Electrocardiogram (EKG or ECG)

The electrocardiogram (EKG or ECG) is a visual depiction of the wave of electrical signal through the myocardium. Its main purpose is as a clinical tool in the work-up of chest pain.

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The typical flow of electrical signal (“wave of depolarization”) through the heart creates the characteristic shape of the EKG tracing. The typical EKG tracing includes the following parts: 1.) P wave, 2.) PR interval, 3.) QRS complex, 4.) ST interval, and 5.) T wave. Each of these segments directly corresponds to some electrical excitation event in the heart. Each wave of depolarization starts with the automatic firing of the SA node, causes atrial and ventricular myocyte depolarization and myocyte contraction, and repeats with every heartbeat. The ultimate result is that these electrical events cause the cyclic contraction and relaxation of the heart, which allows it to pump blood and keep the cardiac cycle going.

EKG tracings typically include the following segments:

1. P wave (orange): Represents the wave of depolarization through the atria ("atrial depolarization"). Since atrial depolarization is initiated by the SA node, if the P wave is absent, it means there is something wrong with the SA node.
2. PR interval (pink): Represents the time the electrical signal is delayed at the AV node. After atrial myocytes depolarize, the electrical signal reaches the AV node, where there is a slight delay before the signal is passed through to the ventricles. This allows time for the atria to contract and empty blood into the ventricles, before the ventricles depolarize and contract. This allows for proper filling of the ventricles. The PR interval therefore represents diastole.
3. QRS complex (blue): Represents the wave of depolarization through the ventricular myocytes ("ventricular depolarization"). Since the ventricles are much larger than the atria, this wave, or spike in the EKG, is usually larger than the P wave. The QRS complex technically also encompasses the depolarization of the His-Purkinje system; however, the His-Purkinje system covers a pretty small area and this depolarization happens really quickly, so it usually doesn't register strongly on the EKG tracing.
4. ST interval (lighter blue/green): Represents the time between ventricular myocyte depolarization and repolarization. This corresponds roughly to the time where the ventricular myocytes are undergoing muscle contraction. The ST interval therefore represents systole.
5. T wave (green): Rpresents repolarization of ventricular myocytes, where they reset and prepare for the next wave of depolarization.

Here is a summary of EKG tracings and the corresponding electrical events in the heart:

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EKG "leads" are the electrical probes that are placed on a patient’s body that measure the flow of electrical signal through their heart. The average, or overall, movement of electrical signal through the heart is from the SA node in the top of the right atrium (RA) down to the botoom left corner of the left ventricle (LV). This average vector of electrical signal through the heart is depicted by the black arrow on our EKG person. The shapes of the tracings in all of the different EKG leads are slightly different because of the placement of the leads in relation to the heart's flow of electrical signal.

Imagine each lead as just a single probe stuck on the patient’s body in the orientation shown on EKG man (this is a simplification of the actual setup). An electrical signal heading toward an EKG lead creates a positive deflection in that lead (a peak on the EKG tracing going up). Electrical signal heading away from an EKG lead creates a negative deflection in that lead (going down).

All EKG leads can be grouped together based on the region of the heart they monitor: 1.) inferior leads, 2.) lateral leads, 3.) anterior (or anteroseptal) leads, and 4.) posterior leads. We will now walk through each of the different groupings of EKG leads, starting with Lead II (the "classic" EKG lead) and aVR (Lead II's polar opposite).

Lead II is what we can think of as our “classic” EKG tracing—Lead II has all positive deflections. This is because Lead II is the most direct recipient of the overall/average "wave of depolarization" through the heart, from the SA node down to the ventricles (electrical signal is almost always traveling directly toward Lead II).

aVR is basically the opposite of Lead II. Since aVR is up at the right shoulder, it measures electrical activity from the perspective of the right atrium. Electrical signal is almost always traveling away from aVR, so aVR has all negative deflections.

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The inferior leads (Leads II, III, and aVF) are on the feet. They measure electrical activity of the heart from below (from the perspective of the inferior aspect of the heart).

As mentioned earlier, all deflections are positive in Lead II (“classic EKG lead”), because electrical signal is almost always flowing toward Lead II. Similarly, everything is mostly positively in aVF, because it’s really near Lead II. Lead III is a little different, just because it is both inferior and on the right side. As you can see, the average movement of electrical current in the heart is more skewed to the left; so from the perspective of lead III, the overall signal is moving at an angle, so you’ll have more equal positive and negative deflections (rather than mostly positive deflections).

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The lateral leads (Leads aVL, I, V5, and V6) are located on the left arm (aVL and I) and the left side of the chest (V5 and V6). They measure electrical activity of the left lateral side of the heart, which mainly includes the left ventricle (LV).

Lateral leads all look really similar, and they all have tall and upright R waves. This is because they primarily measure the depolarization wave across main mass of ventricles, which skews a lot toward the LV because it is much larger than the RV.

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The anterior leads (Leads V1-V4), also sometimes called anteroseptal leads, are located on the chest, directly over the front (or anterior) part of the heart. They measure electrical activity of the anterior heart, which includes the right ventricle and the ventricular septum.

For these leads, you have to imagine the electrical signal that is coming out of the screen towards the leads on the front of the chest. V1 looks a lot like aVR (with mostly negative deflections), because it’s close to the right atrium, and V4 looks like V5, due to its close proximity to the left ventricle. V2 and V3 are more isoelectric (more equal positive and negative deflections) because the average wave of depolarization is running more perpendicular to them (and less directly at them directly, like V4 or V5).

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The posterior leads are ***.

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EKGs are typically performed on patients as part of the work-up for chest pain (angina). The two most commonly tested applications of EKGs are 1.) arrhythmias and 2.) the localization of myocardial infarctions (heart attacks).

In arrhythmias, there is some defect in the electrical conduction system of the heart, and we can see characteristic changes associated with those on EKG.

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In myocardial infarctions, oxygen supply to an area of the heart is cut off, usually due to occlusion of a coronary artery. This leads to ischemia of that area of myocardium, which alters the electrical signaling through that region. We can use EKG to "localize" the myocardial infarction, or determine the area where the ischemia occurred. More information on myocardial infarctions can be found by clicking the above hyperlink, but here is a summary table of myocardial infarction locations, their corresponding EKG leads, and the coronary arteries effected.