Session 6: ECG 1 Flashcards
Briefly explain the conducting system of the heart.
Sinoatrial node going to AV node. Going to bundle of His into right and left bundle branches. They end up in purkinje fibres.
Where is the fibrous ring of the heart and what is its purpose?
It’s in plane between atria and ventricles and consist of a dense connective tissue which forms four fibrous rings. They act as electrical insulators to make the atria contract separately from the ventricles.
If the fibrous ring prevent electrical conductions to reach the ventricles. Then how come the ventricles contract?
Because of the bundle of his. There is a small passageway in the fibrous ring which allows electrical conduction to pass through.
Role of the SA node.
It’s the fastest rate of depolarisation and sets the rhythm (sinus rhythm) of the heart.
Purpose of the AV node.
To slow conduction in order to give time for the atria to contract before the ventricles.
Purpose of the purkinje fibres.
Rapid spread of depolarisation (4 m/s) throughout the ventricular myocardium.
If depolarisation occurs towards an electrode. How will the electrode read it?
As an upwards deflection.
If depolarisation occurs away from an electrode. How will the electrode read it?
As a downwards deflection.
If repolarisation occurs towards an electrode. How will the electrode read it?
As a downwards deflection.
If repolarisation occurs away from an electrode. How will the electrode read it?
As an upwards deflection.
What does the T wave represent?
Upwards deflection because the ventricle repolarises away from the ventricle.
How does SA node depolarisation show up on ECG?
It doesn’t. The signal is not strong enough.
What is the P wave?
Atrial depolarisation.
Explain atrial depolarisation.
Spreads along atrial muscle fibres and internodal pathways.
Explain atrial depolarisation. (Direction as well)
Spreads along atrial muscle fibres and internodal pathways. It does so throughout both right and left atria. Direction is downwards and to the left towards the AV node. It will produce a small p wave.
How long does atrial depolarisation last?
80-100 ms
What does the delay at the AV node look like on ECG?
Like an isoelectric segment (flatline)
What does the conduction via the bundle of His look like?
An isoelectric segment again.
How longs does it take from the start of atrial depolarisation (start of p wave) to the start of ventricular depolarisation?
120-200 ms
What is the first part of the ventricle to depolarise?
The muscle in the interventricular septum.
What does the interventricular septum depolarisation look like on an ECG?
It looks like a small downward deflection. It is termed the q wave. Small case q because it is small.
Why is the q wave a downwards deflection?
Because the depolarisation is moving obliquely away towards the wall of the interventricular septum.
What will depolarise after the muscle of the interventricular septum?
The apex and free ventricular wall.
What does depolarisation of apex and free ventricular wall look like?
A large upwards deflection since it is moving directly towards the electrode. It is large because there is a large muscle mass meaning more electrical activity.
What might an abnormally large R wave indicate?
LV hypertrophy
Why is there a small downwards deflection at the end of the QRS complex (S wave)?
Because the depolarisation will finally spread upwards to the base of the ventricles. It is downward because it is moving away from the electrode. It is small because it is not moving directly away.
How long does the QRS complex last?
Around 80-120 ms.
What happens after the upwards depolarisation?
Repolarisation of the ventricles.
What will repolarisation of the ventricles look like on an ECG?
As an upwards deflection termed the T wave.
Why is repolarisation of the ventricles an upwards deflection?
It begins on the epicardial surface and spreads in the opposite direction to depolarisation. It produces a medium upwards deflection because it is moving away from the electrode.
What is P wave?
Depolarisation of the atria (NOT CONTRACTION)
What is QRS complex?
Depolarisation of the ventricles (NOT CONTRACTION)
How many electrodes are used in ECG?
10 in total. 4 on the limbs and 6 on the chest. It gives in total 12 views of the heart.
How many views do the limb leads give?
6 views even though they are 4 electrodes.
What views do the limbs leads give?
Lead I Lead II Lead III aVR aVF aVL
Show where you will find the 6 views of the limb leads.
Put them at the right degrees and give them their correct name.
In which plane do the chest leads view the heart?
In the horizontal plane.
How many chest leads are there?
6
Name the chest leads.
V1
V2
V3
V4
V5
V6
Which are the antero-septal leads?
