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ECG cardiac infarction and ischemia
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An electrocardiogram - an ECG - or the dutch and german version of the word - elektrokardiogram or EKG, is a tool used to visualize “gram” the electricity “electro” that flows through the heart “cardio”.
An ECG tracing specifically shows how the depolarization wave moves during each heartbeat - which is a wave of positive charge - looks from the perspective of different sets of electrodes.
This particular set of electrodes is called lead II, with one electrode on the right arm and the other on the left leg, so essentially when the wave’s moving toward the left leg electrode, you get a positive deflection, like this big positive deflection correspond to the wave moving down the septum.
To read an ECG there are a few key elements to keep in mind, one of them includes figuring out if part of the heart has is suffering from ischemia or has undergone an infarction.
The term ischemia means that blood flow to a tissue has decreased, which results in hypoxia, or insufficient oxygen in that tissue, whereas infarction goes one step further and means that blood flow has been completely cut off, resulting in necrosis, or cellular death. That typically happens if blood flow has been cut off for about 20 minutes.
In the heart, ischemia and infarction can be transmural, affecting the entire thickness of the myocardium, or subendocardial, affecting just the innermost part of the myocardium - the part just beneath the endocardium.
Out of all four chambers of the heart, the ECG is most sensitive to transmural or subendocardial ischemia or infarction in the left ventricle because that’s the chamber with the thickest walls and therefore has the most cardiac tissue.
Cardiac ischemia means that blood flow to the cardiac muscle tissue has decreased, which can lead to poor oxygen supply or hypoxia. Myocardial infarction means that the blood flow is completely cut off, resulting in cellular death or necrosis of the affected part of the heart muscle.
ECG changes associated with myocardial infarction and ischemia can be difficult to differentiate. Generally, ischemia will present with inverted T waves and flat or downsloping ST-segment depression, whereas myocardial infarction will show ST-segment elevation, T wave inversion, and Q waves.
The best way to differentiate acute myocardial infarction (AMI) from ischemia is to look at the changes in multiple leads. AMI will typically cause ST elevation in two or more contiguous leads, while ischemia may only cause ST elevation in one lead. In addition, AMI will usually cause Q waves, while ischemia typically does not.
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