why is there st elevation in myocardial infarction
Emergency physicians, frequently, deal with patients symptomatic for acute chest pain. In this setting, some aspects such as clinical features and biomarkers of myocardial necrosis may have an important role. In many cases, the accurate diagnosis of myocardial infarction (MI) may be, however, a real challenge. Indeed, the typical clinical presentation may be absent and a non-specific elevation of plasmatic levels of cardiac troponin I could be detectable. The twelve-lead electrocardiogram (ECG) is then an integral part of the diagnostic work up of patient with acute chest discomfort. It is the easiest and available instrument to confirm or exclude the diagnosis of MI and to decide the appropriate treatment strategy. The earliest manifestations of myocardial ischemia typically interest T waves and ST segment.
It is possible to make diagnosis of acute ST segment Elevation Myocardial Infarction (STEMI) when, in a certain clinical context, a new ST segment elevation is detected in at least two continuous leads. In an ECG recorded at a paper speed of 25 mm/s and an amplification of 10 mm/mV, the ST segment elevation from the baseline should be measured 80 ms after the J point and is considered present if the deviation is 0. 2 mV in men and 0. 15 mV in women in V2 V3 leads ( 0. 1 mV in other leads). Despite the high sensitivity, the ST segment deviation has, however, a poor specificity since it may be observed in many other conditions (such as left bundle branch block, hypertrophic cardiomyopathy or left ventricle aneurysm). Furthermore, the problem of equivocal electrocardiographic features is frequent in the departments of emergency care, especially in patients with hypertension or previous history of MI. A wrong diagnosis led patients to unnecessary (invasive or conservative) cares.
Sharkey et al have previously reported the magnitude of the problem, observing that almost 11% of patients with suspected acute coronary syndrome receive unnecessary thrombolytic therapy. A recently published article has investigated the ability to recognize an MI of 15 trained cardiologists coming from various countries (North America, Israel and Europe). They were asked to evaluate some ECG with ST segment elevation ( 0. 1 mV) in at least two contiguous leads and to say if, in presence of symptoms, the modifications were consistent with an MI or related to other causes. Only eight (of 116) ECG were recorded in patients with a real documented STEMI, that was however diagnosed in a variable percentage (7. 8% and 33%).
Moreover, when ST segment elevation was related to other causes, the diagnostic orientation was frequently wrong. Our aim is to overview the main conditions that may electrographically mimic a STEMI. ST depression can be either upsloping, downsloping, or horizontal. Horizontal or downsloping ST depression P0. 5 mm at the J-point in 2Pcontiguous leadsPindicates myocardial ischaemia ( according to theP ). Upsloping ST depression in the precordial leads with prominent Pis highly specific for occlusion of the LAD. Reciprocal change has a morphology that resembles upside down ST elevation and is seen in leads electrically opposite to the site of infarction. Posterior MIPmanifests as horizontal ST depression in V1-3 and is associated with upright T waves and tall R waves.
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