Heparin is usually added to infant cardiopulmonary bypass circuit primes. Ultrafiltration is often used to minimise prime volume before commencing bypass. The extent of heparin removal from bypass primes by ultrafiltration is unknown, however at our institution it was assumed that heparin is freely filtered. The primary aim of this study was to investigate heparin removal during pre-bypass ultrafiltration of a bypass prime for infants. The secondary aim was to investigate the effect of pre-bypass ultrafiltration on heparinization of the patient shortly after commencing bypass.

Patients under 1 year of age having cardiopulmonary bypass were enrolled. Prime solutions contained red blood cells, albumin, PlasmaLyte and 3 IU/ml heparin prior to pre-bypass ultrafiltration. Patient blood samples were collected before and after commencing bypass along with samples of the filtrate and the priming solution. Anti-Xa and antithrombin levels were measured by chromogenic assay.

Nineteen patients were enrolled. Patient weight ranged from 2.4 kg to 7.7 kg. Anti-Xa in the filtrate was 0.94 IU/ml (IQR 0.84 to 1.06 IU/ml). Anti-Xa in the primes was 6.80 IU/ml (IQR 6.68 to 7.84 IU/ml). Anti-Xa once on bypass was 3.31 IU/ml (IQR 2.08 to 4.46 IU/ml). Antithrombin level on bypass was 38 % (IQR 26 to 57 %). On bypass anti-Xa level was associated with patient weight and antithrombin level but not with activated clotting time.

Heparin is not freely filtered from the prime, leading to more heparin being present in the prime than desired. Anti-Xa levels on commencing bypass appear to be predictably influenced by hemodilution such that the gap between total heparin present and anti-Xa activity is wider in smaller patients. The activated clotting time does not differentiate lower levels of anti-Xa activity in the setting of extreme haemodilution.