Theoretical and practical issues in recirculation; assessment of vascular access.

Haemodialysis recirculation is defined as the fraction of cleared extracorporeal blood flow which returns to the inlet of the extracorporeal blood line without systemic equilibration. There are two components of haemodialysis recirculation: the local component is related to access function and placement of access needles; the cardiopulmonary component is a characteristic of the peripheral arterio-venous access where access blood flow bypasses systemic tissue compartments. Identification of access problems requires separation of the two components using newly developed indicator dilution techniques such as ultrasound dilution. If such techniques are not available and recirculation is determined by techniques which measure combined effects of recirculation such as the urea technique, a second recirculation measurement with reversed placement of blood lines will permit us to distinguish between correct and reversed placement of blood lines. The larger of the two recirculation values can be used to identify accesses with insufficient access flow and access recirculation which require immediate intervention.