Interactive Cardiovascular and Thoracic Surgery 3:442-449(2004)
© 2004 European Association of Cardio-Thoracic Surgery
Best evidence topic - Cardiac general |
Can cardiac troponins be used to diagnose a perioperative myocardial infarction post cardiac surgery?
Phil Bothaa,
Darbhamulla V. Nagarajanb,
Philip S. Lewisb and
Joel Dunninga,*
a Department of Cardiothoracic Surgery, Freeman Hospital, High Heaton, Newcastle upon Tyne NE7 7DN, UK
b Department of Cardiology, Stepping Hill Hospital, Stockport, Manchester, UK
* Corresponding author. Tel.: +44-780-15-48-122; fax: +44-780-1548-122
joeldunning{at}doctors.org.uk
Received March 9, 2004;
accepted March 10, 2004
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Abstract
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A best evidence topic in cardiac surgery was written according to a structured protocol. The question addressed was whether Troponin I or T can effectively diagnose a perioperative myocardial infarction after cardiac surgery. Altogether 191 papers were found using the reported search, of which 17 presented the best evidence to answer the clinical question. The author, journal, date and country of publication, patient group studied, study type, relevant outcomes, results, and study weaknesses of these papers are tabulated. We conclude that Troponin I and T can both be used to indicate myocardial damage, with the level correlating well with the level of injury. However until issues such as a ‘gold standard’ for peri-operative MI are addressed, one single cut-off point cannot be recommended for either test.
Key Words: Evidence-based medicine; Thoracic surgery; Troponin; Myocardial infarction; Review
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1. Introduction
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A best evidence topic was constructed according to a structured protocol. This protocol is fully described in the ICVTS [1].
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2. Three- part question
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In [patients following coronary arterial bypass surgery] can [Troponin T or I] be used as diagnostic tool to diagnose [peri-operative MI].
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3. Clinical scenario
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You have just operated on a 72 year old man who was an urgent referral for revascularisation after 2 days of unstable angina despite maximal medical therapy and intravenous nitrates. You had to perform 5 grafts and the operation was long and 3 of the target vessels were small and difficult to graft. You used an on-pump technique with anterograde intermittent cold blood cardioplegia, but as you come off bypass the ECG demonstrates elevated ST segments anteriorly and laterally. You recheck the anastomoses which seem patent and the patient comes off bypass with a moderate dose of adrenaline. He is stable in the intensive care postoperatively but you are concerned that he has had a perioperative MI. Your anaesthetist tells you that Troponin T is unreliable but Troponin I is really good for diagnosis of MI post cardiac surgery but is unable to tell you what level of TnI is sufficient to make the diagnosis.
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4. Search strategy
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Medline 1966—March 2004 using the Ovid interface
[exp coronary artery bypass/OR coronary arter$ bypass.mp OR CABG.mp OR exp cardiac surgical procedures/OR cardiac surgical procedures.mp OR coronary bypass.mp OR cardiac surgery.mp] AND [troponin.mp. OR exp Troponin T/OR exp Troponin/OR exp Troponin I OR TnI.mp OR TnT.mp] AND [exp Myocardial Infarction/OR Myocardial infarct$.mp OR MI.mp OR myocardial damage.mp OR myocardial injury.mp] AND [exp Intraoperative complications/OR postoperat$.mp OR perioperat$.mp.]
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5. Search outcome
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A total of 191 articles were found of which 17 were deemed to be relevant [2–18]. These are summarised in Table 1.
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6. Results
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A very large number of studies of similar quality were found and thus we were unable to reduce the number of best evidence papers included here on the basis of the quality of studies. As may be expected, this large number of papers indicates that there are many difficult issues to address and that no papers have been able to fully answer the question ‘what cut-off point diagnoses a perioperative MI? There are several reasons for the problems encountered by all these papers.
Firstly, when assessing the utility of a biochemical test, the ability to compare it to an established gold standard is paramount. Unfortunately, there is no easily applicable gold standard for the diagnosis of post-operative MI. In these papers, most authors use ECG criteria of newly developed Q-waves, or LBBB or ST changes assessed by cardiologists blinded to the biochemical markers. In addition some papers also diagnosed an MI on Trans-thoracic or trans-oesophageal echocardiography if wall motion abnormalities were seen. Of concern some papers used CK-MB or other rises in biochemical markers to diagnose MI, when the validity of biochemical markers is the very subject in question.
Of note ECG or Echo changes would not allow the diagnosis of smaller or non-transmural infarcts to be tested. Some attempts have been made to define a subgroup of patients with ‘non-specific’ changes of the ST segment or conduction abnormalities on ECG and correlate this to marker activity in some papers. The ‘gold standard’ therefore varied considerably amongst the papers presented here.
Secondly the assay for TnI was initially non-uniform and thus some variation may have been due to the measuring techniques used. In particular the large study by Benoit et al. [13] found a markedly higher TnI level in normal patients. They have been criticised for not accounting for interference of gelatine-based colloid fluid replacement with the immunoassay for TnI. In addition it has more recently been found that the levels of TnI in pericardial blood is higher than serum. Thus if autotransfusion is used, this will artificially raise serum TnI in otherwise normal patients.
Another important issue in these papers is the absence of a correlation between graft patency and myocardial infarction. Holmvang et al. [17] performed an angiogram on all of their patients between days 5 and 7 postoperatively. They found 10 patients with an MI on ECG criteria but only 3 of these patients had vein graft occlusions. In addition 12 patients had an occlusion with only 3 of these showing evidence of myocardial infarction. Peivandi et al. [14] performed an MRI to look at graft patency and found no occluded grafts in the 4 patients diagnosed with peri-operative MI and 1 graft occlusion was found in a patient with minor myocardial injury.
Despite all these shortcomings, the papers do show that both TnI and TnT levels do significantly increase in patients who have a final diagnosis of peri-operative MI (summarised in Table 2). Troponin T tended to be less than 1 in normal patients, however in patients who had an MI, the level was invariably over 1, usually over 3 and occasionally over 10 ng/ml. Troponin I tended to be around 6 ng/ml in normal patients but in patients with a final diagnosis of MI the level was well above 10–20 ng/ml and often was into the high 50 s.
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7. Clinical bottom line
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Troponin I and T can both be used as indicators of myocardial damage following cardiac surgery, with the level correlating well with the level of injury. Until issues such as a ‘gold standard’ for the diagnosis of peri-operative MI are addressed, one single cut-off value cannot be recommended.
doi:10.1016/j.icvts.2004.03.004
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Top
Abstract
1. Introduction