V1-V4
What are V5 and V6 called?
Lateral leads
What do V1 and V2 face?
The right ventricle and the septum also called septal leads.
What do V3 and V4 face?
The apex and the anterior wall of RV and LV.
What do V5 and V6 face?
The left ventricle
Explain how and where to position the 6 chest leads.
Look for the angle of Louis also called the sternal angle.
V1 will be placed to the right (patient view) of the sternum in the 4th intercostal space. Where the 4th intercostal space meets the sternum is where you put V1.
V2 will be placed to the left (patient view) of the sternum in the 4th intercostal space where the intercostal space meets the sternum.
V4 will be put in the 5th intercostal space in level with the mid-clavicular line.
V3 will be put at the midpoint between V2 and V4.
Follow the 5th intercostal space to the axilla. When your fingers are at the beginning below of the axilla this is where you put V5.
V6 will be put below the centre point of the axilla.
Which are the best limb leads to look at problems of the lateral wall of the left ventricle?
Lead I and aVL
Which leads are best at looking at the inferior surface of the heart?
Leads II, III and aVF
If you have muscle necrosis due to occlusion of the right coronary artery. Where would you see this on the limb leads?
In the inferior leads (II, III and aVF)
If you have muscle necrosis due to occlusion of branch of the left coronary artery. Which leads would you look at?
The lateral leads looking at the left side of the heart (I and aVL).
Which ECG leads face inferior surface of ventricles?
(All leads)
II, III and aVF
Which ECG leads face right ventricle and septum?
V1 and V2
Which ECG leads face apex and anterior surface of ventricles?
V3 and V4
Which leads face the lateral surface of ventricle?
Lead I, aVL, V5 and V6
What does the horizontal axis of an ECG signify?
Time in seconds
What does the vertical axis signify of an ECG?
Voltage in mV
How much time does 5 large squares of an ECG signify?
1 second.
How many large squares of an ECG paper is 1 minute?
300 large squares.
How much time is a small square?
40ms
How much time is 1 large square?
200ms
How much time is 5 small squares?
200 ms
Explain how to calculate the heart rate when the rhythm is regular.
Each cardiac cycle. It’s easiest to count from R interval (peak of R) to next R interval.
Remember heart rate is all in relation to minute (bpm). 1 minute is 300 large boxes.
So we find out how many boxes will fit into the R-R interval.
Say 4 large boxes fit into 1 R-R interval.
This gives us 300/4=75
75 bpm
Explain how to calculate heart rate if the rhythm is irregular.
We cannot use R-R interval because of the irregularity.
We calculate the heart rate instead by counting the number of QRS complexes in 6 seconds (30 large boxes).
Then we multiplicate that number by 10.
Say there are 7 QRS complexes in 6 seconds.
7x10 gives 70 bpm.
Where can you find the PR interval on an ECG?
Start of P until the start of Q.
How long should the PR interval be?
120 to 200ms aka 3-5 small boxes.
It is prolonged if it >1 large box.
Common cause of prolonged PR interval.
Delayed conduction through AV node and bundle of His.
Where can you find the QRS interval?
Start of q until the end of s.
How long should the QRS interval be?
The time taken for ventricular depolarisation should be less than 120ms or less than 3 boxes.
Common cause of widened QRS interval.
A depolarisation that arises in the ventricle and not spreading via the rapid conducting His-Purkinje system which is why it takes more time.
Where can you find the QT interval?
Time taken for depolarisation and repolarisation of ventricle.
This means that it is from the start of q until the end of T.
What is corrected QT interval (QTc)?
Since the QT interval varies with heart rate there are calculations to correct it for heart rate.
What is the upper limit of a corrected QT interval?
11 small boxes.
What does a prolonged QTc indicate?
Prolonged ventricular repolarisation.
How do you determine normal sinus rhythm?
Is the rhythm regular (sinus rhythm)
What is the heart rate?
Are there p waves?
Are the the p waves upright in leads I and II?
Is the PR interval normal?
Is every p wave followed by QRS?
Is every QRS preceded by a QRS complex?
Normal width of QRS?
If all criteria are met it is sinus rhythm
What is the limit for sinus bradycardia?
<60 bpm
What is the limit for sinus tachycardia?
>100 bpm
