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

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

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

depolarisation

repolarisation

Wilson’s central terminal http://what-when-how.com/paramedic-care/diagnostic-ecgthe-12-lead-clinical-essentials-paramedic-care-part-1/

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

Limb leads: Cardiac axis https://www.sciencedirect.com/topics/nursing-and-health-professions/electrocardiographic-lead “The ECG Made Easy”, John R. Hampton, Eighth Edition, Elsevier Ltd 2013, pg 10

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

Working out the cardiac axis

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

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

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

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

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

https://www. ecgmedicaltraining https://www.ecgmedicaltraining.com/importance-of-lead-avl-in-stemi-recognition/ “The ECG Made Easy”, John R. Hampton, Eighth Edition, Elsevier Ltd 2013, pg 16

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

https://www.mdedge.com/ccjm/article/95297/cardiology/left-ventricular-hypertrophy-overlooked-cardiovascular-risk-factor

“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

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

“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

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

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

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

https://pulmonarychronicles. com/index https://pulmonarychronicles.com/index.php/pulmonarychronicles/article/view/485/1053

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

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

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

https://codeoneapp.com/2018/07/11/unipolar-bipolar-ecg/ https://www.wikidoc.org/index.php/File:NormalSR.jpg

The T-wave

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

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!

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 http://pedcard.rush.edu/MP/ECG/ECG%201.htm

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?

www.surveymonkey.co.uk/r/29Z3VF5