What defines a non-ST elevated myocardial infarction (NSTEMI)?

A non-ST elevated myocardial infarction (NSTEMI) is characterized by changes on an electrocardiogram (ECG), most notably ST segment depression or T wave inversions, indicating ischemia without the full elevation of the ST segment that is typical in ST elevated myocardial infarction (STEMI). This type of myocardial infarction often occurs due to partial blockage of a coronary artery, leading to reduced blood flow and oxygen to the myocardial tissue, which causes injury and can result in damage to that area of the heart.

The presence of ST depression on the ECG is significant because it helps clinicians identify the ischemic damage and differentiate NSTEMI from other forms of cardiac events. The diagnosis involves a combination of patient symptoms, biomarkers such as cardiac troponins, and ECG changes, with ST depression being an important indicator of myocardial injury.

In contrast, the other options include conditions or signs that do not specifically or exclusively define NSTEMI. For example, complete blockage of a coronary artery is more indicative of a STEMI, while low output failure and full thickness heart damage pertain to broader or different cardiac conditions and complications rather than specifically indicating NSTEMI. Thus, the definition and recognition of NSTEMI hinge significantly on ECG findings,