Activated Coagulation Time

The ACT was first introduced by Hattersley in 1966 and is still the most widely used monitor of heparin effect during cardiac surgery. Whole blood is added to a test tube containing an activator—diatomaceous earth (celite) or kaolin. The presence of activator augments the contact activation phase of coagulation, which stimulates the intrinsic coagulation pathway. ACT can be performed manually, whereby the operator measures the time interval from when blood is injected into the test tube to when clot is seen along the sides of the tube. More commonly, the ACT is automated as it is in the Hemochron and Hemotec systems. In the automated system, the test tube is placed in a device that warms the sample to 37°C. The Hemochron device (International Technidyne Corp., Edison, NJ) rotates the test tube, which contains celite activator and a small iron cylinder, to which 2 mL of whole blood is added. Before clot forms, the cylinder rolls along the bottom of the rotating test tube. When clot forms, the cylinder is pulled away from a magnetic detector, interrupts a magnetic field, and signals the end of the clotting time. Normal ACT values range from 80 to 120 seconds. The Hemochron ACT can also be performed using kaolin as the activator in a similar manner (Fig. 12-1).

Figure 12-1 The Hemochron Response is a dual-chamber point-of-care coagulation monitor that is capable of measuring clotting times that are compatible with Hemochron technology. This system has software capability for calculation, data management, and storage of results.

(Courtesy of International Technidyne Corp., Edison, NJ, http://www.itcmed.com)

The Hemotec ACT device (Medtronic Hemotec, Parker, CO) is a cartridge with two chambers that contain kaolin activator and is housed in a heat block. Blood (0.4 mL) is placed into each chamber and a daisy-shaped plunger is raised and passively falls into the chamber. The formation of clot will slow the rate of descent of the plunger, and this decrease in velocity of the plunger is detected by a photo-optical system that signals the end of the ACT test. The Hemochron and Hemotec ACTs have been compared in a number of investigations and have been found to differ significantly at low heparin concentrations.¹ However, differences in heparin concentration, activator concentration, and the measurement technique make comparison of these tests difficult and have led to the realization that the Hemochron ACT result and the Hemotec ACT result are not interchangeable. In adult patients given 300 U/kg of heparin for cardiopulmonary bypass (CPB), the Hemochron and Hemotec (Hepcon) ACTs were both therapeutic at all time points; however, at two points, the Hemochron ACT was statistically longer. This difference was even more pronounced in pediatric patients, who have higher heparin consumption rates. The apparent “overestimation” of ACT by the Hemochron device during hypothermic CPB may be due to the different volumes of blood that each assay warms to 37°C.

The ACT test can be modified by the addition of heparinase. Using this modification, the coagulation status of the patient can be monitored during CPB while the anticoagulant effects of heparin are eliminated. Because this test is a side-by-side comparison of the untreated ACT to the heparinase ACT, it also has the advantage of being a rapid test for the assessment of a circulating heparin-like substance or for residual heparinization after CPB.

With the introduction of ACT monitoring into the cardiac surgical arena, clinicians have been able to more accurately titrate heparin and protamine dosages. As a result, many investigators report reductions in blood loss and transfusion requirements, although many of these studies used retrospective analyses. The improvements in postoperative hemostasis documented with ACT monitoring are potentially attributed to better intraoperative suppression of microvascular coagulation and improved monitoring of heparin reversal with protamine.

