Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (32 page)

2.   Sudden or unexpected cardiac death, often with symptoms suggestive of myocardial ischemia, and accompanied by presumably new ST elevation, or new LBBB, and/or evidence of fresh thrombus by coronary angiography and/ or at autopsy, but death occurring before blood samples could be obtained or at a time before the appearance of cardiac biomarkers in the blood.

3.   For percutaneous coronary intervention (PCI), patients with normal baseline troponin, an elevation >3× 99th percentile URL is defined as a PCI-related MI. It is important to note the threshold for characterizing NSTEMI post-PCI is higher than in spontaneously presenting NSTEMI (3× 99th percentile). This is due to the fact the 99th percentile is reached in up to 50% of PCI patients but does not carry the same prognosis as chest pain patients. In fact, only 5–8× 99th percentile elevations of CK-MB carry negative long-term prognosis.

4.   For coronary bypass patients with normal troponin, an increase of biomarkers >5× 99th percentile URL with pathologic Q waves, new LBBB, or angiographically documented new graft or native coronary artery occlusion or loss of a viable myocardium by imaging has been designated bypass-related MI.

5.   Pathologic findings of acute myocardial infarction.

   Once the criteria for MI are met, a clinical classification of MI has been established to recognize different etiologies of myocardial necrosis. Each etiology differs in both short- and long-term mortality rates. The classifications are

1.   Type 1 MI: spontaneous MI related to ischemia from a primary coronary event such as atherosclerotic plaque erosion, rupture fissure, or dissection with accompanying thrombus

2.   Type 2 MI: ischemia/necrosis related to increased oxygen demand or decreased supply such as in coronary spasm, embolism, anemia, arrhythmia, hypertension, and hypotension

3.   Type 3 MI: sudden cardiac death meeting criterion as listed above

4.   Type 4 MI: PCI-related MI with further classification as type 4a or type 4b

   Type 4a MI: MI directly related to procedure

   Type 4b MI: MI due stent thrombosis as documented by angiography or autopsy

5.   Type 5 MI: coronary bypass related

   Type 1, 3, and 4b MIs have the highest short- and long-term mortalities and must be triaged and treated aggressively upon presentation. Prognosis for type 2, 4a, and 5 MIs is generally more favorable.

   MI classification is almost always based entirely on clinical context with supporting imaging/autopsy findings if needed. A notable exception may be the use of point of care platelet assays (see Chapter
16
: Platelet Function Assay).
High on-treatment platelet reactivity
in a recently stented patient may suggest sub-optimal antiplatelet therapy or genetically determined resistance to treatment, greatly increasing the risk of stent thrombosis (type 4b MI).

   Any type of MI classification can present as either STEMI or NSTEMI depending upon the severity of ischemic insult.

   Diagnosis

The diagnosis of ACS depends on the likelihood of coronary atherosclerosis, characteristics of the chest pain, abnormalities on ECG, and levels of serum markers of cardiac injury. A rapid assessment of chest pain patients (Figure
3-1
) is required to initiate appropriate and potentially lifesaving treatment and may need to be revisited until a final diagnosis is confirmed.

   The
physical examination
of patients with uncomplicated ACS is usually normal but has the goal of evaluating for precipitating factors (uncontrolled hypertension, anemia, thyrotoxicosis, sepsis), assessing hemodynamic consequences of ACS (CHF, third heart sound, new mitral regurgitant murmur, shock), revealing comorbid conditions that impact treatment decisions (malignancy), and ruling out other chest pain etiologies. A targeted initial exam should evaluate for unequal extremity pulses and aortic regurgitation (aortic dissection), a pericardial rub (pericarditis), pulsus paradoxus (tamponade), or reproducible chest pain with palpation (musculoskeletal).

   The
ECG
should be performed first and within 10 minutes of first medical contact and reviewed for ischemic findings as ECG changes have both diagnostic and prognostic implications.

   ST-segment deviation (depression or elevation) is the most specific sign of ischemia.