UNITED INTERNATIONAL MEDICAL SCHOOLS
Beyond the Peaks - Decoding the QRS
- Muhammad Yaseen Nivas
ECG is one of the most iconic symbols of medicine. From movie scenes with the classic “bip bip beeeeeeeep” to the QRS complex. Medical students at any stage should be able to explain to some level of detail an ECG. In this article, we go over the basics and explore a couple of pathologies that can be identified using the ECG.
What is it?
ECG is short for electrocardiography. Breaking down this compound word into its root components: ‘electro’ comes from a latinised form of a Greek word referring to electricity, ‘cardio’ is a Greek stem meaning heart, ‘graphia’ is a borrowed word from Ancient Greek meaning something written or drawn. All in all ECG is the drawn measurement of the electrical activity of the heart.
Quick Anatomical Review
The electrical conduction system of the heart is responsible for the spontaneous depolarization and thereby contraction of the heart. Depolarization, the reversal of cell membrane polarity, when propagated via action potentials forms the basic unit of electrical signals by cells. The excitatory signal of the heart musculature is created at the sinoatrial node (SA node, also known as the pacemaker) located in the right atrium (for the anatomy nerds, its more specifically in the crista terminalis). The depolarization is then conducted to the Atrioventricular node (AV node), where it then moves onto the Bundle of His into the interventricular septum where it divides into left and right bundle branches and eventually into Purkinje fibers (smallest), which spread the electrical impulses to maximize reach.
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The Leads
Effective electrical visualization of the heart is done by observing the heart in different planes using leads. Why different planes? As we know a 2D image only gives information about …. well 2 dimensions. But the electrical activity of the heart is 3 dimensional, so we use many planes of observation. For example, looking at a cone from the apex, you can only say you see a circle. But a person looking from a different angle can see the protrusion of the apex from the base and can thus identify it as a cone. Similarly, from one ‘lead’ you may not see anything pathological, but the key to the diagnosis could be in another lead!
Standardly, 12 leads are used. 6 chest leads and 6 others, allowing us to make a comprehensive assessment.
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Caption: 6 chest leads. Source: ECG made easy
If a depolarization moves toward a lead, we visualise an upward spike, conversely, if a repolarization wave moves towards a lead, we visualise a downward spike.
The Classic ECG
When one hears ‘ECG’, this is perhaps the first image that comes to mind. The depolarization of the heart chambers are characteristic and have varying magnitudes. The P wave denotes atrial depolarization, the large QRS complex indicates the ventricular depolarization, which make up the bulk of the heart and the T wave represents the REpolarization of the ventricles. Each component is as crucial as the other for diagnosis.
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The PR segment, represents the time taken between atrial and ventricular depolarization. This is significant because there needs to be a sufficient delay for ventricular filling to happen before depolarization. An abnormally short PR segment, could indicate either an alternative conductive path (bypassing the otherwise solitary connection between the atria and ventricles, the AV node) or an abnormally excited AV node.
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Caption: The PR segment is almost nonexistent. The Wolff-Parkinson-White (WPW) syndrome is due to an accessory atrioventricular connection.
On the other hand, an abnormally long PR segment indicates an AV block (more on that here: https://en.wikipedia.org/wiki/Atrioventricular_block).
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Caption: In a second degree AV block, the impulses between the atria and ventricles are so impaired, that there are skipped beats, which is evident from the 1180 ms gap indicated. A first degree AV block would have a long PR segment, but the abnormal delay is not severe enough to result in skipped beats.
Coming to the QRS , being the focal point of attraction with its peak has a lot of diagnostic values, it is the most significant part of the ECG as it represents the depolarization of the bulk of the cardiac musculature: the ventricles. Firstly, the ‘amplitude’ of a lead is calculated using the deflections of the complex.
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This can be used to determine the net direction of depolarization, but that's an article for another day.
Thus, if the QRS interval is higher, it points to a faulty conduction system (because the depolarization is slower).
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From: https://johnsonfrancis.org/professional/very-wide-qrs-complex/
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The T wave represents the repolarization of the ventricles. A lot of pathologies can be detected, but we will just name a few here. Flat T-wave (hypokalemia), Peaked T-wave (hyperkalemia), Inverted T-wave (depends on lead).
Adapted from: https://commons.wikimedia.org/wiki/File:DeBrugada_ecg_characteristics_%28CardioNetworks_ECGpedia%29.png
The U wave (if present) represents the repolarization of the papillary muscles, which when contracted prevent the backflow of blood through the AV valves.
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Deciphering the ECG, particularly when a pathology is present, remains a daunting task for many medical students, it extends further than the ‘bip bip beeeeeeeep’ and is a crucial cornerstone of medicine and indispensable for effective patient care.
References:
1. ​​https://en.wikipedia.org/wiki/PR_interval
2.https://en.wikipedia.org/wiki/Wolff%E2%80%93Parkinson%E2%80%93White_syndrome
3. The ECG Made Easy, John Hampton; Joanna Hampton
