1. ECG: Basic Principles and Use in Myocardial Infarction
Tim Boswell, M4
Presentation created on: 3/7/16
Hi, my name is Tim Boswell and this will be a brief talk on the basics of the ECG. I’m going to focus on how the ECG works by explaining how it is set up on a patient, what the different strips on the ECG paper mean, and a few of the most important patterns. It is my strong belief that if you understand what is going on, you will be able to reason through various presentations and you can rely more on your analytical ability than your memory.

2. Index Case
A 56 year old executive presents to the ER complaining of chest pain that started an hour ago. The pain is sharp, substernal, and radiates to his jaw. He is diaphoretic and appears in distress. His pulse is 48, respiratory rate is 16, and blood pressure is 131/86. An ECG reveals elevation of the ST segments in leads II, III, and aVF. Which vessel is the most likely site of the pathology?
Here is the index case. [read case]. By the end of this presentation, you should understand what the lead grouping II, III, and aVF means as well as the importance of ST elevation and other meaningful ECG patterns.

3. 12 Lead ECG
So, why is it called a 12-lead ECG? Because there are 12 lines or strips on the ECG paper generated by 12 different lead configurations on the patient. Essentially, these are 12 different perspectives: all are looking at the heart, but each is doing so from a different angle. Just imagine looking at 12 pictures of an elephant, all shot from different angles. Each looks a different and tells you something different. A picture taken head on tells you nothing about the tail, etc.

4. 6 Limb Leads 6 Precordial Leads Frontal/Coronal Plane Bipolar
Horizontal/Transverse/ Axial Plane
Unipolar
So the 12 leads come in two types: there are 6 limb leads and 6 precordial or chest leads. Here are a couple generalities about each type and then I will show you the specifics. The limb leads are arranged to give you a perspective of the heart in the frontal plane. The precordial leads view the heart in the horizontal plane. Also, the limb leads are called bipolar leads because they use two physical stickers or leads on the patient and sense the electrical potential difference between the two. The precordial leads, by contrast, are unipolar leads: they are single stickers/leads that just sense electrical activity without comparing to another lead. Let’s take a look at the limb leads first.

5. Limb Leads
3 stickers are placed on the patient to generate the limb lead printouts: one on the right art, one on the left arm, and one on the left leg. As the limb leads are bipolar, they are each defined by 2 stickers. Let me draw this out for you.

6. Limb Leads
Lead I uses the stickers on the right arm and left arm; the left arm is defined as positive and right arm as negative. What that means is that if a wave of depolarization is traveling from right to left, then the deflection on the ECG paper in lead I will be a positive wave. Lead II is between right arm and left leg; lead 3 goes left arm to left leg. So, using these 3 stickers so far (right arm, left arm, left leg), we have can view the heart from 3 different angles in the frontal plane so far. The next 3 leads are a little bit trickier – they are called the augmented leads. I’m going to kind of oversimplify it here, but it think this will make it understandable. For the first augmented lead, the ECG machine uses both the left arm and the left leg as the starting leads (effectively averaging them) and uses the right arm as the second lead. This lead is called aVR (can be remembered by thinking of the R as standing for Right). The next lead uses right arm and left leg as the start and left arm as the finish – avL (L for left). The last lead uses both arms as the start and the left leg as the finish and is called aVF (F for Floor or inFerior). Now we have used 3 stickers to generate 6 leads which allows us to look at the heart from 6 different angles in the frontal plane.

7. Limb Leads Einthoven’s Triangle
If you want to sound fancy, you can talk about Einthoven’s triangle because that’s what this is called. If you rearrange each vector so that they all begin at the same starting point (because each line is really just a direction of focus), it will start to make sense how many different viewpoints you have. And, keep in mind that if an electrical activity of interest occurs going in the opposite direction of the lead’s definition, the ECG paper shows a negative deflection instead of a positive deflection (and vice versa). So, you really have all of these angles covered too (shown by dotted lines). We have a pretty comprehensive view of the heart in the frontal plane this way.
Einthoven’s Triangle

8. 6 Limb Leads 6 Precordial Leads Frontal/Coronal Plane Bipolar
3 stickers
I, II, III, aVR, aVL, aVF
Horizontal/Transverse/ Axial Plane
Unipolar
6 stickers
V1, V2, V3, V4, V5, V6
Recapping the limb leads: you use 3 physical connections on the patient to generate 6 different lines on the ECG strip. Leads I, II, III, aVR, aVL, and aVF. Each of these gives you a slightly different perspective of the heart viewed from different angles in the frontal plane. Now, on to the precordial or chest leads. Each is measured by an individual sticker on the patient’s chest and the 6 leads are called V1 through V6.

