University of Michigan, Ann Arbor, MI, (3)Brown University, Providence, RI, (4)Orthopaedic Surgery, University of Michigan, Ann Arbor, MI
Successful digital replantation has been reported by repairing vessels with a diameter as small as 0.4mm using supermicrosurgery techniques. Blood flow through repaired digital arteries and veins is affected by non-modifiable factors including patient age and co-morbidities such as atherosclerosis, renal disease, diabetes, and smoking. Beyond this, however, modifiable factors including suture size, suture number/technique, blood viscosity, limb position, blood pressure, vasodilatation/temperature, hematocrit, and oxygen saturation can be thought of as under surgeon control (to varying degrees). Our goal is to determine how to optimize modifiable factors to improve blood flow and thus replantation success rates.
Using advanced flow dynamic theory and computer modeling (successfully modeled in larger vessels but never in the microcirculation), we can determine the effect of various conditions on digital blood flow (Figure 1). Linear and geometric impediments to flow can be analyzed.
To maintain blood flow across an anastomosis using velocity vector plots, we have determined that the axial blood velocity increases and a larger pressure drop occurs as the diameter of the artery decreases in size (Figures 2-3). Analyzing axial velocity profiles using Womersley flow, we have observed that the blood flow in the center of a vessel is driven more by blood pressure while the blood flow closer to the vessel wall is more affected by blood viscosity (Figure 4).
When modeling parameters are kept constant, blood viscosity appears to play a larger role in blood flow in a vessel with a diameter of 0.8mm compared to a vessel with a diameter of 1mm (Figure 5). Viscosity plays a larger role in blood flow with an 8-suture technique compared to a 4-suture technique (Figure 6).
Further studies will allow us to determine at what level digital replantation can be successfully performed and at what point it becomes futile based on vessel diameter, suture technique, and adjustments in modifiable factors. By investigating the effect of various parameters on blood flow, we will determine which factors can be adjusted to optimize outcomes, for instance, modifying a patient's temperature, blood pressure, blood viscosity via hematocrit and anticoagulation, and limb position.
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