EKG Interpretation, part 2: Placement of EKG leads, Heart Conduction System and EKG Basics
July 10, 2020 Updated: July 29, 2021 9 min read 1 Comment
In this article, we cover EKG electrode placement when conducting a 12-lead EKG, the steps in the heart conduction system and some EKG basics to know. These basics are important to learn before you really dive into EKG interpretation. The EKG interpretation video series follows along with our EKG interpretation flashcards, which are intended to help RN and PN nursing students study for nursing school exams, including the ATI, HESI, and NCLEX.
12-lead EKG placement
When performing a 12-lead EKG on a patient, we place 10 electrodes on their body. Four electrodes are placed on the patient's limbs and 6 on the patient's chest.
You might be wondering why there are 10 electrodes in a 12-lead EKG, and not 12! Leads actually don’t refer to the electrodes, but rather the perspectives through which we view the heart. So a "12-lead" means looking at the heart from 12 angles, which we obtain through 10 electrodes. There are some different configurations, but most 12-leads are done with 10 electrodes.
Lead placement on patient's limbs
- Lead LA = Left arm
- Lead LL = Left leg
- Lead RA = Right arm
- Lead RL = Right leg
Lead placement on patient’s chest
Leads placed on the patient's chest are placed relative to intercostal spaces (spaces between the ribs), and along vertical anatomical lines. You can think of the intercostal spaces like Y coordinates, or latitudes, and the vertical anatomical lines like X coordinates, or longitudes.
Place lead V1 on the 4th intercostal space (space below the 4th rib), at the right sternal border (where the sternum meets the ribs on the right side). This is the only lead that is placed on the right side of the chest.
Place lead V2 on the 4th intercostal space, at the left sternal border (where the sternum meets the ribs on the left side).
V4 comes before V3! This is simply because V3 is placed in between V2 and V4, so you need to place V4 first to know where V3 should go.
Place lead V4 on the 5th intercostal space (space below the 5th rib), at the midclavicular line (the line running vertically down the body from the midpoint of the clavicle). You may remember that this is the location where you can measure apical pulse.
Place lead V3 between V2 and V4.
Place lead V5 level with V4 at the anterior line (the line running vertically down the body where the armpit meets the chest in the front).
Place lead V6 level with V5 at the left mid-axillary line (the line running vertically down the body from the middle of the armpit).
The chest EKG placement locations are often near a patient's inframammary fold, which is the point where the breast meets the chest. This area may be obscured by larger breasts that descend over that fold.
One important thing to keep in mind when performing an EKG on a patient with larger breasts is that you may need to lift the breast in order to place the leads correctly. We recommend bringing a second RN or CNA to assist you. It’s very important to communicate the procedure with your patient beforehand and explain that their breast will need to be lifted in order to place the electrodes.
Heart conduction system
Definitions of terms in the heart conduction system
|Atria||Upper two chambers of the heart|
|Atrioventricular (AV) node||Electrical connection between the lower and upper chambers of the heart|
|Bundle branches (left, right)||Offshoots of the bundle of His leading to the left or right ventricle|
|Bundle of His||A bundle of muscle fibers that transmit electrical impulse|
|Sinoatrial (SA) node or Sinus node||A group of cells in the heart that act as the heart's natural pacemaker by spontaneously producing an electrical impulse.|
|Ventricles||The lower two chambers of the heart|
|Ventricular myocardium||The muscle layer of the ventricles|
Steps in the heart conduction system
- The SA node initiates an electrical impulse, which stimulates the atria to depolarize. Atrial depolarization is recorded as a P wave on the EKG
- The electrical impulse travels down to the AV node, where there is a DELAY to allow the blood in the atria to empty into the ventricles. This pause is recorded as the PR segment on the EKG, shown as the short flat line after the P wave.
- The electrical impulse then travels down rapidly to the bundle of His, which transmits the impulse to the left and right bundle branches, where it travels to the ventricular myocardium. Depolarization of the ventricular myocardium triggers ventricular contraction. Ventricular depolarization is recorded as the QRS complex on the EKG. Atrial repolarization also occurs during this time.
- Repolarization of the ventricular myocardium then occurs. The ST segment represents the initial phase of ventricular repolarization, and the T wave represents the rapid phase of ventricular repolarization.
Box size on an EKG strip
EKG strips are split into big boxes, which are then each further split into smaller boxes.
Each big box on an EKG strip represents 0.2 seconds in duration.
- 5 big boxes = 1 second
- 30 big boxes = 6 seconds
- 300 big boxes = 60 seconds
Each small box on an EKG strip represents 0.04 seconds in duration.
- 5 small boxes = 1 big box (5 x 0.04 seconds = 0.2 seconds)
- 25 small boxes = 1 second
- 150 small boxes = 6 seconds
- 1500 boxes = 60 seconds
|Big boxes||Small boxes||Seconds|
|⅕||1||.04 or 1/25|
|1||5||0.2 or ⅕|
Normal duration and amplitude of EKG components
The amplitude of the P wave can be up to 2.5 mm (2.5 small boxes high) and duration between 0.06 - 0.12 seconds (1.5 - 3 small boxes).
The duration of the PR interval is normally between 0.12 - 0.20 seconds (3-5 small boxes).
The duration of a normal QRS complex is between 0.04 - 0.10 seconds. QRS complexes with a duration < 0.12 seconds (3 small boxes) are considered normal.
