Changing systems for measuring activated clotting times: Impact on the clinical practice of heparin anticoagulation during cardiac surgery

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Abstract

Background

The activated clotting time (ACT) is a standard monitor for heparin anticoagulation during cardiopulmonary bypass (CPB). This study determines the effect of upgrading our ACT system on our clinical practice with regards to the conduct and safety of heparin anticoagulation during cardiopulmonary bypass.

Methods

We compared the intraoperative heparin doses required for all adult cardiac surgery patients (n=1240) and postoperative bleeding for a subset of primary aortocoronary bypass (CABG) surgery procedures (n=285) from cohorts before and after the change in ACT systems.

Results

The heparin dose needed to exceed our target ACT of 480 sec for the duration of CPB was higher (45 000 vs. 40 000 units; p<0.0001), and the mean ACT during CPB was lower (557 vs. 618 sec; p<0.05) using the new ACT system. Furthermore, this coincided with decreased postoperative bleeding in the CABG subset (median value of 417 vs. 575 ml over 12 h; p<0.0005).

Conclusions

We demonstrated that the introduction of the Actalyke® ACT system significantly altered our clinical practice by increasing the heparin dose required to exceed our target ACT during CPB. Prospective study to determine the effect of Actalyke® ACT system monitoring on hemostasis after cardiac surgery is merited.

Introduction

Activated Clotting Time (ACT) test systems are used to monitor heparin anticoagulation for over 500,000 cardiac operations per year in the U.S. alone. First described in 1966 [1], the ACT remains a standard point-of-care monitor for high dose heparin anticoagulation during cardiac surgery with extracorporeal circulation/cardiopulmonary bypass (CPB). Because the ACT responds to factors other than heparin [2], [3], the relationship between heparin levels and the ACT is variable [4] and could lead to underdosing of heparin during CPB [5]. It is important to adequately dose heparin during CPB to avoid sub-clinical thrombosis, leading to consumptive coagulopathy and increased bleeding, or the possibility of thrombus formation and systemic embolization [6].

Compared to fixed dose heparin anticoagulation, ACT-guided heparinization can reduce postoperative coagulopathy, blood transfusion, and blood loss when using the concept of a target ACT for safe and adequate anticoagulation during CPB [7], [8], [9]. The concept of a target ACT of >480 sec, to protect against underdosing the heparin during CPB, was introduced in 1975 [10] and this is our institutional standard. The numerous ACT systems commercially available use various activators and clot detection mechanisms, producing different results [11], [12]. We replaced the existing ACT system (Hemochron 801, International Technidyne, Edison, NJ) with an ACT technology that utilizes earlier clot detection methods and 3 clot activators (Actalyke® with MaxACT® activation, Helena Laboratories, Beaumont, TX), which has improved precision and records ACT values approximately 20% lower than the Hemochron 801 system [11]. This could lead to an increased requirement for heparin to maintain our target ACT during CPB, thereby affecting our day to day clinical practice. A meaningful clinical effect of this change could be inferred by comparing postoperative bleeding before and after the change in ACT systems [6], [13]. The aim of this current study was to determine the effect of the introduction of the Actalyke® ACT system on clinical practice by comparing heparin dosing during CPB and postoperative bleeding before and after the change in ACT systems.

Section snippets

Study design

This was a retrospective cohort study at a University cardiac surgical center spanning the time period before and after the change in ACT systems. With IRB approval, all patients above the age of 18 years undergoing cardiac surgery with CPB were included in the study. The operation was not specified as the same ACT target (>480 sec) is followed for all cardiac surgery patients. The following exclusion criteria were used: preexisting coagulopathy, emergency procedure (including administration of

Results

In the Hemochron 801 (old ACT) cohort, 584 patients were included between June 2001 and December 2001; in the Actalyke® cohort (new ACT), 620 patients were included between March 2002 and October 2002. Overall the procedures performed were CABG (69%), combined CABG/valve procedure (10%), and valve procedure alone (21%). Demographic variables, coexisting conditions and procedural factors were similar between the 2 groups (Table 1), except the incidence of preoperative myocardial infarction. As

Discussion

Our results demonstrate that increased heparin administration was required in the Actalyke® ACT group to exceed our target ACT of >480 sec, therefore the change in ACT systems did affect our day to day clinical practice. While we increased our heparin dosing, this did not appear to adversely affect bleeding, as we observed a decrease in postoperative blood loss after CABG surgery coincident with the higher heparin dosage used with the Actalyke® ACT system.

Our observations are consistent with

References (19)

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    Another observational study showed that many ACT tests correlated poorly with heparin level as assessed by anti-Xa plasma activity [28]. Patteril and associates [29] demonstrated that after switching their cohort population to a newer ACT device, the new instrument yielded a lower mean ACT value compared with temporal controls (557 versus 618 seconds, p < 0.05), and a higher dose of unfractionated heparin was needed to achieve a minimum ACT of 480 seconds [29]. A certain level of validation has been performed for other ACT instruments as well [30].

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