Recirculation, urea disequilibrium, and dialysis efficiency: peripheral arteriovenous versus central venovenous vascular access.

When accurate, non-urea-based methods of measuring recirculation are used, recirculation is usually absent in arteriovenous (AV) accesses. When urea-based methods are used to measure recirculation in AV accesses, falsely elevated recirculation rates are common. These errors are due to AV and venovenous disequilibrium (peripheral vein method), delayed systemic sampling (two-needle methods), and errors in urea measurement (all methods). The literature suggests that recirculation in central venovenous (CV) catheters is approximately 5%. The methods used for these determinations have all been urea based. However, there are few theoretical problems in using urea-based measurements for measuring recirculation in this setting, making it more likely that these values are accurate. When hemodialysis via CV and AV accesses are compared, equilibrated Kt/V values differ significantly for the same single-pool Kt/V when 15-second postdialysis blood urea nitrogen values are used for modeling, but differ minimally when 2-minute postdialysis samples are used. The impact of transient retrograde blood flow in the superior vena cava on recirculation and whether dialysis efficiency is influenced by the exact site of CV catheter placement (superior vena cava v right atrium) is uncertain.