Most of us probably know that anesthetists use the activated coagulation time (ACT) to monitor unfractionated heparin therapy during coronary bypass graft surgery, and that the heparin plasma concentration is raised to 2.5 units/mL during surgery. The ACT is a time-honored adaptation of the Lee-White clotting time in which fresh whole blood is pipetted to a 12 X 75 mm tube containing a negatively-charged particulate activator. The interval between blood collection and clot formation is typically 110–150 seconds. While it can be performed manually, most cardiac operating rooms are equipped with a Hemochron®, from ITC Inc, which provides a semiautomatic ACT.
Yesterday, Prof. Bernadette Rodak, Indiana University, asked me about the target therapeutic range, and checking around, I realized most laboratorians don’t know, because the test is invariably performed in the operating room. I contacted Laura Taylorat UAB Academic Health Sciences Center and Dave McGlasson at Wilford Hall USAF Medical Center. I learned anesthetists try to hit 1.5–2.0 X the baseline, for instance 165–300 seconds. The amount of heparin needed to reach and maintain a certain ACT varies as does the body’s clotting potential at that ACT. If there are clotting or bleeding problems, the dosages and ACT may need to be adjusted accordingly. After surgery, the ACT may be maintained within a narrow range (for instance, 175 – 225 seconds) until the patient has stabilized.
Thanks to “Bunny” for the question, and Laura and Dave for helping me with the answer, which I hope is useful information for everyone.
Hi Siddhartha and George. If as George writes, “the heparin
Hi Siddhartha and George. If as George writes, “the heparin plasma concentration is raised to 2.5 units/mL during surgery,” and as Dave McGlasson says, “anesthetists try to hit 1.52.0 X the baseline,” I would assume that amount of heparin would be “terminal time” on your instrument for aPTT. Modifying the “terminal time” settings in your instrument would eliminate that problem, but I would think that using the aPTT for monitoring heparin levels that high would be relatively insensitive (if you have ever seen an aPTT heparin therapeutic range study based on .3 to .7 u/mL heparin you know the associated issues at hand and how much more complicated that would be at these levels). The lab would also be hard pressed to meet any kind of turn around time required by the OR.
Regards,
Herb Crown
St. Louis University Coagulation Reference Lab
Added by George: Thank you, Herb, I agree. For the heparin levels used in cardiac surgery, the ACT is the only choice available. There exist no PTT correlations at heparin levels above 1.0 units/mL for the reason Herb suggest, the PTT instrument will “time out” at those concentrations.
Dear George,
Any studies on the correlation between ACT a
Dear George,
Any studies on the correlation between ACT and APTT ? And which is better for monitoring a patient on heparin ? Thanks, Siddhartha