CVS ECG and associated abnormalities

Question 1

What signal is recorded when an extracellular electrode i placed near a myocardial cell during systole?

Answer 1

During systole each cell fires an action potential comprising of a depolarisation and repolarisation.
Extracellular electrodes record changes in membrane potential so see 2 signals per systole - depolarisation & repolarisation

Question 2

State the rules to work out whether a signal recorded will be upwards or downwards

Answer 2

If depolarisation is moving towards the electrode the signal will be upward, and if depolarisation is moving away from the electrode the signal will be downward.

If repolarisation is moving towards the electrode the signal will be downward, and if repolarisation is moving away from the electrode the signal will be upward.

The more muscle depolarising and the more more directly towards the electrode it is moving, the bigger the amplitude.

Question 3

Describe the positions of the 12 ECG electrodes

Answer 3

Limb leads - provide a vertical view of the heart
Ride Your Green Bike (Red starting at Right upper then working around clockwise)

Chest leads - provide horizontal view of the heart
1- 4th IC space right sternal edge
2- 4th IC space left sternal edge
4- 5th IC space midclavicular line
3- in the middle of 2 and 4
5- level with 4 at anterior axillary line
6- level with 4 at midaxillary line (directly under armpit)

Question 4

How do you calculate the heart rate from a rhythm strip?

Answer 4

Regular heart rhythm:
300 / no. of squares in R-R interval

Irregular heart rhythm:
No. of QRS complexes in 30 squares (10 secs) x 6

Question 5

What is a ventricular ectopic beat and how will it appear on an ECG?

Answer 5

When ventricular cells gain pacemaker activity there will be ventricular contraction before the ventricles would usually depolarise.

• The QRS complex appears taller and broader
• Ventricular ectopic beats can appear every other beat, every third beat etc or in clusters

Question 6

What is atrial fibrillation and how will it appear on an ECG?

Answer 6

The muscles are not contracting in a coordinated way and there is no regular stimulus reaching AV node so other pacemakers must generate rhythm.
-The p wave will be absent and replaced by irregular fibrillation waves

Question 7

What is ventricular fibrillation and how will it appear on an ECG?

Answer 7

Uncoordinated contraction of ventricles causing it to quiver rather than contract

• appears as squiggly lines with no recognisable PQRST pattern
• abnormal, chaotic, fast

Question 8

What are the different types of heart block?

Answer 8

Heart block is a communication problem between the atria and ventricles.

First degree heart block - conduction delay through AV node
Second degree heart block - type 1, some but not all atrial beats get through to the ventricles, type 2 excitation sometimes fails to pass through AV node or bundle of His so not all atrial contractions followed by ventricular contraction

Third degree heart block - complete heart block, normal atrial contractions but ventricles have to generate their own signal through ectopic pacemaker

Question 9

What factors can interfere with recording the ECG?

Answer 9

Lead misplacement
Muscle contraction eg shivering, talking, coughing
Interference from other currents
Poor electrode contact e.g. sweat, hair, cable pull

Question 10

What is the appropriate length of the PR and QRS interval?

Answer 10

PR interval - start of P to start of Q
3-5 small squares

QRS interval - start of Q to end of S
Less than 3 small squares

Question 11

How does first degree heart block appear on an ECG?

Answer 11

P wave normal QRS normal

PR interval prolonged

Question 12

How does second degree heart block appear on an ECG?

Answer 12

Type 1:
PR interval gets longer and longer until a QRS it dropped

Type 2:
PR interval normal
There is a sudden QRS drop
High risk of progression to complete heart block, requires a pacemaker

Question 13

How does third degree heart block appear on an ECG?

Answer 13

P waves are a normal rate but have no relation to QRS
Ventricular pacemaking v slow - 30-40bpm
Wide QRS
Heart rate too slow to maintain blood pressure and perfusion - URGENT pacemaker required

Question 14

What is ventricular tachycardia and how does it appear on an ECG?

Answer 14

A run of 3 or more ventricular ectopic beats

Appears as tall squiggles - no structure

Question 15

How does an ECG appear during myocardial ischaemia during exercise?

Answer 15

Appears normal at rest but changes during exercise so need to do an exercise ECG
Causes ST depression - looks like a ‘little step’ between S and T

Question 16

How does an ECG appear during a MI?

Answer 16

3 ECG features:

1. Pathological Q waves due to myocardial necrosis (really deep and more than 1 small square wide)
2. ST segment elevation due to subepicardial injury
3. T wave inversion

Question 17

How would you be able to tell if there was right or left ventricular hypertrophy from an ECG?

Answer 17

Look at QRS in leads I, II and III
In left ventricular hypertrophy, lead I is positive and lead III is negative - they are Leaving each other so Left axis deviation

In right ventricular hypertrophy, lead I is negative and lead III is positive - they are Reaching towards each other so Right axis deviation

Question 18

What is left and right bundle branch block?

Answer 18

Right bundle branch block - the signal is not carried by the right bundle of his to RV, impulses reach RV from LV so widened QRS

Leat bundle branch block - signal not carried to ventricle by left bundle of his. Impulses spread to LV from RV so it contracts later, wide QRS and inverted T wave