ORIGINAL ARTICLES A New Method of Measuring Heparin Levels in Whole Blood by Protamine Titration Using a Heparin-Responsive Electrochemical Sensor Joyce A. Wahr, MD, Jong-Hoon Yun, PhD, V~ctor C. Yang, PhD, Lai Ming Lee, Bin Fu, BS, and Mark E. Meyerhoff, PhD Objective: To determine the ability of a new electrochemi- cal sensor to determine heparin levels in whole blood and to correlate the heparin levels as determined by this heparin- responsive sensor (HRS) with heparin levels as determined by the Hepcon assay system (Medtronic Hemotec, Parker, CO). Design; Methods comparison study. Setting: A large academic medical center. Interventions: The heparin levels of 162 samples from 24 patients undergoing cardiopulmonary bypass were deter- mined by the HRS system and by the Hepcon system. In 21 samples, heparin levels as measured by anti-Xa activity were determined as well, Measurements and Main Results: HRS-determined values correlated highly with Hepcon-determined values (r = 0.942) and with anti-Xa determined values (r = 0.905}. Bias -+ precision comparing the HRS and Hepcon methods was 0,211 + 0.478 U/mL. Conclusions: HRS determined that whole blood heparin levels correlate well with Hepcon-determined levels, These limited results indicate that further development and testing of this new technology are warranted. Copyright © 1996by W.B. Saunders Company KEY WORDS: anticoagulation, heparin concentration, hemo- stasis, potentiometric sensors H EPARIN-INDUCED anticoagulation remains a criti- cal component of cardiac surgery. Cardiopulmonary bypass (CPB) systems actwate the coagulation process and result in widespread thrombosis without systemic anticoagu- lation. Based on work by Bull et al and Young et al, current recommendations for management of anticoagulation dur- ing CPB include maintenance of activated coagulation time (ACT) levels of 450 seconds or greater. 1,2 Despite the long record of safety associated with this protocol, thrombin activation and fibrin production occur even when appropri- ate ACT values are maintained. 3-5In addition, many factors such as hypothermla and hemodilutlon influence ACT values but not heparin concentration. Monitoring of hepa- rm levels may provide valuable additional reformation about the state of antlcoagulation during CPB. Appropriate management of anticoagulation during CPB requires immediately available results (5 to 10 minutes). The only currently available method for accurately and rapidly determining heparin levels in whole blood is the Hepcon HMS assay system (Medtronic Hemotec, Parker, CO), which has been shown to correlate well with laboratory- determined plasma anh-Xa levels. 6 The Hepcon uses clot formation of whole blood as an end point. Each assay cartridge contains four to six channels that contain different amounts of protamine as well as dilute thromboplastin. The channel in which the amount of protamine most closely neutralizes the heparin in the blood sample is the channel that will clot first. Each Hepcon cartridge tests a limited range of heparin levels. If the earhest channel to dot is the first (or last) channel, the true heparm level is either that of the channel, or any level below (or above) that channel. Often, multiple cartridges must be run, resulting in a system that is time-consuming, labor-intensive, and expensive. Nonetheless, the Hepcon assay is the most reliable and rapid point-of-care method, providing results within 60 to 120 seconds. Polymer membrane type ion-selectwe electrodes (ISE) are widely used in biomedical instruments to determine whole blood levels of chnicaUy sigmficant ions. The poten- tiometric response of the electrode to a specific anion can be precisely defined. Application of the same technology to quantification of larger biomolecules has been more diffi- cult. A polyanionic molecule such as heparin would not be expected to generate an analytically useful potentiometrlc response owing to the high charge (z = -70) of heparin. Nonetheless, preliminary reports have described an electro- chemical sensor composed of a polyvinyl chloride mem- brane impregnated with tridodecylmethylammonmm chlo- ride (TDMAC), which exhibits a large and reproducible potentiometric nonequllibrium response to heparin and other polyions with high charge density. 7,8 The presence of other anions in blood can interfere with the potentiometric signal caused by heparin. Accurate determination of hepa- rin levels with this heparm-responsive sensor (HRS), there- fore, requires quantitative titration with protamine. If the protamine added to the test sample does not fully neutral- ize the heparin present, a potential change, proportional to the amount of excess heparin, will be noted. If the prot- From the Departments of Anesthesiology, Pharmaceuttcs, and Chemtstry, The Untversttyof Mtchtgan, Ann Arbor, M[ Supported m part by NIH Grants HL 38353 and GM 28882 and by Medtromc Hemotec. Address repnnt requests to Joyce A. Wahr, MD, Department of Anestheszology, 1G323-0048, Umversztyof Mwhtgan Hospttals, 1500 E Me&cal CenterDr, Ann Arbor, 34148109-0048. Copyright © 1996 by W B Saunders Company 1053-0770/96/1004-000253 00/0 Journal of Cardlothoraclc and VascularAnesthesla, Vol 10, No 4 (June), 1996' pp 447-450 447