Basic Concepts
Conduction System of the Heart
1. Sino-Atrial (SA) Node
SA Node is a cell that’s found within the right atrium of the heart. Electrical conduction begins here in the SA node.
2. Atrio-Ventricular (AV) Node
As the conduction starts in the SA node, it goes directly to the AV node which is located within the border of the right atrium and the right ventricle. The AV node is known as the “gate-keeper” of the heart as it decides what impulse to send through. That being said, if the SA node sends a weak impulse due to a block for example between the 2 nodes, the AV node can conduct its impulse.
3. Bundle of His
Also known as atrioventricular bundle, are cardiac muscle fibers that conduct electrical impulses received from the AV node and bifurcate into two separate cells (one to the left and the other to the right) which is referred to as Left and Right Bundle Branches.
4. Purkinje Fibers
As the left and right bundle branches extend towards the apex of the heart, they will then be termed as the Purkinje Fibers. These fibers simultaneously activate the left and right ventricles by directly stimulating the myocardium. So, if we make a pattern out of the conduction system of the heart, it goes like this: SA Node —> AV Node —> Bundle of His —> L and R Bundle Branches —> Purkinje Fibers
Inherent Rate
These cells have the capabilities to fire electrical impulses in case there is a blockage in one or more components of the conduction system of the heart. If the SA node generates a normal impulse up to the Purkinje fibers, it conducts a heart rate of 60 – 100 beats per minute (bpm). However, if there is a blockage from the SA node, the AV node can conduct its electrical impulse and can generate 40 – 60 bpm; same with the Bundle of His. If the AV node and the Bundle of His fail, the left and right bundle branches and the Purkinje Fibers can conduct 20 – 40 bpm.
EKG Pattern
1. P Wave
P wave represents atrial depolarization where the two atria are contracting.
2. QRS Complex
The QRS complex represents ventricular depolarization where the two ventricles are contracting.
3. T Wave
T wave represents ventricular repolarization where the two ventricles are relaxing. You might ask, where is the atrial repolarization or the relaxation of the atria? Well, the answer to that is within the QRS complex. Since the ventricles contract stronger than the atria, the QRS complex which represents the contraction of the ventricles masks the atrial repolarization.
Segments and Intervals
Now that we are done with the basics, let us move on to the segments and intervals. There are many segments and intervals found in an EKG result, namely the PR interval, the PR segment, the QRS complex, the ST segment, and the QT interval. But since we are discussing the basics, let’s just talk about the PR interval and the QRS complex as most of the anomalies are found within these two segments and intervals. The other segments are important too but the PR interval and the QRS complex are what we need to read the EKG results in a fast and effective manner. Before we go through the PR interval, let’s first discuss the EKG strip.
The EKG strip contains big boxes with broad lines and inside each big box are 5 small boxes. Each of the small boxes is equivalent to 0.04 seconds. Hence, in 5 small boxes, it is equivalent to 0.20 seconds.
The 6-Step Method How to Interpret Electrocardiogram Results
1. Identify and examine the P wave
A normal P wave is present and upright. P waves should be present before each QRS complexes.
2. Measure the PR interval
The PR interval or PRI should be 0.12 – 0.20 seconds.
3. Measure the QRS complex
The QRS complex should be around 0.06 – 0.12 seconds.
4. Identify the rhythm
Identify if the strip is giving a regular or irregular rhythm. This can be done through the use of a caliper. To do it, put a piece of paper over the strip but make sure the “R” in the QRS complex is visible. Put a marker in the paper where the “R’s” are and then measure each “R’s” in the strip. If they have the same distance from the last pair of “R’s” then it is a regular rhythm; but if not, then it is irregular.
5. Determine the heart rate
Use a 6-second method to identify an irregular heart rate. To do this, we must make sure that we have a 6-second strip. We would know that an EKG result has a 6-second strip when there are 3 lines on the top present in the strip.
Looking at the image above, just count the “R’s” inside the 3 lines and multiply it by 10. That would provide a result of the heart rate. In this example, the heart rate is 60bpm as there are 6 “R’s” found inside the 3 lines.
For a regular rhythm, it best to use the Big Boxes method. This is done by counting the big boxes between 2 “R’s” and dividing it to 300. Say for example in the image below, there are 5 boxes between the 2 “R’s”. Hence, 300/4 will yield 75 – thus, 75bpm.
6. Interpret the strip
Using all the data that were gathered from the previous steps, it’s time to interpret the EKG strip. Always stick to the fundamental concepts like the normal heart rate which is 60-100bpm or in the EKG strip, it will be seen as 3 – 5 big boxes between the 2 “R’s”. Tachycardia is an HR of >100bpm, or in the EKG strip, would mean that there are <3 big boxes between the 2 “R’s”. Bradycardia is an HR of <60bpm, or in the EGK strip, would show that there are >5 big boxes between the 2 “R’s”.
Through familiarizing with these concepts, we will be able to identify if the heart rate is normal, tachy, or brady. Next, check if the QRS complex is narrow or wide. The important concept is that if the QRS complex is <0.12secs (<3 small boxes), then it means it’s a narrow QRS complex. If it’s >0.12 secs (>3 small boxes), then it is a wide QRS complex. Check if the Rhythm is regular or irregular. With these 6 basic steps, one will be able to determine if there are abnormalities in the heart such as major arrhythmias, and coronary ischemia or infarction
The six Tachyarrhythmias
This is a concept by Michael Spinner where in his article, he categorized the 6 tachyarrhythmias that we can easily spot by simply using the 6-step method:
| Tachyarrhythmia | Rate | QRS | Rhythm | P waves |
| Sinus tachycardia | Fast | Narrow | Regular | Yes |
| Supraventricular tachycardia or SVT (AVNRT) | Fast | Narrow | Regular | No |
| Atrial Fibrillation | Fast | Narrow | Irregular | No |
| Atrial Flutter | Fast | Narrow | Regular | Sawtooth |
| Ventricular tachycardia | Fast | Wide | Regular | No |
| Ventricular fibrillation | Fast | Wide | Irregular | No |
What’s Your Take
Being proficient in interpreting EKG results doesn’t come overnight. It needs continuous practice and every day that we do it, we will get better and better. In the beginning, it may seem daunting especially when the result is very far from the ideal image that a normal sinus rhythm would show; but slowly we will be able to improve our abilities to read and interpret it – until such time that it becomes muscle memory.
When our eyes are continuously exposed to the EKG strips, we develop the skill of pinpointing the abnormalities at a much faster rate. But until such time, we should be patient with counting the small boxes, counting the big boxes, marking the 2 R’s, and measuring it using a kardex or if you really want to be ideal, use a caliper. There will come a time when we become so proficient at reading EKG strips, we won’t even need to do these things anymore.
These tips are just a guide for the health care workers to have a basic understanding and knowledge of how Electrocardiogram results are interpreted. There is a lot more to learn when it comes to electrocardiogram but this is a fundamental step to sharpen our skills in reading EKGs.
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