ECG: The Electro Cardio Gram

Question 1

Describe the conducting system of the heart?

Answer 1

Specialised cells are able to generate action potentials and conduct the impulse very rapidly to all subendocardial regions of the ventricles, resulting in depolarisation of myocytes (ventricles depolarise from endocardium to epicardium), enabling coordinated contraction of the atria and ventricles.

Question 2

Why does the Sino Atrial Node (SAN) set the rhythm of the heart?

Answer 2

It has the fastest rate of depolarisation.

Question 3

Where does an impulse pass after being derived from the SAN and why is this necessary?

Answer 3

The AtrioVentricular Node (AVN) is continuous with the Bundle of His, which is the only conducting pathway between the atria and ventricles, as impulses can’t travel through the fibrous ring, which the muscles are attached to.

Question 4

Where are the right and left bundle branches and what are the Purkinje fibres?

Answer 4

The right and left bundle branches lie subendocardially in the interventricular septum.
The purkinje fibres are fine branches of the BoH, where there’s rapid spread of depolarisation throughout the ventricular myocardium (4m/s).

Question 5

In what order to the components of the ventricles depolarise?

Answer 5

Myocytes of the interventricular septum are depolarised first, then the apex and r and left free walls, with the base of the ventricles the last to depolarise - repolarisation happens in reverse order.

Question 6

What does an ECG do?

Answer 6

It records changes on the extracellular surface of cardiac myocytes during the wave of depolarisation and repolarisation, from the surface of the body, using electrodes pasted on the skin.

Question 7

What changes in the cardiac myocytes produce a dip on an ECG?

Answer 7

A wave of depolarisation travelling away from the electrodes view or a wave of repolarisation towards it.

Question 8

What changes in the cardiac myocytes produce a peak on an ECG?

Answer 8

A wave of depolarisation towards and electrode or a wave of repolarisation away from an electrode produces a peak?

Question 9

Why might there be a straight line on an ECG?

Answer 9

If the myocytes are isoelectric, even if they are all depolarised.

Question 10

What difference does it make now directly a wave of depolarisation is travelling towards an electrode?

Answer 10

If a wave is travelling directly to (instead of obliquely towards) a view, there will be a taller complex. A wave at 90 degrees to an electrode will show a biphasic (little up then little down) or no complex.

Question 11

What is the first electrical event in the heart and how does this show on an ECG?

Answer 11

The SAN depolarisation, nut there’s insufficient signal to register on a surface ECG.

Question 12

Describe the depolarisation of the atria and its effect on the ECG.

Answer 12

Depolarisation spreads along the atrial muscle fibres and intermodal pathways (throughout L and R atria), downwards and to the left (towards the AVN). This produces a small upward deflection of the p wave, as it’s travelling towards the recording positive electrode.

Question 13

What follows the p wave on an ECG and why?

Answer 13

There is delay at the AVN with conduction slowed down, so there’s time for the atrial contraction to fill the ventricles - the signal is very small, meaning an isoelectric flat line (fibrous ring between atria and ventricles means no direct contact between their myocytes).

Question 14

What is the point of the fibrous ring between the atria and ventricles?

Answer 14

The fibrous ring is only crossed by the Bundle of His, so depolarisation can only travel down it and is thereafter rapidly conducted by the His-Purkinje system (also contributing to the isoelectric segment from AVN depolarisation).

Question 15

What, on an ECG, is a q wave and how is it produced?

Answer 15

Depolarisation of the myocardium, first to do so being the muscle in the interventricular septum, which does so from left to right, producing a small downward deflection, the q wave, as the wave is moving obliquely away.

Question 16

What is the result on an ECG of depolarisation of the apex and free ventricular wall and why?

Answer 16

A large upward deflection, known as the R wave, because depolarisation is travelling directly towards the electrode and a large muscle mass means more electrical activity (correspondingly taller as the LV hypertrophies).

Question 17

What is the S wave on an ECG and what is it caused by?

Answer 17

A small downward deflection to finish the QRS complex, caused by the depolarisation finally spreading up to the base of the ventricles. It is moving away, but not directly.

