When working up a case of acute coronary syndrome it is usually the case that CK and CK-MB are tested.
In a typical ST elevation MI, CK and the MB fraction will rise. However, now with the more specific cardiac biomarkers of Troponin-I and Troponin-T it is now possible to identify myocardial damage even when CK and MB fractions do not indicate an MI.
Such cases can involve the Non-ST elevation MI (NSTEMI) situation in which such patients may also have a normal ECG. I have seen this on a few occasions in the UK.
Other ECG abnormalities can be shown by non-specific ST changes, such as flattening, sinusoidal ST morphology and little else on occasion.
On some occasions the CK does not rise BUT the troponin is positive >0.1 consistent with an infarction.
Hence, measuring CK-MB fraction in a patient with a suspected acute coronary syndrome would seem obsolete especially when there is a more specific cardiac marker available such as the troponin test and which remains elevated for longer.
The Troponin test is not immediately positive and can take at least 6 hours before it rises, but it can persist for up to 10 days.
Hence, if you suspect an acute coronary syndrome (ACS), in addition to checking a CK level, it would also help in assessing a troponin level after 6 hours, if that utility is available.
Troponin levels have a direct relationship with 30 day mortality and patients should be treated as high risk for further cardiac events.
Troponin levels can be raised in renal failure and the higher the level of renal failure the higher the troponin. However, renal failure is also associated with increasing cardiac risk and hence, a raised troponin in renal failure should not be ignored.
CK and Troponin can also be raised by myocarditis and pulmonary embolism and hence, in situations when patients develop chest pain diagnoses such as AMI, PE and myopericarditis should be considered if these cardiac markers are raised.
Finally, patients with chest pain can be risk stratified into low risk and high risk groups which predicts further cardiac events after either NSTEMI / Unstable Angina or following an STEMI. Such TIMI scoring systems can predict the likelihood of further cardiac events within a 14 or 30 day period respectively. High scoring patients need further and more intensive investigation and treatment. The NSTEMI / UA scoring system can produce a High Risk score even when cardiac enzyme markers are normal and hence, patients should be treated with the similar treatment intensity as those patients who do have raised cardiac markers as the risk scores can be the same.
The STEMI scoring system does not include cardiac markers as the assumption is that these are already raised because of the nature of an STEMI. However, on occasions, STEMI can be aborted with successful intervention treatment and where there is no cardiac enzyme rise although this is the exception rather than the rule.
Hence, for example, a 65 year old patient with > 3 coronary artery disease risk factors and who is taking aspirin and who develops cardiac chest pain without ST change and with no rise in cardiac enzymes scores 3 points making this a High Risk patient and with an approximate 13% 14 day risk of death or new / recurrent MI or severe recurrent ischaemia requiring urgent revascularisation. Such patients in England are very common. Typical patients are elderly angina sufferers who have cardiac risk factors and who are taking aspirin and many other medications for angina. However, they should be taken seriously as they are High Risk for further events. As a result, such patients should receive intensive medical therapy such as heparinisation, additonal anti-platelet therapy e.g. clopidogrel, anti-anginal medication, statin therapy and early percutaneous coronary intervention if available.
It is therefore safer to keep such patients in hospital rather than sending them home when the CK / CK-MB / Troponin T are negative, until their condition has stabilised on treatment and it is deemed safe for them to leave after a thorough work up and exclusion of serious pathology.
The take home message here is if the CK / CK-MB is normal, it does not exclude acute coronary syndrome. Always check a troponin at least 6 hours after an episode of chest pain-- you may be horrified to find it raised despite the normal CK.
Lastly, even if the cardiac enzymes are not raised, it does not exclude the patient from being High Risk for a future cardiac event with 14 days, as predicted with the NTSEMI / UA TIMI scoring system. Such patients should be fully assessed, investigated and treated the same as for any patient with a cardiac event with raised cardiac markers.
Please check out the NSTEMI / UA and STEMI TIMI scoring systems online by clicking on the words outlined in blue above which will link through to those internet pages respectively.