Inflow stenosis obscures recognition of outflow stenosis by dialysis venous pressure: Analysis by a mathematical model

Background. Recent studies have shown that inflow stenosis of haemodialysis grafts is more common than previously realized. The influence of inflow stenosis on graft haemodynamics and venous pressure (VP) surveillance has not been previously systematically studied. Methods. We used a well-established mathematical model to determine the relation between inflow stenosis and static VP (adjusted for mean arterial pressure, VP/MAP), outflow stenosis and artery and vein luminal diameters. We applied low, median and high ratios of artery/vein diameters from 94 patients with grafts. The median ratio was 0.77, indicating that the artery was generally narrower than the vein. Results. The model shows that inflow stenosis reduces VP/MAP. More importantly, however, as outflow stenosis progresses, fixed inflow stenosis causes a delayed increase in VP/MAP followed by a rapid increase at critical outflow stenosis. When both stenoses progress together, their relative rates determine whether and how rapidly VP/MAP increases. The increase in VP/MAP is remarkably abrupt when the rate of inflow stenosis approaches that of outflow stenosis. No increase occurs when inflow stenosis progresses as fast or faster than outflow stenosis. Conclusion. Inflow stenosis exerts its most important haemodynamic effect through its interaction with outflow stenosis. As outflow stenosis progresses, inflow stenosis causes a delayed and then rapid increase in VP/MAP at critical outflow stenosis. This increase may not be detected before thrombosis unless measurements are very frequent. Inflow stenosis has an important impact on graft haemodynamics and surveillance because of its location in the relatively narrow inflow tract.