Cardiac Enzymes as diagnostic markers for an acute MI

[DrMetal]

Is it a fair question for Step II, to be asked which Cardiac enzyme (CKMB or Troponin) should be used as a diagnostic marker in the context of an acute MI?

I suppose it depends on when you're looking:

If you're looking for an MI that occurred 5 days ago, Troponin would be the correct answer, b/c it stays elevated longer.

What if you're looking for an MI that occurred < 24 hrs ago? Troponin peaks faster, but the CKMB peak is greater in amplitude. Also something to consider is that troponin in more specific for the heart, right? So, can you make a distinction, can you really call one better than the other to serve as a diagnostic marker within the first 24 hours? [according to Harrisons: "Many hospitals are using cTnT or cTnI rather than CKMB as the routine serum cardiac marker for diagnosis of STEMI, although any of these analytes remain clinically acceptable."]

Thoughts?

Thanks,

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[shan564]

The main reason to check cardiac enzymes in the first few hours after an MI is to establish a baseline. Your diagnosis will be based on the ECG, not the cardiac enzymes.

 

[DrMetal]

The main reason to check cardiac enzymes in the first few hours after an MI is to establish a baseline. Your diagnosis will be based on the ECG, not the cardiac enzymes.

Understood, and thank you for your reply, but I don't think you answered my question.

My question is: Can one claim that one of these enzymes (CKMB vs. Troponin) is a "better" marker for the diagnosis of MI < 24 hrs ? Is this even a valid question or distinction? (I think not, b/c both enzymes are ordered on the same cardiac enzyme panel, and you'd trend both)

I'm asking b/c in some of the board review books, they'll make comments stating that CKMB is the 'best' marker, but troponin is more specific (b/c troponin is only found in the heart). They'll also point out that troponin might peak faster (at 6 hours), while CKMB might peak at (6 to 12 hours). All I'm wondering is, why do these review books go to such extents to draw a distinction between Troponin and CKMB, when in fact we trend both, and in fact you can't be too sure of which enzyme peaking first?

 

[ASAPRocky]

The main reason to check cardiac enzymes in the first few hours after an MI is to establish a baseline. Your diagnosis will be based on the ECG, not the cardiac enzymes.

completely WRONG.

Normal EKG with elevated Troponin = admission for MI.

 

[Raryn]

completely WRONG.

Normal EKG with elevated Troponin = admission for MI.

=admission for observation/MI ruleout. Unless that Troponin is sky-high (or continuing to rise), it doesn't mean much.

 

[DirtyNetter]

=admission for observation/MI ruleout. Unless that Troponin is sky-high (or continuing to rise), it doesn't mean much.

...Unless their TIMI score is high, then it's off to the cath lab they go (even with a moderate to low troponin).

Sabatine's green book covers this pretty well.

 

[ASAPRocky]

=admission for observation/MI ruleout. Unless that Troponin is sky-high (or continuing to rise), it doesn't mean much.

Ummm for EM, the big decision with chest pain is can you send home or do you r/o admission/observation.

So yes it means a lot

 

[aagupta2]

So the topic of cardiac enzymes and diagnosis of MI seems very board relevant for our CK exam. So let's take a step back and think through what we are trying to do. When someone comes in with an acute MI, none of the markers are elevated yet, so we rely on the clinical picture and EKG for insight, and since this is not sacrosanct we will hold the patients for at least 3 serial enzymes even with low clinical suspicion since we don't want to miss one and get sued. EKG is good, however it is not what we want to hang our hat on.

Now to the business of the enzymes. Troponin rises early and stay elevated for a week. The reason we get Ck-mb is because it rises and falls in a much shorter time frame. This makes ck-mb able to show reinfarction since a patient with recurrent pain in the next couple days post-mi may have reinfarcted, but troponins will be still elevated whether re-infarction occurred or not. TCAL:Troponins, ck-MB, AST,LDH (in that order if I remember correctly) will all rise, soon after an MI but not soon enough to wait on treatment. Troponins and CK-MB are more specific for the heart, but we are not going to wait for them to start treatment. MONA (morphine, oxygen, nitrates, asprin) first, ask questions later.