The normal amplitude of the ST segment is usually isoelectric (0 mm), but may be between -0.5 and 1mm.
The normal amplitude of the T wave is up to 10 mm (10 small boxes high).
|P wave||.06 - .12 seconds||2.5mm or 2.5 small boxes|
|PR Interval||.12 - .20 seconds||n/a|
|QRS complex||.04 - .10 seconds||n/a|
|ST segment||n/a||-0.5 - 1.0mm, or flat|
|T wave||n/a||10mm, or 10 small boxes|
In this video, we are going to talk about placement of the EKG electrodes when performing a 12-lead EKG. Then we'll talk about the different steps in the heart conduction system, and how those steps are reflected on an EKG waveform, and then we're going to talk about some EKG basics that you'll need to know before we go into interpretation of EKGs.
So when we are doing a 12-lead EKG, we will place 10 electrodes. So 4 of those electrodes will go here on the limbs, and 6 will go on the chest.
For the limb leads, we would place LA on the left arm, LL on the left leg, RA on the right arm, and RL on the right leg.
Then moving over here to the chest, V1, in red, will go in the 4th intercostal space, at the right sternal border. This will be the only electrode that goes on the right side of the chest.
Then V2, here in yellow, will also be placed in the 4th intercostal space, but it will be at the left sternal border.
Then we're going to skip to V4, here in blue. That is placed in the 5th intercostal space, at the midclavicular line. So here's the clavicle here. If you go straight down from the middle of that to the 5th intercostal space, that's where V4 goes.
Then V3 is placed in between V2 and V4.
V5, in orange, is placed level with V4 at the left anterior axillary line.
And then lastly, V6 here in purple is placed level with V5 at the left mid-axillary line.
One thing to keep in mind, if your patient is female and they have larger breasts, then you're going to need to lift that breast up to be able to place these electrodes. If that is the case, I would really recommend bringing a second RN or CNA in to assist you. And you definitely want to communicate effectively with your patient before just getting in there and lifting their breast. Just explain the procedure and that you're going to need to lift their breast to place these electrodes.
Now let's talk about the steps in the heart conduction system and how these steps are reflected here on the EKG waveform. So these steps are described in detail on Card 2 in my EKG interpretation flashcard set.
So starting off, the SA node, or sinus node, initiates an electrical impulse which causes the atria to depolarize and to contract. So the P wave here reflects atrial depolarization.
Then the impulse travels down to the AV node, where there is a delay to allow the blood from the atria to empty into the ventricles. So this delay or pause is reflected as this flat line after the P wave, which is the PR segment.
Then the impulse travels rapidly to the bundle of His, and then to the left and right bundle branches, and then to the ventricular myocardium. This causes the ventricular myocardium to depolarize, which causes the ventricles to contract. So the QRS complex here represents the time where the ventricles are depolarizing. The other thing that's happening during this time is atrial repolarization.
Then we have ventricular repolarization. So the ST segment, the flat line after the QRS segment, represents the initial phase of ventricular repolarization, and the T wave here represents the rapid phase of ventricular repolarization.
Okay, now let's talk about some EKG basics which are important to know before we get into EKG interpretation. So the information I'm going to talk about here can be found on card 5 in my EKG interpretation deck.
Let's first talk about small and big boxes.
Each small box on an EKG strip represents 0.04 seconds in duration. There are 5 small boxes in 1 big box, so each big box is 0.2 seconds in duration.
There are 5 big boxes in 1 second. And if we're looking at a 6-second strip, that will have 30 big boxes. And if we're talking about a full minute or 60 seconds, that will have 300 big boxes or 1,500 small boxes.
And those numbers are going to be important to remember, especially when we are calculating heart rate in my next video.
Now let's look at the different EKG components here in the waveform and talk about what we can expect in terms of amplitude or height and what we can expect in terms of duration or width of each of the components.
So when we are looking at the P wave, the P wave should be up to 2.5 millimeters or 2 and a half small boxes high. In terms of duration, it should be between 0.06 and 0.12 seconds in duration. So it looks like our P wave is exactly 3 small boxes wide, approximately, which means it's 0.12 seconds in duration.
And then, when we're looking at the PR interval, which includes the P wave as well as that PR segment right after the P wave, that PR interval should be between 0.12 and 0.2 seconds in duration, which translates to 3 to 5 small boxes wide. So it looks like our PR interval is exactly 5 small boxes wide, which means it's 0.2 seconds in duration.
Then looking at the QRS duration, the QRS duration should be between 0.04 and 0.1 seconds in duration. However, it is generally accepted that as long as the QRS duration is under 3 boxes wide, which is 0.12 seconds in duration, then it is considered normal.
Then when we're looking at ST segment after the QRS complex, this should be roughly flat; it should be isoelectric. It can vary slightly, so it may be 1 small box above baseline or a half of a small box below baseline. But in general, it should be flat, zero millimeters.
Then the T wave can have an amplitude up to 10 millimeters in height, or 10 small boxes in height.
So out of all of these numbers I've just thrown at you, the most important ones to remember are the PR interval, which should be between 3 and 5 small boxes in duration, and the QRS complex, which should be under 3 small boxes in duration.
Now that we have those basics out of the way, we are ready to interpret our strips. So hang in there with me, and I'll see you soon!
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