9. Precordial Leads
6 stickers are placed across the chest to serve as the precordial leads. By sensing the electrical activity in their positions, they effectively look into the chest at the heart, allowing them to tell you information about the heart’s electrical activity in the horizontal plane.

10. Precordial Leads Lead Placement V1
Right border of sternum, 4th intercostal space V2
Left border of sternum, 4th intercostal space V3
Between V2 and V4 V4
Left midclavicular line, 5th intercostal space V5
Left anterior axillary line, 5th intercostal space V6
Left midaxillary line, 5th intercostal space
One day when you are working face-to-face with patients, you may need to know the specific lead locations on the chest (although an ECG technician usually does this for you).

11. 6 Limb Leads 6 Precordial Leads
Here is a reminder of the 12 leads that make up the 12-lead ECG.

12. 6 Limb Leads 6 Precordial Leads Frontal/Coronal Plane Bipolar
3 stickers
I, II, II, aVR, aVL, aVF
Horizontal/Transverse/ Axial Plane
Unipolar
6 stickers
V1, V2, V3, V4, V5, V6
Here is a summary of how the 12 leads provide different viewing angles of the electrical activity of the heart.

13. Lead Groupings V1, V2 V3, V4 I, aVL, V5, V6 II, III, aVF
Let’s talk now about lead groupings like the one that we saw in our index case. Knowing from which direction you are viewing the heart will help you to make sense of these groupings. Leads V1 and V2 are placed centrally on the chest and look at the interventricular septum. Leads V3 and V4 view the anterior heart. Leads V5 and V6 see the left lateral aspect of the heart best. Also, note that leads I and aVL have leftward components to their vectors, so they see the left lateral heart as well. The 3 leads that look downward, II, III, and aVF, look at the inferior part of the heart.
I, aVL,
V5, V6
II, III, aVF

14. Lead Groupings
Here is another helpful picture that summarizes which leads have the best angle of perspective of certain regions of the heart. Here is an ECG strip. The lines corresponding to lead are organized in a standard fashion with leads I, II, and III in the left most column, etc. Some people like to memorize a chart like this one to remember important lead groups. Understanding the principles behind the lead grouping will help you remember them, whether you memorize a chart like this or not.

15. Transmural infarction Subendocardial infarction
Important Patterns
Pattern
Appearance
Meaning
Q waves
ST elevation
ST depression
T wave inversion
Infarction, old or new
Transmural infarction
If you recall our index case, the patient’s ECG revealed ST elevation in leads II, III, and aVF. We now know that the lead grouping is indicating that the inferior aspect of the heart is affected. What does ST elevation (or the other patterns, for that matter) mean?
Subendocardial infarction
Ischemia

16. Coronary Circulation Grouping Region Vessel V1, V2 Septal V3, V4
Anterior
I, aVL, V5, V6
Lateral
II, III, aVF
Inferior
LAD
LAD
LCX
Knowing the region of the heart affected allows you to determine which vessel is likely involved.
RCA

17. Index Case
A 56 year old executive presents to the ER complaining of chest pain that started an hour ago. The pain is sharp, substernal, and radiates to his jaw. He is diaphoretic and appears in distress. His pulse is 48, respiratory rate is 16, and blood pressure is 131/86. An ECG reveals elevation of the ST segments in leads II, III, and aVF. Which vessel is the most likely site of the pathology?
Putting it together: the patient in our index case has ST elevation in leads II, III, and aVF which suggests transmural infarction of the inferior heart, most likely because of a lesion in the RCA. Why is he bradycardic (with pulse 48)? Branches off the RCA supply both the SA node and the AV node so his pacing centers are likely ischemic and not functioning correctly. What does this man need? Cath lab for likely percutaneous intervention (stent, etc.).

18. Summary The End ECG basics and principles Lead groupings
Important patterns
The End