Rat Epigastric Flow-Through Flap as a Modulated Arteriovenous Fistula: Model for the Radial Artery Flow-Through Flap in Distal Arterial Bypass

ABSTRACT

The authors describe a rat flap model that is useful for flow studies. It is an epigastric flow-through flap that mimics the clinical use of a radial artery flow-through (RAFT) flap that has been used as an adjunct to a distal lower extremity arterial bypass graft to improve patency when there is potential high outflow resistance. The hypotheses were that this RAFT flap serves two purposes: 1) it allows additional blood flow through the skin flap and drainage via the vena comitans to increase the blood flow through the bypass graft and help to maintain bypass graft patency; and 2) it acts as a modulating arteriovenous fistula in which the additional flow through the vena comitans of the flow-through flap fluctuates with distal arterial outflow resistance.

The rat epigastric flow-through flap model was designed to test these hypotheses. High outflow resistance was induced by sequentially ligating the outflow vessels of the rat femoral artery. Using this model, an increase in blood flow to the skin via the epigastric artery of the flow-through flap was demonstrated as outflow obstruction increased. Then, the patency rates of the flow-through flap bypass were compared to an interpositional arterial graft. The flow-through flap maintained patency while the arterial interposition bypass thrombosed, with near total outflow obstruction induced by serial ligation of the outflow vessels (75 percent patent anastomoses at 1 week for flow-through flap vs. 0 percent for the arterial graft). This flow study demonstrates the inherent ability of the flow-through flap to divert blood flow through the skin capillaries when there is high arterial outflow resistance.

The authors believe that a flow-through flap such as the RAFT flap can be an important adjunct to the traditional distal arterial bypass in a subset of patients with high outflow resistance in the recipient artery.

KEYWORDS

Rat epigastric flow-through flap - bypass graft patency - high outflow resistance

REFERENCES

• 1 Chun J K, Marin M L. Radial artery flow-through (RAFT) flap for distal lower extremity arterial bypass for limb salvage (in press).  Plast Reconstr Surg . 2003; 
• 2 Lau H, Cheng S W. Long-term outcome of aortofemoral bypass for aortoiliac occlusive disease.  Ann Acad Med Singapore . 2000;  29 434-438
• 3 Syrek J R, Calligaro K D, Dougherty M J. Do distal arteriovenous fistulae improve rates of prosthetic infrapopliteal arterial bypasses?.  Ann Vasc Surg . 1998;  12 148-152
• 4 Sogaro F, Galeazzi E, Amroch D, Ganassin L. Pantaloon vein graft technique in tibial revascularization with arteriovenous fistula for limb salvage.  Cardiovasc Surg . 1996;  4 377-380
• 5 Ascer E, Gennaro M, Pollina R M. Complementary distal arteriovenous fistula and deep vein interposition: a five-year experience with a new technique to improve infrapopliteal prosthetic bypass patency.  J Vasc Surg . 1996;  24 134-143
• 6 Robinson P H, van der Lei B, Jongebloed W L. Prosthetic microvenous grafting into the femoral vein of the rat: considerations concerning the anastomotic technique.  Br J Plast Surg . 1989;  42 538-543
• 7 Dardik H, Berry S M, Dardik A. Infrapopliteal prosthetic graft patency by use of the distal adjunctive arteriovenous fistula.  J Vasc Surg . 1991;  13 685-690
• 8 Cuadros C L, Hughes J E. Patency of synthetic microvenous conduits: an experimental study in the femoral vein of the rat.  Plast Reconstr Surg . 1986;  78 378-382
• 9 Calligaro K D, Ascer E, Torres M, Veith F J. The effect of adjunctive arteriovenous fistula on prosthetic graft patency: a controlled study in a canine model.  J Cardiovasc Surg . 1990;  31 646-650
• 10 Gibson K D, Caps M T, Gillen D. Identification of factors predictive of lower extremity vein graft thrombosis.  J Vasc Surg . 2001;  33 24-31
• 11 Hida M, Iida T, Shimbo T. Report on blood access for hemodialysis in the Department of Transplantation: Tokai University School of Medicine and a satellite hospital (in Japanese).  Tokai J Exp Clin Med . 1982;  7 497-504
• 12 West J C, Bertsch D J, Peterson S L. Arterial insufficiency in hemodialysis access procedures: correction by “banding” technique.  Transplant Proc . 1991;  23 1838-1840

Maintenance of distal lower extremity perfusion with restricted runoff is a major challenge to vascular surgeons. Over the past five decades, various means have been established to augment perfusion of the ischemic limb. Nevertheless, the limitation of runoff may frustrate attempts at limb salvage, since the combination of a high resistance and a low flow distal runoff circuit will result in predictable thrombosis of a bypass graft. This is particularly true if prosthetics are employed for bypass, since the critical thrombotic threshold for these graft materials is much lower than that for autologous vein. However impressive the progress in peripheral vascular surgical techniques in providing pulsatile flow to the ischemic lower extremity, the limiting factor remains the capacity of the distal arterial circuit to accept adequate inflow to nourish the extremity.[1]

The theoretical basis for using distal arteriovenous fistulas (dAVF) as adjuncts to crural reconstructions, in order to maintain graft patency and distal perfusion, is based on the need to reduce the vascular overload being presented to the distal circuit and, at the same time, to keep graft flow over the critical thrombotic threshold level.[2] Establishing such a vent results in an increased flow in the graft, most of which is then directed into the high capacitance venous circulation. The amount of blood that can be accepted by the arterial runoff, albeit limited, will perfuse distally and reverse the ischemic state. We have been impressed by the minimal increment of blood required to effect this change. Intra-operative flow studies have demonstrated a trebling of graft flow with an adequate dAVF. The predominant flow is through the fistula, but arterial flows of less than 60 ml/min have been maintained and have resulted in limb salvage.

