Understanding and Interpreting the Electrocardiogram

Written By Vaishnavi Kolluru

The electrocardiogram (ECG/EKG) is a clinical method used to visualize heart contraction patterns for diagnostic purposes. The detector placed on the body allows tracking of the movement of depolarization and therefore contraction through the heart. The events of contraction are as follows. 

First, the sinoatrial (SA) node is the pacemaker of the heart, so it initiates and creates the contraction pace according to autonomic input from the pons and medulla oblongata. This rate normally ranges between 60 to 100 depolarizations per minute. To decrease the pace of SA node depolarization-repolarization sequences, the parasympathetic fibers (both preganglionic and postganglionic) release acetylcholine as a hormone. To increase the pace, the adrenal medulla synthesizes and releases catecholamine hormones (mainly epinephrine and norepinephrine) to activate a sympathetic cardiac response.

Each depolarization spreads through the atrium from the SA node. Then, the charge spreads towards the apex of the heart. This happens to be towards the direction of the electrical detector in an ECG test. The AV node pauses this charge spread, and then sends the electrical signals through a system of electrically conductive fibers. This is atrial systole, and is visible on the ECG as the P-wave.

The bundle of His branches down from the AV node, and splits into the right and left (anatomical directions!) bundle branches. These bundle branches further split into Purkinje fibers, which spread into the heart muscle of the ventricles. Notice below that the orientation of the fibers is away from the apex of the heart, so you should see a depression in the ECG when positive charge spreads in that direction. However, the depolarization towards the apex is stronger than the depolarization away from the apex due to the fibers, since some of the fibers are in the direction of the apex of the heart. So you actually see the QRS complex, which is indicative of ventricular systole.

To “reset the heart,” cells repolarize. However, they repolarize in the opposite direction of the apex of the heart, so essentially, negative charges are moving away from the detector. This appears as a net positive in the ECG, as a T-wave.*

* So um, this may be false. Different sources are saying different things. I’ve discussed this with a friend, and they said the lead on the upper body (since there are multiple) detects negative charge, so repolarization in the direction of the upper body lead would create a positive deflection. This is something I’m confused about still, so if you know the answer, please reach out to me at k27.vaishnavi@gmail.com!

ECG abnormalities indicate improper heart function. Below are various ECGs:

The ECG labeled (a) depicts a normal ECG.

The one labeled (b) has two P-waves, and a shortened T-wave. This indicates that there is a 50% discrepancy between SA and AV node communication. In other words, they are unable to share ions, so their beating frequencies don’t mesh. Now we have two rhythms occurring at different times. Thus, we see initiation but no follow-through on the second P-wave, since the node interplay is faulty here.

The one labeled (c) is experiencing a full block, in which the signals created by the SA node are completely independent from the signals from the AV node. Thus, each independent component is evenly spaced from repetitions of itself, but they are not in order.

To clarify, the SA node creates P-waves. The AV node creates the QRS complex and T-wave.

These AV blockages impede communication between the SA and AV node. Examples of causes include fibrosis (tissue thickening/scarring) and sclerosis (calcification/thickening), which can cause physical blockage between the SA and AV nodes.

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Vaishnavi Kolluru
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