ECG

Question 1

What is an ECG?

Answer 1

Electrocardiogram
It is a graphic representation of electrical potential difference changes against time; showing the electrical changes of the myocardium throughout the cardiac cycle

Question 2

What is a myocyte?

Answer 2

Cardiac cell

Question 3

How does myocardium contract?

Answer 3

Myocardium has electrical currents that sequentially depolarise individual cells and results in the change in cellular morphology that allows muscle contraction

Question 4

What is required for a myocyte to contract?

in terms of potential difference

Answer 4

The potential difference across the semi permeable membrane must change from negative —> positive in relation to the inside of the cell.
This occurs through the flow of ions through specialised ion channels in the cellular membrane and also freely through gap junctions

Question 5

Describe when an electrical cardiac field is present and how it arises

Answer 5

Present during repolarisation and depolarisation (ABOVE RESTING POTENTIAL)
Repolarisation and depolarisation causes different currents to flow across the cell membrane at various points = dipole to be produced
This flow of current along the cell surface sets up an external electrical field around the dipole

Question 6

Describe when an electrical field is NOT present and explain why

Answer 6

When the cell is repolarised or depolarised (BELOW RESTING POTENTIAL)
Below resting potential means there is NO difference in membrane potential = no electrical field

Question 7

Name the frontal plane leads used in an ECG

Answer 7

• Lead I 
	• Lead II
	• Lead III
	• aVR 
	• aVL 
	• aVF
Each bipolar lead records the difference in electrical potential between TWO limb leads

Question 8

Name the chest leads used in an ECG and describe their placement

Answer 8

• V1: 4th Intercostal, right of sternum
  
  • V2: 4th intercostal, left of sternum
  • V3: mid point between V2 + V4
  • V4: 5th intercostal, mid clavicular line
  • V5: in line horizontally with V4, anterior axillary line
    • V6: in line horizontally with V4, mid axillary line

Question 9

Why do we need 12 ECG leads?

Answer 9

To look at the different dimensions of the heart
Inferior activity is observed using II, III and aVF
Septal activity is observed using V1 and V2
Anterior activity is observed using V3 and V4
Lateral activity is observed using V5 and V6
Posterior activity is observed using I and aVL

Question 10

outline the electrical conductivity of the heart

Answer 10

• SAN initiates electrical activity in right atrium
  
  • Bachmans bundles takes the electrical impulse from the right atrium to the left atrium
  • Atrium contract simultaneously
  • Delay at AV node to allow ventricles to fill completely
  • Electrical activity goes down the bundle of his down the septum to the apex
  • Depolarisation spreads upwards through the purkyne fibres

Question 11

Describe the different parts of an ECG

Answer 11

P wave: atrial depolarisation
PR segment: AV node delay
QRS complex: ventricular depolarisation (atria repolarise simultaneously)
ST segment: time during which ventricles are contracting and emptying
T wave: ventricular repolarisation
TP interval: time during which the ventricles are relaxing and filling

Question 12

When do coronary arteries supply the myocytes?

Answer 12

During diastole, the coronary arteries supply the myocytes with oxygen

Question 13

What 2 main things can an ECG tell us?

Answer 13

Heart size (taller R peak = bigger heart)

Heart rate

Question 14

What can a long QT segment result in?

Answer 14

Sudden death

Question 15

What is tachycardia?

Answer 15

A heart rate that exceeds the normal resting heart rate

Question 16

What is bradycardia?

Answer 16

A heart rate that is abnormally slow compared to normal resting heart rate

Question 17

What are some ways to identify a normal sinus rhythm?

Answer 17

Check for positive P wave prior to QRS complex
Normal PR interval (< 200ms)
Normal QRS duration (< 0.11 s)
Normal QT interval ( <0.44 ms)

Question 18

What are some normal variations in heart rate?

Answer 18

Athlete = lower heart rate
Children = faster heart rate
Sinus arrhythmia is NORMAL in younger people

Question 19

What is meant by an arrhythmia?

Answer 19

An abnormal cardiac rhythm, there are 2 types:
Conduction abnormalities (eg- block)
Abnormal impulse initiation (eg- ectopic)

Question 20

What is sino atrial block?

Answer 20

SA node fails to initiate impulse = no P wave/ QRS complex (sino atrial block)

Question 21

What is 3rd degree AV block?

Answer 21

An abnormal heart rhythm (arrhythmia) that happens when the electrical impulses that control your heartbeat are delayed or blocked.It happens when the electrical impulses that tell your heart when to beat don’t pass between the top and bottom chambers of your heart as they should

Question 22

What is meant by abnormal impulse initiation?

Answer 22

Every other beat is ventricular ectopic
Can occur due to excess caffeine intake
Long term can cause ventricular tachycardia

Question 23

What is ventricular tachycardia?

Answer 23

Impairs cardiac output = hypotension + collapse + acute cardiac failure
Due to extreme heart rates and lack of coordinated atrial contraction

Question 24

What causes myocardial ischaemia?

Answer 24

Inadequate oxygen supply

Question 25

What is angina?

Answer 25

Chest pain on exercising caused by chronic arterial narrowing

Question 26

What is meant by injury current effect?

Answer 26

Ischaemic myocytes have reduced membrane potentials compared to normal healthy myocytes. The difference in potential between the ischaemic region and healthy region displaces the ST segment. ST depression is caused by injury potential difference.