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

A fistula consists of a donor artery (inflow), outflow vein, a low flow - highresistance connection (the peripheral vascular bed); the fistula itself (a low resistanceconnection); and a parallel set of inflow and outflow conduits (the arterial and venouscollaterals). Creating an AV fistula (high flow, low resistance) causes a transient drop inproximal arterial pressure that is quickly compensated by an increase in the heart rate andcardiac output. Temporary occlusion of an AV fistula by manual compression results in atransient increase in blood pressure and a fall in heart rate and cardiac output (Branham-Nicoladoni sign). After AV fistula creation, both the proximal arterial and proximalvenous flow increase several fold. In all cases, the direction of blood flow in the proximalartery remains centrifugal (toward the periphery) and the direction of blood flow in theproximal vein remains centripetal (toward the heart). The direction of blood flow in thearterial and venous collaterals remains predictable with arterial collateral flow directedperipherally and venous collateral flow directed centrally. The direction of blood flow inthe artery distal to the fistula varies and may be antegrade, retrograde, or bidirectional.The reversal of blood flow in the artery just distal to the fistula is common, and NOTsufficient to diagnose ISS.Pressure gradients govern the direction and magnitude of blood flow in the arterydistal to the fistula and are related, in complex ways, to the hemodynamic resistances ofthe proximal artery, the arterial collaterals, the fistula, and the peripheral vascular bed(ddP = QR, where ddP is the pressure gradient, Q is the flow, and R is the resistance).Sumner’s description of the relationship using a simple electrical analogue (Ohm’s Law)provides the best conceptual understanding. In this model the artery immediately distal tothe fistula is represented by the cross arm of a Wheatstone bridge (Fig. 3B). The problemof flow through the cross arm of the Wheatstone bridge was first solved by CharlesWheatstone in 1843. Traditional methods of solving the equation require solving sixsimultaneous loop equations. A more recent technique, Maxwell’s method, uses both ofKirchoff ’s rules and reduces the number of simultaneous equations to three. The solutiondemonstrates that the pressure gradient between the two points is dictated by theresistance ratios of inflow to fistula and collateral to peripheral vascular bed. Thepressure in the artery at the level of the fistula (Fig. 3A, point C) is a function of the ratiobetween the resistance of the proximal artery (Fig. 3A, segment BC) and the fistula (Fig.3A, segment CE). The pressure in the artery distal to the fistula (Fig. 3A, point D) is afunction of the ratio of resistance of the arterial collaterals (Fig. 3A, segment BD) to thedistal vascular bed (Fig. 3A, segment DE). The importance of these relationships cannotbe overemphasized because the pressure gradient between these two points will dictatethe direction and magnitude of the blood flow in the artery distal to the fistula.Using this model, one can predict the effect of varying individual portions of thecircuit. For example, the model correctly predicts that increasing the peripheral vascularresistance favors the development of steal; increasing fistula resistance would favorantegrade flow in the distal artery. From a practical standpoint, deprivation of adequatedistal perfusion by the presence of either retrograde flow or low magnitude antegradeflow in the artery distal to the fistula defines ischemic steal syndrome.1134