ECG cardiac infarction and ischemia

Definitions & Key takeaways

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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Introduction0:00–0:56

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.

Ischemia and Infarction0:56–1:52

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.
Alright, let’s start with subendocardial ischemia, which might happen when there’s incomplete blockage - let’s say 70% - in a coronary artery.

Subendocardial Ischemia1:52–3:46

In that situation, at rest there’s enough blood flowing through to meet the demand of the myocardium, but during exercise there’s not enough to meet the increased demand of the myocardium.
In that situation, the subendocardial tissue get ischemic because it’s the last bit of tissue to get blood from the tiny branches of the coronary arteries as they make their way through the ventricular wall from outside to inside.
This condition is called stable angina, because there’s ischemia which causes chest pain with exercise and it disappears with rest.
And there’s also unstable angina, which can occur with incomplete blockage of a coronary artery, but in this case, chest pain appears at rest.When there’s subendocardial ischemia in a region, it causes ST depressions in the corresponding lead on the ECG.
An ST depression is when the J-point, which is where the QRS complex meets the ST segment, goes down by at least 0.5mm or ½ of a small box.
The ST depression can be upward sloping, downward sloping, or horizontal. In subendocardial ischemia, ST depressions are usually widespread, often affecting leads I, II, and V4-6.
With such widespread involvement, it’s difficult to determine which coronary artery caused the ST-depression. As a side note, if the ST segment is depressed and curved, it may be due to the “digitalis effect” which is when the patient takes the medication digoxin, rather than being due to subendocardial ischemia.On the other hand, subendocardial infarction occurs if the coronary artery remains blocked for around 20 minutes, and the subendocardial cells begin to die, resulting in necrosis.

Subendocardial Infarction3:46–4:20

And with necrosis, the subendocardial cells leak cardiac enzymes, like troponins and CK-MB, into the blood. With a subendocardial infarction, there is no ST elevation on the ECG, so this is called a non-ST elevation myocardial infarction, or an NSTEMI for short.
Instead, the ECG can show ST depression and T-wave inversion.Now let’s put these concepts into ECG perspective! On ECG, both unstable angina and NSTEMI’s may show ST-depression as well as T wave inversions.

ST Depression4:20–5:31

The difference is that with an NSTEMI, cardiac enzyme levels in the blood are elevated, whereas with unstable angina, the damage resolves in a short time window, so there is no cardiomyocyte death - and therefore no elevated cardiac enzymes.
With an NSTEMI, the ST-depression looks similar to the ones in subendocardial ischemia, and the T wave inversions are usually symmetric and at least 1 mm or 1 little box deep and are most noticeable in the chest leads, but can also appear in the limb leads.
They also have to occur in at least two contiguous leads, For example, lead V2 and V3 are contiguous, but leads V3 and V5 are not contiguous.
There can also be a dominant R wave, meaning that the R wave has a higher amplitude than the S wave. Now, it turns out that a little bit of T wave inversion can be normal in leads III, aVR, and V1, but any sign of T wave inversion in leads V2-V6 is abnormal.Alright next there’s transmural ischemia.
When a coronary artery is narrowed from an atherosclerotic plaque buildup over time, there are two parts to that plaque - a hard fibrous cap and the soft cheese-like interior.

Transmural Ischemia5:31–6:29

As blood flows past the plaque, the fibrous cap can sometimes rip and that cheesy interior is exposed and considered thrombogenic - meaning that blood clots form on it very quickly, which can quickly stop blood flow!
Transmural ischemia can occur secondary to vasospastic angina, also called Prinzmetal angina, which is when a coronary blood vessel spasms and narrows, cutting off blood flow to that cardiac region.
Vasospastic angina occurs independent of traditional risk factors, like atherosclerosis, and can be triggered by tobacco, alcohol or cocaine use.
Finally, we have transmural infarction, which occurs when a coronary artery becomes completely blocked for greater than about 20 minutes, which causes transmural necrosis.
Within minutes a transmural infarction leads to ECG changes. The most common would be T wave inversions, but there might also be hyperacute T waves, which are large asymmetric T waves that appear in at least two contiguous leads.

Transmural Infarction6:29–7:20

The classic sign of transmural infarction is ST elevation, and when that happens it’s called an ST-elevation myocardial infarction, or STEMI.
On ECG, the ST elevation at the J point must be over 1mm in any two contiguous leads except V2 or V3, where it has to be over 2mm.
On an ECG, both a STEMI and vasospastic angina can cause ST elevations, but with vasospastic angina, the elevation is transient.
Another key difference is that with a STEMI, the ischemia is so severe that damaged heart cells die - resulting in myocardial necrosis - and leak out certain enzymes like troponin and CK-MB.

ST elevation7:20–8:06

But ST elevation can also be caused by other conditions like, left ventricular hypertrophy, and pericarditis. For example, a 20-year-old physically fit person with a viral illness that gradually develops chest pain and is found to have ST-elevation on ECG is more likely to have pericarditis than an ST-elevation myocardial infarction.
Besides ST elevation and T-wave inversions, a transmural infarction can also cause pathologic Q waves to appear on the ECG.
Normally, depolarization in the ventricle spreads from the endocardium to the epicardium, and most of that ends up pointing toward the positive electrode, which means a positive deflection.

Q waves8:06–8:59

With infarcted tissue, though, it doesn’t conduct electricity, which means that the electrode essentially sees through the infarcted tissue, like it was a hole, to the other ventricular wall, the depolarization wave that moves through the healthy wall on the opposite side of the heart, and since those are mostly moving away from the positive electrode, you end up with this big, negative pathologic q wave.
So pathologic Q waves are those

Infarction Location8:59–9:52

Post-Infarction ECG9:52–10:26

Review10:26–8:45