We don't diagnose MI with markers, we get the serial enzymes so we can get the patient we don't think is having an MI out of the hospital with out fear. EKG early can help us say this is cardiac chest pain, and if in doubt error on the side of over treating. Think of the enzymes to not make he diagnosis, but we get them to rule out MI if they are negative after several hours, hence the need for serial enzyme test before we send home mr. Cocaine associated chest pains.

So while it seems that "ASAP" and "shan" are in disagreement, but they are not as they are both right. Someone comes in with chest pain you get enzymes for baseline AND you admit them for observation and serial enzymes. If someone just had onset of chest pains you get enzymes and if it is acute enough you don't expect ANY enzyme to have risen yet. However getting the reading early helps you to be sure of the rise you see after several hours. EKG can altered more immediately, but if there is previous heart damage, or structural defects the impression from it can be more convoluted.

Hope this helped and didn't confuse matters more. Good luck on your test.

 

[DrMetal]

So the topic of cardiac enzymes.

Nice post, that was helpful. So, I guess CKMB is the more pertinent enzyme, b/c (as you pointed out), it is a more 'dynamic' enzyme, in the sense that it is indicative of re-infarcts.

But, to answer my own question, within the first 24 hours, both Troponin and CKMB are important to trend. Which is more important (with respect to the original MI is probably a moot point.

 

[coralis]

Nice post, that was helpful. So, I guess CKMB is the more pertinent enzyme, b/c (as you pointed out), it is a more 'dynamic' enzyme, in the sense that it is indicative of re-infarcts.

But, to answer my own question, within the first 24 hours, both Troponin and CKMB are important to trend. Which is more important (with respect to the original MI is probably a moot point.

troponins would be the answer for acute MI since they are more sensitive and more specific then CK-MB. CK-MB is great to look at for re-infarction since troponins stay up longer. This has been researched and a scientific consensus reached at this point in time. In the old days they used to look at Myoglobin which raises faster than any of the above but it is not specific. Then they switched to LDH1/2 flip, then CK-MB, now troponins.... historic trend based on specificity for myocardial injury.

 

[OveractiveBrain]

Nice post, that was helpful. So, I guess CKMB is the more pertinent enzyme, b/c (as you pointed out), it is a more 'dynamic' enzyme, in the sense that it is indicative of re-infarcts.

But, to answer my own question, within the first 24 hours, both Troponin and CKMB are important to trend. Which is more important (with respect to the original MI is probably a moot point.

Initial MI = Troponin (more specific for cardiac tissue)
Reinfarct after CABG = CKMB

See www.onlinemeded.org/cards1 regarding diagnosis of initial MI (troponins better)

and

see www.onlinemeded.org/surgery8 regarding reinfarct after CABG (CK better)

=admission for observation/MI ruleout. Unless that Troponin is sky-high (or continuing to rise), it doesn't mean much.

This quote refers to the practice in academic centers where "just a little bump" doesn't go to cath. In the private setting, Troponins above normal go to cath, often emergently. While it is true the degree of elevation is taken into account, a positive troponin doesn't get stressed, at least not without a cardiologist writing down, in a chart, "no cath, stress first." This is subtle, and not intended to be learned for USMLE.

Again, for Step II:
1. Original MI = Troponin
2. Reinfarct after revascularization = CKMB

 

[OveractiveBrain]

We don't diagnose MI with markers, we get the serial enzymes so we can get the patient we don't think is having an MI out of the hospital with out fear.

No we don't. Elevated Troponins get a discharge diagnosis of "NSTEMI" as in, non ST segment elevation myocardial infarction. Emphasis on myocardial infarction. If they have progressive symptoms of CAD, but no elevation of enzymes, they get the discharge diagnosis of unstable angina. Emphasis on the absence of myocardial infarction. If there is ST segment elevation, they get the diagnosis of STEMI.

What you should take away is

No/Low Risk for CAD - Good to go after their stress
Stable Angina - Good to go after their stress... was it even angina at all?
Unstable Angina - Control with meds, cath at some point
NSTEMI - Optimize medically, cath this admission
STEMI - HOLY CRAP BALLS! ACTIVATE TEH CATH LABZ! GET TEH FELLOW DOWN HERE NOWZ! TIME IS MUSCLE! ONE! ONE! ONE!

 

[DrMetal]

thanks for the discussion all, this was very useful.