Question 18

Ventricular repolarisation begins on the epicardial surface and spreads in the opposite direction to depolarisation, producing a medium upward deflection on an ECG, because the repolarisation is moving away from the electrode. What is this called?

Answer 18

The T wave.

Question 19

All components of an ECG reading may not be seen in all leads, but generally, what shows atrial and what shows ventricular depolarisation?

Answer 19

The P wave is atrial depolarisation and the QRS complex is ventricular depolarisation.

Question 20

Recording an ECG requires ____ electrodes: ___ on the limbs and ____ on the chest, connected by cables to the machine. They give ___ views to heart aka ________.

Answer 20

10
4
6
12
'Leads'

Question 21

Leads looking at the inferior surface of the heart are best for looking at problems there e.g. Muscle necrosis from an inferior MI. Which leads look at the inferior heart?

Answer 21

Leads II, III and aVF look at the inferior surface of the heart.

Question 22

Where do leads I and aVL look and so which problems are they most equipped to detect.

Answer 22

Leads I and aVL look at the left side of the heart and are the best limb leads for looking at problems in the lateral left ventricle.

Question 23

From which planes do the different leads view the heart?

Answer 23

The 6 limb leads (from 4 limb electrodes), give views in the vertical plane, whereas the chest leads (from 6 electrodes), give 6 views of the heart in the horizontal plane.

Question 24

Which are the septal leads, facing the right ventricle and septum?

Answer 24

Leads V1 and V2 are the septal leads.

Question 25

Where do leads V3 and V4 face?

Answer 25

Leads V3 and V4 face the apex and anterior walls of the ventricles - ‘anterior leads’.

Question 26

Where do leads V5 and V6 look?

Answer 26

They are the ‘lateral leads’, facing the left ventricle.

Question 27

Looking at a graph from an ECG, there are small and large squares. 1 large square = ___ mm squared and is 5 small boxes across. 5 large squares =___ second, so 1 large square =___ seconds and 1 small square = ____ seconds.

Answer 27

5
1
0.2
0.04

Question 28

How do you calculate heart rate from an ECG graph if the rhythm is regular?

Answer 28

Each PQRST complex is 1 cardiac cycle, so you count the number of large boxes between complexes (R-R) = X. There are 300 large boxes to a minute, so 300 divided by X = HR.

Question 29

How do you calculate heart rate from an ECG graph if the rhythm is irregular?

Answer 29

Count the number of QRS complexes in 6 seconds (30 large squares), then x10.

Question 30

How do you measure the PR interval and what should it be?

Answer 30

Count the small squares from the beginning of P to just before Q. It should be 0.12-0.2 seconds (3-5 small squares). If it doesn’t conduct quick enough then there may be ischaemia or the perhaps heart block.

Question 31

QRS interval, is the width of the complex, what should it be?

Answer 31

It should be <0.12 seconds (3 small squares).

Question 32

The QT interval varies with HR and therefore must be corrected (using charts etc). Once it’s corrected, what are the upper limits of QR intervals for men and women?

Answer 32

In adult males, 0.45s and in adult females, 0.47s (11-12 small squares). It’s from the beginning of the Q to the end of the T.

Question 33

What is normal sinus rhythm?

Answer 33

It means that depolarisation is initiated by the sinus node?

Question 34

What sort f things might you check to make sure that the heart is in sinus rhythm?

Answer 34

Is the rhythm regular? Is the PR interval normal? Is every P wave followed by a QRS? Is every QRS preceded by a P wave? Normal QRS width? Sinus rhythm with rate <60bpm is sinus bradycardia (sinus tachycardia >100).

Question 35

These are the colours of the electrodes of an ECG: yellow, red, green, black, red, yellow, green, brown, black, blue,
where should they be placed?

Answer 35

Yellow on the left upper limb (lemon), Red for the Right upper limb, green for the left lower limb, black for the neutral right lower limb.
C1 (V1) is red and goes in the 5th intercostal space, right of the sternum, C2 is yellow and the same but to its left, C3 is green and goes in the middle of C2 and C4, which is brown and in the 5th intercostal space down the midclavicular line, V5 is black and on the same level at the anterior axillary line and V6 is blue on the same level, midaxillary line.