Analysis of the Hemodynamics of a Bypass with Adjunctive Arterio–Venous Fistula Using a Direct Electrical Circuit Model

Abstract

Background: The purpose of this work was to study, in a theoretical electric model the effect that an adjunctive arterio–venous fistula (AVF) produces on the flow patterns of a distal infrapopliteal bypass. Method: A theoretical electric model that mimics a femoro–infrapopliteal bypass with an adjunctive AVF was designed. Based on Thevenin's Theorem, flows in different areas of the circuit (bypass, collaterals, AVF, segment of artery distal to bypass anastomosis, arteriolar system) were calculated at different resistance values for AVF, collaterals, and bypass. Results: This model demonstrated the following: (1) An adjunctive AVF with large resistance (smaller vein diameter) maintains distal flow, as opposed to a low-resistance AVF. (2) Good collateral flow, in addition to low-resistance AVF, can produce retrograde flow in the segment of the artery distal to the AVF. (3) True distal flow (arteriolar flow) in the presence of high collateral resistance can decrease with decreasing resistance of the AVF, but it will never become retrograde. Conclusion: The effect of an adjunctive AVF on the hemodynamics of a bypass and its distal runoff vessels depends on the resistance of the AVF and the collateral vessels. The presence of AVF always increases the bypass flow and in this respect it may promote graft patency. However, Bypass grafting in the face of a low collateral resistance is of dubious value and the addition of a fistula in this situation is detrimental.