L12: ECG

Question 1

ECG recording

Answer 1

Bipolar: between 2 points (+, - electrodes)
Unipolar: 1 point on body surface (exploring electrode) relative to ground (indifferent electrode)

Recording: sum of all electrical activity of heart (all individual movements of potential waves)

Question 2

ECG events

Answer 2

P wave: atrial depolarisation (just before atrial contraction)
QRS complex: ventricular depolarisation (just before isovolumetric contraction)
T wave: ventricular repolarisation (during reduced ejection)

Q: interventricular septum
R: main body of ventricular muscle (predominantly left ventricle)
S: upper septum and high posterior wall (predominantly right ventricle)

Question 3

Vectors in ECG

Answer 3

Depolarisation
Towards: +ve
Away: -ve

Repolarisation
Towards: -ve
Away: +ve

90 degree = no deflection

Question 4

12 leads

Answer 4

Bipolar leads (-ve —> +ve):
Lead I: right arm —> left arm (左右手)
Lead II: right arm —> left leg
Lead III: left arm —> left leg (左手左腳)

Unipolar leads: (+ve)
- Limb leads
VR: right arm
VL: left arm 
VF: left leg

- Precordial leads
V1: 4th ICS, R sternum
V2: ***4th ICS, L sternum
V3: between V2, V4
V4: ***5th ICS, MCL (apex)
V5: 5th ICS, AAL
V6: ***5th ICS, MAL

Question 5

Directions of view of Limb leads

Answer 5

Limb leads (unipolar, bipolar) view heart in vertical plane

Unipolar: from direction of exploring electrode

Bipolar: from halfway between +ve and opposite of -ve electrode

Question 6

Directions of Cardiac vector

Answer 6

Atrial depolarisation: SA node to AV node
- Left and downwards, Small magnitude (+45 degree) (VD and VR 中間)

Ventricular depolarisation: inner to outer ventricular muscle (majority)
- Left and downwards, Large magnitude (+60 degree) (向下咁滯)

Ventricular repolarisation: outer to inner ventricular muscle
- Left and downwards, Medium magnitude (+35-40 degree)

Question 7

ECG appearance

Answer 7

aVR: all negative deflections (all three vectors pointing away from aVR)
aVL: very little deflections (all three vectors 90 degree to aVL)
aVF: all positive deflections (all three vectors align with aVF direction)

Lead I to III: all positive deflections
Lead I: big T wave ~ size of QRS (align with Lead I)
Lead II: very prominent QRS wave (ventricular depolarisation exactly align with Lead II)
Lead III: very small T wave (ventricular repolarisation 90 degree with Lead III)

Question 8

Directions of view of Precordial leads

Answer 8

View the heart in the horizontal plane (more concerned with QRS complex)
***Closer to electrode —> larger deflection

V1: 4th ICS, R sternum —> over R ventricle —> S wave > R wave
V2: 4th ICS, L sternum —> over R ventricle —> S wave > R wave
V3: between V2, V4 —> over interventricular septum —> S wave = R wave
V4: 5th ICS, MCL (apex) —> over interventricular septum —> S wave = R wave
V5: 5th ICS, AAL —> over L ventricle —> R wave > S wave
V6: 5th ICS, MAL —> over L ventricle —> R wave > S wave

Question 9

Types of information obtained from ECG

Answer 9

1. Rate (Lead II)
2. Rhythm (duration of events, relationship between events)
3. Coordination of electrical activity in different regions
4. Abnormal pacemaker activity
5. Heart block
6. Myocardial hypertrophy (extra large ECG cycle)
7. Ischaemic/Injury (depressed ST segment; MI: elevated ST segment)