To the Editor:—
In the 92nd volume of Anesthesiology, Latham et al. 1reported the use of recombinant hirudin (r-hirudin) as a cardiopulmonary bypass (CPB) anticoagulant in two patients with a history of heparin-induced thrombocytopenia of type II. Based on our experiences with the use of r-hirudin in this clinical setting, we would like to comment. Latham et al. 1used the activated partial thromboplastin time for intraoperative monitoring of hirudin. We have recently shown that activated partial thromboplastin time is not an adequate method to monitor plasma concentrations of r-hirudin greater than 1 μg/ml; ecarin clotting time is the method of choice to monitor hirudin during CPB. 2Another promising device is the HMT TAS analyzer (Cardiovascular Diagnostics, Raleigh, NC) 3for measurement of ecarin clotting time and activated clotting time, which is now commercially available.
Latham et al. 1described that the blood in the extracorporeal circuit clotted immediately after discontinuation of CPB. R-hirudin blood concentrations at this time might have been borderline. To keep r-hirudin blood concentration greater than 2.5 μg/ml, we administer additional 5-mg boluses during CPB. When CPB is stopped, 5 mg r-hirudin is administered into the CPB system, which is then run as a closed circuit until the blood can be returned to the patient. Any remaining volume in the machine is prepared by a cell saver to eliminate r-hirudin. In patients with renal impairment or high r-hirudin blood concentrations, we use hemofiltration with a cellulose acetate filter membrane and a cutoff point of 50,000 Da toward the end of CPB. 4,5
The first patient described in the case report 1had a history of heparin-induced thrombocytopenia type II 6 yr previously. Although the platelet factor 4 enzyme–linked immunosorbent assay was negative before surgery, r-hirudin was chosen as anticoagulant during CPB. Our strategy in patients with a history of heparin-induced thrombocytopenia type II but negative heparin-induced platelet aggregation test results is to treat these patients with unfractionated heparin during CPB and standard protamine protocol. After the end of surgery, we initiate an r-hirudin infusion for the first postoperative days to keep the activated partial thromboplastin time values between 40 and 60 s. Using this protocol, we treated six patients without thromboembolic, bleeding, or allergic complications.