1. Lecture 2
Why do ECGs look like they do?
Part 2

2. ‘The Medical Registrar’ Facebook account, 04/09/2019, credit @jamestoml1

3. Recap! Why do the QRS complexes look like they do?

4. depolarisation

5. repolarisation

6. Wilson’s central terminal

7. Chest leads: Big bulky LV Septal fascicle
“The Only EKG Book You’ll Ever Need”, Malcolm S. Thaler, Eighth Edition, Wolters Kluwer Health, 2014, pg 188

8. Limb leads: Cardiac axis
“The ECG Made Easy”, John R. Hampton, Eighth Edition, Elsevier Ltd 2013, pg 10

9. Cardiac axis
To understand the shape of the QRS complexes in the limb leads, we need to understand about cardiac axis
The cardiac axis represents the net direction of the wave of depolarisation during each heartbeat in this coronal plane
Any questions? Does this all look familiar?
“The ECG Made Easy”, John R. Hampton, Eighth Edition, Elsevier Ltd 2013, pg 10
“The Only EKG Book You’ll Ever Need”, Malcolm S. Thaler, Eighth Edition, Wolters Kluwer Health, 2014, pg 188

10. Working out the cardiac axis

11. Left axis deviation is the 60 between aVL and the top of the circle
In a normal heart, the cardiac axis should be between aVF and aVL (120˚)
We know that a positive trace means that the axis is pointing towards that lead (overall)
Left axis deviation is the 60 between aVL and the top of the circle
Right axis deviation is the left 180˚ of the circle
Extreme right axis deviation (or ‘northwest QRS axis’) is the top-left 90˚
“The ECG Made Easy”, John R. Hampton, Eighth Edition, Elsevier Ltd 2013, pg 16

12. Step 1: lead I Is lead I positive or negative?
a) If negative, you have confirmed right axis deviation
b) If positive, you can exclude right axis deviation
If you have right axis deviation, how could you determine if it was extreme right axis deviation?
Move onto step 2
“The ECG Made Easy”, John R. Hampton, Eighth Edition, Elsevier Ltd 2013, pg 16

13. Lead II: right angle to aVL
Step 2: lead II
Lead II: right angle to aVL
Lead I is positive, so then:
- a positive lead II confirms normal axis
- a negative lead II confirms left axis deviation
“The ECG Made Easy”, John R. Hampton, Eighth Edition, Elsevier Ltd 2013, pg 16

14. (use aVF to determine if extreme RAD)
Lead II
+ve
-ve
normal
LAD
Lead I
RAD
(use aVF to determine if extreme RAD)

15. The shape of the QRS complexes in the limb leads is related to the cardiac axis
So, we can actually work out roughly where the axis lies simply by looking at the ECG trace
The axis will be roughly perpendicular to the lead in which the S-waves and R-waves are equal
“The ECG Made Easy”, John R. Hampton, Eighth Edition, Elsevier Ltd 2013, pg 10

16. https://www. ecgmedicaltraining
“The ECG Made Easy”, John R. Hampton, Eighth Edition, Elsevier Ltd 2013, pg 16

17. Left axis deviation (LAD)
Main causes: hypertrophy and heart block
1) Left ventricular hypertrophy
More current will spread into the LV on each depolarisation
This drags the axis to the left
So left ventricular hypertrophy causes left axis deviation
We will cover this more thoroughly in the ‘heart block’ and ‘hypertrophy’ lectures

19. “The Only EKG Book You’ll Ever Need”, Malcolm S
“The Only EKG Book You’ll Ever Need”, Malcolm S. Thaler, Eighth Edition, Wolters Kluwer Health, 2014, pg 188

20. Left axis deviation (LAD)
2) Heart block:
If one of the fascicles supplying the LV is blocked, then the depolarisation will have to spread to the LV via a different route
We will cover this more thoroughly in the ‘heart block’ and ‘hypertrophy’ lectures

