Design of a protamine bioreactor for extracorporeal heparin removal.

Abstract

The removal of the anticoagulant heparin from the blood after an extracorporeal circulation (ECBC) using a protamine bioreactor, a device containing hollow fibers immobilized with protamine, is proposed. Extracorporeal circulation is used in many surgical procedures such as cardiopulmonary bypass, hemofiltration, and hemodialysis. After the procedure, heparin is neutralized by injection of protamine to prevent bleeding. Possible side effects of protamine range from mild (rash and an increase in pulmonary artery pressure) to severe (bronchospasm, hypotension, cardiovascular collapse and even death). Patients previously exposed to protamine through surgery or protamine-containing insulin have a higher risk. Under ECBC conditions, 288 mg heparin needed to be removed with a 50g bioreactor. Thus, fibers with the capacity to adsorb 5.76 mg heparin/g fiber at 4 U/mL and 2000 mL/min were required. To ascertain whether or not the approach was feasible, an <italic>in vitro</italic> plug flow model along with recirculation and <italic>ex vivo</italic> two-compartmental models were developed. The recirculation model of a protamine bioreactor with ideal heparin adsorption capacity (K_EQC_S = 144) during ECBC revealed that a larger but reasonably sized reactor was required. Next, fibers with better heparin adsorption capacity were synthesized and characterized using linear isotherms. Protamine, polylysine-protamine (PLLP), diamino polyethylene glycol-protamine (dPEGP), and polylysine-diamino polyethylene glycol-protamine (PLL-dPEG-P) fibers were synthesized using cyanogen bromide and 4-(oxyacetyl)phenoxyacetic acid as immobilization methods. Protamine fibers adsorbed a maximum of 20 +/- 1.4 mg heparin/g fiber with K_EQC_S of 8.7 (or 0.348 mg heparin/g fiber at 4 U/mL). Polylysine, used as a surface amplification agent, increased maximum PLLP fiber adsorption capacity to 39.0 +/- 8.8 mg heparin/g fiber with K_EQC_S of 8.8 (0.352 mg heparin/g fiber at 4 U/mL). Diamino polyethylene glycol as a spacer arm increased maximum dPEGP fiber adsorption capacity to 29.6 +/- 7.7 mg heparin/g fiber with K_EQC_S equal to 6.6 (0.264 mg heparin/g fiber). PLL-dPEG-protamine fibers unexpectedly did not increase protamine immobilization or heparin adsorption of fibers. Of all the fibers synthesized, the highest adsorption capacity attained had K_EQC_S equal to 14, much less than the goal of K_EQC_S equal to 144. Even though the desired adsorption was not achieved, the experimental data from the various protamine fibers was used to verify the model in order to account for addition mass transfer effects within the system.