ACT monitoring of heparinization is not without pitfalls, and its use has been criticized because of the extreme variability of the ACT and the absence of a correlation with plasma heparin levels (Fig. 12-2). Many factors have been suggested to alter the ACT, and these factors are prevalent during cardiac surgical procedures. When the extracorporeal circuit prime is added to the patient’s blood volume, hemodilution occurs and may theoretically increase ACT. Evidence suggests that this degree of hemodilution alone is not enough to actually alter ACT. Hypothermia increases ACT in a “dose-related” fashion. It has been shown that although hemodilution and hypothermia significantly increase the ACT of a heparinized blood sample, similar increases do not occur in the absence of added heparin. The effects of platelet alterations are a bit more problematic. At mild to moderate degrees of thrombocytopenia, the baseline and heparinized ACT are not affected. It is not until platelet counts are reduced to below 30,000 to 50,000/μL that ACT may be prolonged. Patients treated with platelet inhibitors such as prostacyclin, aspirin, or platelet membrane receptor antagonists have a prolonged heparinized ACT compared with patients not treated with platelet inhibitors. This ACT prolongation is not exclusively related to decreased levels of platelet factor 4 (PF4) (PF4 is a heparin-neutralizing substance), because it also occurs when blood is anticoagulated with substances that are not neutralized by PF4. Platelet lysis, however, significantly shortens the ACT due to the release of PF4, and other platelet membrane components, which may have heparin-neutralizing activities. Anesthesia and surgery also decrease the ACT and create a hypercoagulable state, possibly by creating a thromboplastic response or through activation of platelets.

Figure 12-2 Anticoagulation measured at baseline (−60 minutes), heparinization (−30 minutes), and six time points after institution of cardiopulmonary bypass (CPB). Note the close correlation between the anti–factor Xa (Xa) activity and whole blood heparin concentration (WBHC), which does not parallel the change in Hemochron (HC ACT) or Hemotec activated coagulation time (HT ACT).

(Modified from Despotis GJ, Summerfield AL, Joist JH: Comparison of activated coagulation time and whole blood heparin measurements with laboratory plasma anti-Xa heparin concentration in patients having cardiac operations. J Thorac Cardiovasc Surg 108:1076-1082, 1994.)

During CPB, heparin decay varies substantially and its measurement is problematic because hemodilution and hypothermia alter the metabolism of heparin. In a CPB study, the consumption of heparin varied from 0.01 to 3.86 U/kg/min and there was no correlation between the initial sensitivity to heparin and the rate of heparin decay. In the pediatric population, the consumption of heparin is increased above that of adult levels. The heparin administration protocol for pediatric patients undergoing CPB should account for a large volume of distribution, increased consumption, and a shorter elimination half-life. In monitoring the effects of heparin in pediatric patients, the minimum acceptable ACT value should be increased or an additional monitor should be used.

Heparin Resistance

Heparin resistance is documented by an inability to raise the ACT of blood to expected levels despite an adequate dose and plasma concentration of heparin. In many clinical situations, especially when heparin desensitization or a heparin inhibitor is suspected, heparin resistance can be treated by administering increased doses of heparin in a competitive fashion. If an adequately prolonged clotting time is ultimately achieved using higher-than-expected doses of heparin, a better term than heparin resistance would be heparin tachyphylaxis or “altered heparin responsiveness.” During cardiac surgical procedures, the belief that a safe minimum ACT value of 300 to 400 seconds is required for CPB is based on a few clinical studies and a relative paucity of scientific data. However, inability to attain this degree of anticoagulation in the heparin-resistant patient engenders the fear among cardiac surgical providers that the patient will experience a microvascular consumptive coagulopathy or that clots will form in the extracorporeal circuit.

Many clinical conditions are associated with heparin resistance. Sepsis, liver disease, and pharmacologic agents represent just a few (Table 12-1). Many investigators have documented decreased levels of antithrombin III (ATIII) secondary to heparin pretreatment, whereas others have not found decreased ATIII levels.² In patients receiving preoperative heparin infusions, lower baseline ACT was the only risk factor found for predicting heparin resistance compared with patients not receiving preoperative heparin.

Table 12-1 Disease States Associated with Heparin Resistance

ATIII = antithrombin III; DIC = disseminated intravascular coagulation.

