The delivery of hemodialysis is defined by the dialysis prescription. This prescription includes the duration of treatment, dialysate flow, blood flow, and the specific dialyzer. However, many patients are underdialyzed due, in part, to problems with hemodialysis access recirculation, resulting in dialysis delivery being less than that prescribed . (See "Prescribed versus delivered dialysis: Importance of dialysis time".)
One setting in which the presence of significant recirculation should be suspected is when there is an inadequate reduction in the postdialysis blood urea nitrogen (BUN), which should be less than 40 percent of the predialysis value. (See "Kt/V and the adequacy of hemodialysis", section on 'Urea reduction ratio'.)
Hemodialysis access recirculation occurs when dialyzed blood returning through the venous needle reenters the extracorporeal circuit through the arterial needle, rather than returning to the systemic circulation. It is important to measure hemodialysis access recirculation for two reasons:
MECHANISMS OF RECIRCULATION
Access recirculation occurs in conditions of low access flow. The most common cause is the presence of high-grade venous stenoses, which obstruct venous outflow, leading to backflow into the arterial needle.