Ok, you sent me down a rabbit hole, but it was worth it because it turns out that the heparin used in HELP is utilized entirely in the filtering/precipitation process outside of the patient and the heparin is removed before the plasma is returned to the patient. This means that the mechanism of HELP has to do solely with the removal of clotting factors and other components from the serum.
https://vpjournal.net/article/view/3015
[QUOTE = The main effect of H.E.L.P.-apheresis is the elimination of atherogenic lipoproteins due to precipitation. During H.E.L.P.-apheresis, atherogenic lipoproteins and plasma fibrinogen precipitate on-line in the presence of high heparin doses and acetate buffer. Primarily, blood passes through the plasma filter (surface area 0.3-0.5 m2, rate 60-80 mL/min). Red blood cells are returned to the patient, and plasma is mixed with acetate buffer (pH = 4.85) in the ratio 1:1 and with heparin solution (100 U/mL). This acidic mixture (pH = 5.12) reaches the precipitating filter with the rate 20-30 mL/min (25%-30% of blood flow) and precipitates there with further deposition of insoluble sediments of LDL, Lp(a), triglycerides and fibrinogen. Heparin excess is eliminated from plasma on heparin adsorber (DEAE of cellulose). Bicarbonate dialysis is used for restoration of plasma pH. After that plasma is returned to the patient in combination with red blood cells.