Patients receiving preoperative heparin therapy traditionally require larger heparin doses to achieve a given level of anticoagulation when that anticoagulation is measured by the ACT. Presumably, this “heparin resistance” is due to deficiencies in the level or activity of ATIII. Other possible causes include enhanced factor VIII activity and platelet dysfunction causing a decrease in ACT response to heparin. In vitro addition of ATIII enhances the ACT response to heparin. ATIII concentrate is now available and represents a reasonable method of treating patients with documented ATIII deficiency.³

Heparin-Induced Thrombocytopenia

The syndrome known as heparin-induced thrombocytopenia (HIT) develops in anywhere from 5% to 28% of patients receiving heparin. HIT is commonly categorized into two subtypes. HIT type I is characterized by a mild decrease in platelet count and is the result of the proaggregatory effects of heparin on platelets. HIT type II is considerably more severe, most often occurs after more than 5 days of heparin administration (average onset time, 9 days), and is mediated by antibody binding to the complex formed between heparin and PF4. Associated immune-mediated endothelial injury and complement activation cause platelets to adhere, aggregate, and form platelet clots, or “white clots.” Among patients developing HIT II, the incidence of thrombotic complications approximates 20%, which in turn may carry a mortality rate as high as 40%. Demonstration of heparin-induced proaggregation of platelets confirms the diagnosis of HIT type II. This can be accomplished with a heparin-induced serotonin release assay or a specific heparin-induced platelet activation assay. A highly specific enzyme-linked immunosorbent assay for the heparin/PF4 complex has been developed and has been used to delineate the course of IgG and IgM antibody responses in patients exposed to unfractionated heparin during cardiac surgery. Bedside antibody tests are being developed that may speed the diagnosis of this condition.

The options for treating these patients are few. If the clinician has the luxury of being able to discontinue the heparin for a few weeks, often the antibody disappears and allows a brief period of heparinization for CPB without complication.⁴ Changing the tissue source of heparin was an option when bovine heparin was predominantly in use. Some types of low-molecular-weight heparin (LMWH) have been administered to patients with HIT, but reactivity of the particular LMWH with the patient’s platelets should be confirmed in vitro. Supplementing heparin administration with pharmacologic platelet inhibition using prostacyclin, iloprost, aspirin, or aspirin and dipyridamole has been reported, all with favorable outcomes. Tirofiban with unfractionated heparin has been used in this clinical circumstance. Plasmapheresis may be used to reduce antibody levels. The use of heparin could be avoided altogether through anticoagulation with direct thrombin inhibitors such as argatroban, hirudin, or bivalirudin. These thrombin inhibitors have become standard of care in the management of the patient with HIT II. Monitoring their effects during CPB is more complex.

Measurement of Heparin Sensitivity

Even in the absence of heparin resistance, patient response to an intravenous bolus of heparin is extremely variable. The variability stems from different concentrations of various endogenous heparin-binding proteins such as vitronectin and PF4. This variability exists whether measuring heparin concentration or the ACT; however, variability seems to be greater when measuring the ACT. Because of the large interpatient variation in heparin responsiveness and the potential for heparin resistance, it is critical that a functional monitor of heparin anticoagulation (with or without a measure of heparin concentration) be used in the cardiac surgical patient. A threefold range of ACT response to a 200-U/kg heparin dose and similar discrepancy in heparin decay rates was documented, and therefore, the use of individual patient dose-response curves is needed to determine the optimal heparin dose. This is the concept on which point-of-care individual heparin dose-response (HDR) tests are based.

An HDR curve can be generated manually using the baseline ACT and the ACT response to an in vivo or in vitro dose of heparin. Extrapolation to the desired ACT provides the additional heparin dose required for that ACT. Once the actual ACT response to the heparin dose is plotted, further dose-response calculations are made based on the average of the target ACT and the actual ACT (Fig. 12-3). This methodology was first described by Bull and associates and forms the scientific basis for the automated dose-response systems manufactured by Hemochron and Hemotec. The Hemochron RxDx system uses the heparin-response test (HRT), which is an ACT with a known quantity of in vitro heparin (3 IU/mL). Using an algorithm that incorporates the patient’s baseline ACT, estimated blood volume, and HRT, a dose-response curve is generated that enables calculation of the heparin dose required to attain the target ACT. The patient’s heparin sensitivity can be calculated in seconds/IU/mL by dividing the HRT by 3 IU/mL.