21. “The Only EKG Book You’ll Ever Need”, Malcolm S
“The Only EKG Book You’ll Ever Need”, Malcolm S. Thaler, Eighth Edition, Wolters Kluwer Health, 2014, pg 189

22. Left axis deviation (LAD)
2) Heart block: If one of the fascicles supplying the LV is blocked, then the depolarisation will have to spread to the LV via a different route Left anterior hemiblock or left bundle branch block cause LAD LAD can also be seen in inferior MIs, WPW or ASD It can also be normal in short, overweight individuals
We will cover this more thoroughly in the ‘heart block’ and ‘hypertrophy’ lectures

23. Right axis deviation (RAD)
The same applies: hypertrophy & heart block 1) Right ventricular hypertrophy: Pulmonary pathologies, such as pulmonary embolism or pulmonary hypertension 2) Heart block: left posterior hemiblock RAD also seen in left lateral MI and WPW You may see minor RAD in tall, thin individuals
Right heart strain: deformation of the right ventricular muscle
“The Only EKG Book You’ll Ever Need”, Malcolm S. Thaler, Eighth Edition, Wolters Kluwer Health, 2014, pg 190

24. The P-wave
Remind me, what does the P-wave represent?

25. https://pulmonarychronicles. com/index

26. RA LA
The SAN is found in the junction between the SVC and the right atrium
When the SAN fires, the depolarisation spreads directly into the right atrium
In order to depolarise the left atrium, the depolarisation spreads along Bachmann’s bundle
The P-wave is actually made up of two components: the depolarisation of the right atrium, which happens first, followed by that of the left atrium
P-wave

27. So, the wave of depolarisation from the SAN travels:
- from right to left, along Bachmann’s bundle
- inferiorly, towards AVN

28. Therefore, we expect upwards, positive P-waves in leads that measure:
- the left heart border: I, aVL, V5 and V6
- the inferior heart border: II, III and aVF
So, conveniently, the overall direction of atrial depolarisation mimics our ideal cardiac axis
Lead II should have the most positive P-wave due to its inferior, left-sided position – this is why we use it as the rhythm strip
We see downwards, negative P-waves in aVR because this measures the superior, right heart border
And we see biphasic P-waves in V1 as it's perpendicular
V2-4 are variable, but mostly positive
“The ECG Made Easy”, John R. Hampton, Eighth Edition, Elsevier Ltd 2013, pg 10
“The ECG Made Easy”, John R. Hampton, Eighth Edition, Elsevier Ltd 2013, pg 11

29. https://codeoneapp.com/2018/07/11/unipolar-bipolar-ecg/

30. The T-wave

31. The T-wave represents ventricular repolarisation
A wave of positive repolarisation creates the opposite effect on an ECG trace compared to depolarisation
So a wave of repolarisation moving away from our positive ECG electrodes creates a positive, upwards deflection and vice versa

32. Remember: QRS complex = ventricular depolarisation T-wave = ventricular repolarisation Normally, repolarisation of the myocardium occurs in the last area of the heart that has been depolarised It then travels back along the same paths (ventricular walls  apex  septum) So essentially, ventricular repolarisation is the opposite of ventricular depolarisation And because repolarisation causes an opposite ECG deflection compared to depolarisation…
The reverse of the reverse is the same!

33. Whilst the leads with big S-waves should have inverted T-waves
Thus, the same leads that have a positive, upwards R-wave should also have a positive, upwards T-wave
Whilst the leads with big S-waves should have inverted T-waves
This is a general rule however, and is not always seen in practice
T-waves are usually in the opposite direction to the last deflection in the QRS complex

34. To summarise… why do ECGs look like they do?
QRS complexes:
chest leads: two factors (big bulky LV and septal fascicle)
limb leads: learn the cardiac axis model
P-waves:
left to right (RA to LA, along Bachmann’s bundle)
inferiorly (towards AVN)
T-waves: easy! – will often be in the same direction as the biggest deflection in the QRS complex
“The ECG Made Easy”, John R. Hampton, Eighth Edition, Elsevier Ltd 2013, pg 16
Any questions?