6. The ECG

Question 1

What is the difference between cells of the heart conducting system and myocytes?

Answer 1

Conducting system cells = specialised myocytes that have lost contractile ability but are able to generate APs.
Myocytes = contractile ability but unable to generate APs.

Question 2

Describe how excitation normally spreads through the myocardium.

Answer 2

1. depolarisation at SAN followed by depolarisation of atrial myocytes
2. wave of depolarisation delayed by ~120-200ms at AVN - allows for atrial contraction
3. spread from atria to ventricles via Bundle of His
4. rapid conduction to all parts of ventricles via His-Purkinje system: depolarisation of interventricular septum… of apex and free ventricle wall (from endocardial to epicardial surfaces)… of base of ventricles
5. ventricular myocardial cells repolarise once all have depolarised, in opposite sequence to depolarisation (epicardial to endocardial cells)

Question 3

What is the ‘electrical axis’ of the heart?

Answer 3

‘overall’ direction of depolarisation wave, directed towards apex, slightly to left of IV septum in normal heart

Question 4

What is an ECG?

Answer 4

Records changing electrical field on extracellular surface of cardiac myocytes during wave of depolarisation and repolarisation, from the surface of the body using electrodes pasted on skin.

Question 5

What does the nature of an ECG signal depend on?

Answer 5

the direction of spread of the electrical field relative to the position of the recording electrode

Question 6

State the rules governing the nature of an ECG signal recorded by the positive electrode.

Answer 6

1- depolarisation spreading towards a positive electrode = upward deflection
2- depolarisation spreading away from a positive electrode = downward deflection
3- repolarisation spreading towards a positive electrode = downward deflection
4- repolarisation spreading away from a positive electrode = upward deflection

Question 7

What does the amplitude (height) of an ECG deflection depend on?

Answer 7

1. amount of muscle mass (increased mass… larger electrical depolarisation signal… greater vertical deflection)
2. how directly wave of activity travels towards electrode:
   - directly towards/away = large complex
   - obliquely towards/away = smaller complex
   - 90 degree angle to electrode = biphasic or no complex

Question 8

Which part of the ECG trace indicates SAN depolarisation?

Answer 8

insufficient signal to register on surface ECG but equal to start of P wave

Question 9

Which part of the ECG trace indicates atrial depolarisation?

Answer 9

P wave: small upward deflection as travelling towards +ve electrode

Question 10

Which part of the ECG trace indicates AVN delay?

Answer 10

isoelectric (flat line) segment between P and Q wave

Question 11

Which part of the ECG trace indicates depolarisation of bundle of His?

Answer 11

isoelectric (flat line) segment between P and Q

Question 12

Why is the Bundle of His important in electrical conduction?

Answer 12

fibrous ring between atria and ventricles prevents contact between atrial and ventricular myocytes - bundle of his is only connection (so ischaemia in this region can affect ventricular contraction)

Question 13

Which part of the ECG trace indicates IV septum depolarisation?

Answer 13

Q wave: small downward deflection as septum depolarises from left to right, moving obliquely away from lead II

Question 14

Which part of the ECG trace indicates apex and free ventricular wall depolarisation?

Answer 14

R wave: large upward deflection as depolarisation moves directly towards electrode (large because of large muscle mass - more electrical activity)

Question 15

How would left ventricular hypertrophy be indicated on an ECG trace?

Answer 15

taller R wave

Question 16

Which part of the ECG trace indicates depolarisation of ventricle bases?

Answer 16

S wave: small downwards deflection as moving away from electrode, but not directly away

Question 17

Which part of the ECG trace indicates ventricular repolarisation?

Answer 17

T wave: medium upward deflection as wave of repolarisation moving away from electrode (spreads in opposite direction to depolarisation, from epicardial to endocardial celsl)

Question 18

How many electrodes are used in an ECG and how many view/leads of the heart does this give?

Answer 18

10 electrodes: 4 on limbs, 6 on chest

Give 12 views of the heart

Question 19

In which planes do the limb and chest electrodes view the heart?

Answer 19

Limb leads = coronal plane

Chest leads = transverse (horizontal) plane

Question 20

Describe the 6 leads of the limb electrodes.

Answer 20

Bipolar limb leads:

• I: right arm (-) to left arm (+)
• II: right arm (-) to left leg (+)
• III: left arm (-) to left leg (+)

Unipolar limb leads:

• aVR: heart to right arm (+)
• aVF: heart to left leg (+)
• aVL: heart to left arm (+)

Question 21

Which lead provides the typical ECG trace?

Answer 21

II

Question 22

What would the traces of lead I and aVL look like compared to lead II?

Answer 22

smaller amplitude of curves as more oblique angle (~90 degrees for aVL)

Question 23

What would the trace of lead aVR look like compared to lead II?

Answer 23

~ mirror image as view in opposite direction

Question 24

Where are the chest electrodes placed?

Answer 24

C1 - 4th intercostal space, right sternal border
C2 - 4th intercostal space, left sternal border
C3 - midway between C2 and C4
C4- 5th intercostal space, midclavicular line
C5- midway between C4 and C6
C6 - 5th intercostal space, midaxillary line

Question 25

Which limb leads look at the inferior surface of the heart?

Answer 25

II, III and aVF

Question 26

Which limb leads look at the left side of the heart?

Answer 26

I and aVL

Question 27

Which chest leads look at the anterior heart/right ventricle and septum?

Answer 27

V1 and V2

Question 28

Which chest leads look at the anterior apical region?

Answer 28

V3 and V4

Question 29

Which chest leads look at the left ventrical/lateral aspect?

Answer 29

V5 and V6

Question 30

Why is the QRS complex negative in V1 and positive in V6?

Answer 30

R wave measures depolarisation of ventricular walls - occurs simultaneously in both sides so records net effect of opposing wave direction. LV wall is much thicker so determines curve direction (away from V1 = downwards deflection, towards V6 = upwards deflection)