Physical determinants of line pressures during apheresis

Line pressure changes are inherent to apheresis procedures, although little work has been done to determine their physical basis. We developed a mathematical model in which fluid pressure within apheresis tubing is primarily dependent upon viscosity and hematocrit (HCT). Return line flow rates and pressures during plasma collections for peripheral blood stem cell (PBSC) harvest and during plasma exchange procedures using the COBE Spectra were recorded and used to model return line pressures. Pressure estimates were similar to measured values for both PBSC plasma collection (r2 = 0.70) and plasma exchange (r2 = 0.86). HCT and viscosity thus appear to be primary physical determinants that underlie these pressure changes.