Over the past decade, there has been a slow but sure increase in the number of experimental and clinical studies dealing with the creation of a distal arteriovenous fistula.[3] [4] [5] [6] [7] Teodorescu et al.[8] have now described an intriguing concept and procedure that appear to be able to increase blood flow through a bypass graft and thereby maintain graft patency, as well as permitting additional flow through a skin flap with drainage via the vena comitans. Furthermore, the authors postulate that the skin paddle with its own afferent and efferent vascular system acts as a modulator of blood flow, by permitting fluctuation of blood flow in relation to distal arterial outflow resistance. Clinical experience by the same group with a small group of patients requiring revascularization with the aid of a radial artery flow-through (RAFT) graft conduit appears to substantiate the authors' hypothesis and experimental data provided in this manuscript. There are, however, several aspects of their experimental and clinical preparation that require critical analysis and clarification. The authors describe their flow-through flap as serving a function similar to an arteriovenous fistula, but having the significant advantage of being self-modulating. This conclusion is a quantum leap from the experimental data which are limited and, in fact, represents an assumption based on an observed relationship rather than fact.

A clear difference between the dAVF and flow-through flap concepts is the existence of a tissue bed between the arterial and venous systems of the latter that provides its own resistance factor to flow, compared to the former where there is direct flow from the arterial system into the venous drainage. Chun and associates believe that the arteriovenous component of the flap tissue bed can act as an auto-modulator of blood flow. That this does not occur in the direct dAVF model is truly unknown, despite the authors' assertion. I would suspect that there is indeed a variable venous return in even the direct dAVF model which would relate to the runoff resistance.

The authors' work is provocative and needs to be amplified with additional studies that would demonstrate flow in the venous circuit by delayed arteriography and duplex sonography. These studies would aid in defining the uniqueness of this model in being a true arteriovenous fistula with an interposed capillary bed in the skin paddle. Although there is some support from the authors' hemodynamic data, there is a missed opportunity here to prove the modulation of flow other than by an increase associated with staged ligation of runoff vessels, all performed in a very small group of animals. The authors' technique is, in fact, an experimental model of the free flap procedure, an established procedure generally performed by plastic surgeons. I have no doubt that the flow rates are increased, as described in their work, but is this model truly an arteriovenous fistula? Further analysis is necessary to define the resistance across the tissue flap capillary bed, as opposed to flow that occurs in a direct open arteriovenous fistula.

This work is an important contribution in that the authors appropriately emphasize the critical inverse relationship of distal runoff resistance and graft patency. Adjunctive measures to alter this relationship, that is, improved graft patency with a fixed high runoff resistance, have included such measures as use of patches, cuffs, and fistulas. Although the former two can impact on flow patterns, it is only a fistula or analogous flow system that can alter hemodynamics and physiology, and thereby affect volume flow and velocity flow. It appears that the use of a flow-through flap may offer similar advantages, although without being a direct arteriovenous fistula. The authors are certainly to be congratulated for bringing this novel concept for securing limb salvage to our attention. Further effort is essential to define the hemodynamics of this model in relation to direct flow fistulas, and thereby eventually enable precise clinical deployment.

REFERENCES

• 1a Dardik H, Sussman B, Ibrahim I M. Distal arteriovenous fistula as an adjunct to maintaining arterial graft and patency for limb salvage.  Surgery . 1983;  94 478-485
• 2a Dardik H. The use of an adjunctive arteriovenous fistula in distal extremity bypass grafts with outflow obstruction. In: Kempczinski RF, ed. The Ischemic Leg Chicago, IL: Year Book Medical Publishers Inc., 1985: 463-474
• 3a Cuadros C L, Hughes J E. Patency of synthetic microvenous conduits: an experimental study in the femoral vein of the rat.  Plast Reconstr Surg . 1986;  78 378-382
• 4a Sogaro F, Galeazzi E, Amroch D, Ganassin L. Pantaloon vein graft technique in tibial revascularization with arteriovenous fistula for limb salvage.  Cardiovasc Surg . 1996;  4 377-380
• 5a Ascer E, Gennaro M, Pollina R M. Complementary distal arteriovenous fistula and deep vein interposition: a five-year experience with a new technique to improve infrapopliteal prosthetic bypass patency.  J Vasc Surg . 1996;  24 134-143
• 6a Calligaro K D, Caps M T, Gillen D. Identification of factors predictive of lower extremity vein graft thrombosis.  J Vasc Surg . 2001;  33 24-31
• 7a Qin F, Dardik H, Pangilinan A. Remodeling and suppression of intimal hyperplasia of vascular grafts with a distal arteriovenous fistula in a rat model.  J Vasc Surg . 2001;  34 701-706
• 8a Teodorescu V J, Chun J K, Morrisey N J. Radial artery flow-through graft: a new conduit for limb salvage.  J Vascular Surg . 2003;  37 816-820