Program Book - Keck School of Medicine of USC - University of ...

within the peripheral circulation, but also within the total system. In the simplest termsthe bypass graft functions as a low-resistance collateral in parallel circuitry (Fig. 4B).Recalling that the inverse of the total resistance of a parallel circuit is the sum of theinverse of the individual resistors, 1/Rtotal = 1/R1 1 1/R2 the total resistance of the parallelcircuit is always lower than that of each individual resistor. By reducing the ratios ofresistance between the systemic circulation and the fistula, the brachial shunt fraction isdecreased and peripheral perfusion is augmented. The overall reduction in resistance inthe system augments the total blood flow in the extremity such that fistula blood flowsare maintained. If a distal bypass graft were performed without concomitant ligation ofthe artery distal to the fistula, it could certainly augment the retrograde flow in the distalbrachial artery. In the face of ligation the blood flow is left with no other choice but tofollow the pressure gradient downstream to the peripheral vascular bed. Some vascularsurgeons remained concerned about ligating the artery below the fistula origin, but inpractice, this component of the procedure rarely leads to significant adverse sequelae.Figure 4.Key points to DRIL procedure:1. Use autogenous vein whenever possible2. Exclude inflow stenosis before proceeding with DRIL procedure3. Originate the bypass well proximal to the pressure sink zone near the AVF origin.In practice, this might need to be at least 5-10 cm. In an upper extremity fistula,there is a continuous drop off in arterial pressure from the origin of the ipsilateralsubclavian artery to the large pressure sink zone at the origin of the fistula itself.If the bypass is originated too close to the pressure sink zone, the inflow will beinadequate to restore adequate distal perfusion (somewhat analogous to1178