Figure 12-3 Construction of a dose-response curve for heparin. ACT = activated coagulation time.

(From Bull BS, Huse WM, Brauer FS, et al: Heparin therapy during extracorporeal circulation: II. The use of a dose-response curve to individualize heparin and protamine dosage. J Thorac Cardiovasc Surg 69:685-689, 1975.)

The RxDx system also provides an individualized protamine dose using the protamine response test (PRT). This is an ACT with one of two specific quantities of protamine, depending on the amount of circulating heparin suspected (2 IU/mL or 3 IU/mL). Using the patient’s heparinized ACT, the PRT, and an estimate of the patient’s blood volume, the protamine dose needed to return the ACT to baseline can be calculated based on a protamine-response curve. Jobes and coworkers reported that the heparin dose directed by the RxDx system resulted in ACT values well above the target ACT. In their patients, in vivo heparin sensitivity was higher than in vitro sensitivity. RxDx also resulted in lower protamine doses, lower postoperative mediastinal tube losses, and reduced transfusion requirements compared with a ratio-based system of heparin/protamine administration.⁵ In a larger study that standardized the treatment of heparin rebound, the reduced protamine dose was confirmed; however, the reductions in bleeding were not substantiated. The use of a protamine dose-response curve has been shown to successfully reduce the protamine dose in vascular surgery compared with standard weight-based protamine dosing.⁶

The Hepcon HMS system uses the HDR cartridge in the Hepcon instrument (Fig. 12-4). Each cartridge houses six chambers. Chambers 1 and 2 contain heparin at a concentration of 2.5 U/mL, chambers 3 and 4 contain heparin at a concentration of 1.5 U/mL, and chambers 5 and 6 do not contain heparin. Once information regarding patient weight, height, and CPB prime volume is entered, the information that can be obtained from this test includes the baseline ACT (chambers 5 and 6) and an HDR slope. The dose-response slope, which is the increase in ACT from 1.5 U/mL to 2.5 U/mL heparin, is extrapolated to the desired target ACT or target heparin concentration and the heparin dose is calculated.

Figure 12-4 The HMS heparin management system has an automated dispenser that places the appropriate volume of whole blood into each chamber of the test cartridge. A variety of assays can be performed in this instrument, depending on the cartridge used.

(Courtesy of Medtronic Inc., Parker, CO, http://www.medtronic.com/cardsurgery/bloodmgmt/hmsplus.html)

Heparin Concentration

Proponents of ACT measurement to guide anticoagulation for CPB argue that a functional assessment of the anticoagulant effect of heparin is mandatory and that the variability in ACT represents a true variability in the coagulation status of the patient. Opponents argue that during CPB the sensitivity of the ACT to heparin is altered and ACT does not correlate with heparin concentration or with anti–factor Xa activity measurement. Heparin concentration can be measured using the Hepcon HMS system (Medtronic Hemotec), which uses an automated protamine titration technique. With a cartridge with four or six chambers containing tissue thromboplastin and a series of known protamine concentrations, 0.2 mL of whole blood is automatically dispensed into the chambers. The first channel to clot is the channel whose protamine concentration most accurately neutralizes the heparin without a heparin or a protamine excess. Because protamine neutralizes heparin in the ratio of 1 mg of protamine per 100 U of heparin, the concentration of heparin in the blood sample can be calculated. A cartridge that monitors heparin concentration over a wide range can be used first, followed by another cartridge that can measure heparin concentrations within a narrower range. The maintenance of a stable heparin concentration rather than a specific ACT level usually results in higher doses of heparin being administered because the hemodilution and hypothermia on CPB increase the sensitivity of the ACT to heparin. The measure of heparin concentration has been shown to more closely correlate with anti–factor Xa activity measurements than the ACT during CPB, although the precision and bias of the test may not prove to be acceptable for exclusive use clinically.

