Ask the Clinical Instructor

I’ll take the opportunity to answer your question by explaining the Activated Clotting Time (ACT) test and how it applies to what we do in the lab. The ACT was initially developed as a screening test for patients with bleeding disorders. In the mid-70’s it began its use as a guide to the administration and reversal of heparin during coronary artery bypass grafting (CABG). The concept of “no clotting” carried over from CABG to angioplasty, since equipment is in the artery and could create clots if the blood was not “thinned.” That is why ACT is used in current coronary interventional procedures — to monitor the clotting time.
There are many ‘clotting’ tests (aPTT, for example) that are more accurate, but in a percutaneous coronary intervention (PCI), the ACT is the most rapidly available test to evaluate the anti-coagulation status of the blood. (Note: If you have taken the Cardiovascular Credentialing International [CCI] Registered Cardiovascular Invasive Specialist [RCIS] exam, you may recall a question concerning this point.)
In a ‘normal’ patient who is not on anti-coagulation therapy, the ACT would be somewhere around 107 seconds (± 13 seconds). The desired ACT level for a patient undergoing CABG would be somewhere between 400-600 seconds. This was developed by tests that revealed that at this level, clots would not form in the heart-lung machine bypass circuits. In a PCI with heparin alone, the desired ACT level is 250-350 seconds. In a PCI where heparin and a IIb/IIIa agent is administered, the ACT is to be targeted at 200-250 seconds.¹

There are two systems that seem to be more prevalent in the cath lab. One is the Hemochron (International Technidyne Corp, Edison, NJ) and the other is the Medtronic HemoTec (Medtronic HemoTec, Inc., Englewood, CO) (Figures 1-2). These systems work quite differently from one another, but their main result is the activated clotting time in seconds by analyzing how long it takes for fibrin fibers to develop.
It is important to note that these systems are not interchangeable. If you are utilizing one brand on a procedure, you would not want to switch to another brand in the middle of the procedure. There is up to a 30% variance between the two.¹
ACTs are generally used for one of three reasons:

1. Assessment of the patient anticoagulant status when they arrive in the cath lab after receiving a heparin bolus and/or drip;
2. Maintenance of the ACT during the PCI;
3. Optimal time for sheath removal after the procedure (generally 150 to 180 seconds).

To answer your original question about why the cardiologist performed the exam on the patient arriving from the ER, I can only assume it is because the patient received a heparin dose in the emergency room. Generally, patients who come to the cath lab for a diagnostic procedure, such as an outpatient, for example, would not receive an anti-coagulant until it was decided that an angioplasty was needed. In these cases, the ACT would not be performed until approximately 10 minutes after the heparin administration.
This brings up another debate that I can present, but not resolve. What about the use of bivalirudin (Angiomax, The Medicines Company, Parsippany, NJ) in PCI? There is debate in the literature regarding whether the current ACT systems developed for heparin monitoring can be used to accurately portray the anticoagulation status of patients who have received bivalirudin.2-5
Because of the established performance and reliability of bivalirudin, ACTs are often not needed. However, a common practice is to perform an ACT test on the patient 10 minutes after the administration of bivalirudin. Often, you will get a result of “High” or “Out of Range.” This assures that the IV access was good and that the medication made it into the circulatory system. The cardiologist could then proceed with the case.
The debate is AFTER the procedure, whether post-procedure results are accurate and correlate to the true anti-coagulation status. Bivalirudin studies state that the ACT should be low enough to pull the sheath 2 hours after the medication has been stopped. This is due to the half-life of bivalirudin. However, if you have taken care of a patient with a sheath in for longer than 2 hours, and someone performs an ACT, the results can often be higher than 150. In patients with renal failure, the ACT can be substantially higher for a longer period of time. More studies and documentation on this will be required before definitive changes in practice occur.
The ACT exam is not without its flaws. Patients can respond differently to heparin administration, to the point that the ACT will be ‘off’ from what is expected. There can be a wide individual variation in results, even with the same sample and the same operator on exactly the same equipment. The ACT is intended as a general guideline and not an exact result. The ACT is influenced by many outside factors, including, but not limited to: temperature of the tube/slide, patient hypothermia, hemodilution, too little/too much sample, etc. An important safety rule: if the patient is administered a loading dose of heparin/bivalirudin, but the ACT fails to show an increase (over 180 seconds) immediately confirm that the dose actually made it into the patient. This could mean checking the IV patency, the IV line and the medication actually administered. Clotting complications can occur if one blindly assumes that the patient is anti-coagulated.
If there is ever a doubt on the accuracy of the ACT, perform another test. A result is generally available in less than 5 minutes. The proper analysis of ACT is an important part of any PCI, and helps to assure a basic level of safety for the patient after the administration of potent anti-coagulant medications.

References

1. Libby P, Bonow RO, Zipes DP, Mann DL. Braunwald’s Heart Disease: A Textbook of Cardiovascular Medicine. 8th Edition. Philadelphia: Elsevier Science 2007, 1-2183.
2. Cheneau E, Canos D, Kuchulakanti PK, et al. Value of monitoring activated clotting time when bivalirudin is used as the sole anticoagulation agent for percutaneous coronary intervention. Am J Cardiol 2004;94:789–792.
3. Lincoff AM. Pilot study of bivalirudin versus heparin during percutaneous coronary intervention with stenting and GP IIb/IIIa blockade:results of the REPLACE 1 trial (abstr). J Am Coll Cardiol 2002;39(suppA).
4. Lincoff AM, Kleiman NS, Kereiakes DJ, et al for the REPLACE-2 Investigators. Long-term efficacy of bivalirudin and provisional glycoprotein IIb/IIIa blockade vs heparin and planned glycoprotein IIb/IIIa blockade during percutaneous coronary revascularization: REPLACE-2 randomized trial. JAMA 2004;292:696-703.
5. Bejarano J, Muniz AJ. Use of bivalirudin instead of heparin during a percutaneous coronary intervention in a patient with severe thrombocytopenia. J Invasive Cardiol 2004 Sept;16(9):535-536.