caveat:
More flow is better than low flow. Low flow is better than no flow.
The ‘fem-pop’ bypass graft is a common surgical procedure to treat insufficient blood flow to the leg. In the vast majority of cases the flow to the leg is improved and symptoms are alleviated. More flow is better than low flow. In 7-10% of cases the graft clots off shortly after the procedure resulting in no flow. This may result in loss of part of the leg. Low flow is better than low flow.
The loss of perfusion following the operation should make us wonder why the situation worsens after the procedure and thus wonder what can be done to predict who is about to clot and what can be done to prevent the clot.
To state the important factor as succinctly as possible,
**** Blood is a non-Newtonian fluid!! ****
This means that the viscosity depends on the conditions of flow. The viscosity of blood is constant over a wide range of volume flow conditions such as high flow in a wide lumen tube and low flow in a narrow tube. The viscosity, however, increases if the flow is low in a wide lumen tube.
Consider a situation where there exists a long segment of narrow tube and an appropriately low rate of flow through the tube. We now replace the middle length of the tube with a much larger lumen of tube. The outer two-thirds of tubing maintain the appropriately low flow while the blood in the middle third flows at a low rate in a much larger lumen. The viscosity of this blood increases which leads to even lower flow. This lower flow leads to a further increase in viscosity and this results in an even further reduction in flow. If this spiral continues then the result is stoppage of flow and clotting. The clotting occurs as a result of the diminishing flow, not as a deficiency of anti-coagulants. The rational treatment of this viscosity problem is to address the non-Newtonian issue of the blood.
The relevance to the fem-pop bypass graft is obvious.
Fluid flows through a tube in a layered or ‘laminar’ fashion. The fluid in the center of the tube flows the fastest while the layer at the wall of the tube is motionless relative to the wall. There is a velocity gradient between the minimum of zero at the wall and the maximum velocity at the center. As the layers pass over each other the layers interact with each other. This is mainly due to the red blood cells in adjacent layers being reversibly cross-linked by the long protein molecule fibrinogen. The faster these layers slide over each other the less opportunity for this cross-linkage. The opportunity for this cross-linkage can also be reduced simply by reducing the concentration of red blood cells in the blood, or reducing the concentration of fibrinogen or by reducing both.
This downward spiral is an issue in a small minority. Deciding who might benefit from viscosity directed treatment can be aided with ultrasound analysis of the graft site and adjacent vasculature. The ultrasound can reveal both the lumen size of the vessels and the flow velocity gradient of the blood within the lumen.