In a prospective randomized trial, Despotis and colleagues demonstrated that by using a transfusion algorithm in association with Hepcon-based heparin management, chest tube drainage was minimally reduced and transfusion of non–red blood cell products could be significantly reduced relative to a group of patients who had ACT-based heparin management.⁷ They attributed their results to better preservation of the coagulation system by high heparin doses because the doses of heparin administered in the Hepcon group were nearly twice the doses used in the ACT management group. The Hepcon, remains one of the more sensitive tests for detecting residual heparinization after protamine reversal because the heparin concentration can be measured by protamine titration to levels as low as 0.4 IU/mL.

High-Dose Thrombin Time

A functional test of heparin-induced anticoagulation that correlates well with heparin levels is the high-dose thrombin time (HiTT; International Technidyne Inc., Edison, NJ). The TT is a clotting time that measures the conversion of fibrinogen to fibrin by thrombin. The TT is prolonged by the presence of heparin and by hypofibrinogenemias or dysfibrinogenemias. Because the TT is sensitive to very low levels of heparin, a high dose of thrombin is necessary in the TT to accurately assay the high doses of heparin used for CPB. The HiTT is performed by adding whole blood to a prewarmed, prehydrated test tube that contains a lyophilized thrombin preparation. After the addition of 1.5 mL of blood, the tube is inserted into a Hemochron well and the time to clot formation is measured. In vitro assays indicate that HiTT is equivalent to the ACT in evaluation of the anticoagulant effects of heparin at heparin concentrations in the range of 0 to 4.8 IU/mL. Unlike ACT, HiTT is not altered by hemodilution and hypothermia and has been shown to correlate better with heparin concentration than the ACT during CPB. While on CPB, heparin concentration and HiTT decrease while the Hemochron and the Hepcon ACT increase. Another potential advantage of HiTT monitoring occurs for patients receiving aprotinin therapy. In the presence of heparin, aprotinin augments the celite ACT, possibly because its kallikrein-inhibiting capacity prolongs activation of the intrinsic coagulation pathway by XIIa. This should not be interpreted to represent enhanced anticoagulation. The kaolin ACT is less affected by aprotinin therapy than the celite ACT, perhaps because kaolin, unlike celite, activates the intrinsic pathway by stimulation of factor XI directly. Others have suggested that kaolin binds to aprotinin and reduces the anticoagulant effect of aprotinin in vitro. However, the heparinized kaolin ACT is still somewhat prolonged in the presence of aprotinin. HiTT is not affected by aprotinin therapy and can be used as a measure of heparinization for CPB patients receiving aprotinin therapy. The high-dose thromboplastin time is another measure of anticoagulation that is not affected by aprotinin therapy. The high-dose thromboplastin time is a whole blood clotting time in which celite is replaced by 0.3 mL of rabbit brain thromboplastin to which 1.2 mL of blood is added. This test measures the time to coagulation via activation of the extrinsic pathway. This pathway of coagulation is also stimulated during pericardiotomy due to the rich thromboplastin environment of the pericardial cavity.

Protamine Effects on Coagulation Monitoring

Reversal of heparin-induced anticoagulation is most frequently performed with protamine. Biologically, protamine binds to positively charged groups such as phosphate groups and may have important properties in angiogenesis and immune function. Different successful dosing plans have been proposed. The recommended dose of protamine for heparin reversal is 1 to 1.3 mg protamine per 100 U heparin; however, this dose often results in a protamine excess.

In addition to hemodynamic sequelae, protamine has adverse effects on coagulation.⁸ Large doses prolong the WBCT and the ACT, possibly via thrombin inhibition. In animals and in humans, protamine has been associated with thrombocytopenia, likely due to activation of the complement cascade. The anticoagulant effect of protamine may also be due to inhibition of platelet aggregation, alteration in the platelet surface membrane, or depression of the platelet response to various agonists. These alterations in platelet function result from the presence of the heparin-protamine complex, not protamine alone. Protamine-heparin complexes activate ATIII in vitro and result in complement activation. The anticoagulant effects of free protamine occur when protamine is given in doses in excess of those used clinically; however, the risk of free protamine being the cause of a hemostatic defect is small, given the rapid clearance of protamine relative to heparin.

Monitoring for Heparin Rebound