Basic Electrocardiography

Question 1

Why are ECGs important?

Answer 1

Because the electrical stimulus proceeds the physical occurrence e.g. ventricular contraction

1st heart sound is at QRS whilst 2nd one is at T wave ~

Question 2

What does an ECG lead trace represent?

Answer 2

An aggregate of the entire cardiac current measured from a single point over time; each lead is unique to + identified by its position

Question 3

What ions contribute to the electrical potential of the heart?

Answer 3

Na+: depolarization

K+: repolarization

Question 4

What does each wave of a ECG trace represent?

Answer 4

P: atrial depolarisation

QRS complex: depolarization of ventricle + atrial repolarization

T: ventricular repolarization

Question 5

How is cardiac electrical conduction measured?

Answer 5

• Electrical signal generated by an AP reduces as the distance to the recording lead increases
• Depolarisation wave toward electrode records upward/positive deflection
• Depolarisation wave away from electrode records downward/negative deflection

Question 6

What is signal amplitude related to?

Answer 6

Myocardial mass

Question 7

What and how do different factors affect signal amplitude?

Answer 7

• Small structures e.g. SA + AV node & bundle of His will not produce a measurable signal
• Left ventricular signal dominates right (normal QRS axis shows this; -30o-90o)
• Atrial signal smaller than ventricular
• Atrial repolarisation not visible
• Changes if muscle mass changes

Question 8

What are the frontal ECG leads?

Answer 8

I: L + R arm
II: R arm + L foot
III: L arm + L foot

AVF: lead comes off of I + goes to L foot
AVL: lead comes off of II + goes to bony prominence of L arm on wrist
AVR: lead comes off of III + goes to bony prominence of R arm on wrist

Question 9

How many ECG leads are there in total?

Answer 9

12 leads BUT 10 stickers

Question 10

What ECG sticker represents the earth lead?

Answer 10

R foot

Question 11

Where are the chest wall ECG leads placed?

Answer 11

V1: 4th R ICS at sternal margin
V2: 4th ICS at sternal margin
V3: Midway between V2 + V4
V4: 5th ICS in MCL
V5: L anterior axillary line on same horizontal plane as V4
V6: L mid-axillary line on same horizontal plane as V4 + V5

Question 12

What timeframe do the different size squares on an ECG represent?

Answer 12

Small = 0.04 secs
Large = 0.2 secs
So 5 large squares = 1 sec

Question 13

How do you calculate the heart rate from an ECG?

Answer 13

300/number of large blocks between QRS complexes

Count 30 big squares (6 seconds) + count no. of R waves within this x 10 = HR per minute (accounts for irregularity)

Question 14

How can you tell if sinus rhythm is normal from an ECG?

Answer 14

P waves equidistant before every regularly spaced QRS

Question 15

What is sinus rhythm?

Answer 15

Normal heart beat with respect to the heart rate and rhythm

Question 16

Why do the ECG leads on the right have a smaller positive deflection than those on the left?

Answer 16

Further away from left ventricle (where majority of muscle mass is)

Question 17

What is the QRS (electrical) axis a measure of?

Answer 17

The ventricular activity in the frontal (vertical) plane (normally directed leftward + downward)

Question 18

How do you work out the QRS (electrical) axis?

Answer 18

Determined by observing the QRS complex in leads I, II, III, AVF, AVL + AVR:

1. Look first for lead whos +ve & -ve deflections sum closest to 0 (equiphasic)
2. Main QRS axis will be at R angles to this
3. Dominant deflection in this lead (+ve/-ve) will indicate axis direction

Question 19

What are the 2 commonest abnormalities of the QRS complex?

Answer 19

1. Changes in muscle bulk e.g. hypertrophy due to hypertension or stenotic valve lesions OR muscle destruction due to CAD or myocarditis
2. Abnormalities of electrical conduction e.g. damage to conduction system (often ischaemic), damage to sinus node OR abberant or multiple foci of generation (AF)

Question 20

Where does a normal QRS axis lie?

Answer 20

-30o (+AVL) to +90o + AVF +ve

Question 21

Where do deviated QRS axis lie?

Answer 21

LAD: (+AVF) -30o to -90o (-AVF) - I is +ve + AVF -ve

RAD: +90o (+AVF) to +180o (-I) - I is -ve + AVF is +ve

Question 22

Where does a indeterminate QRS axis lie?

Answer 22

-180o to -30o

Question 23

If there was a right sided QRS axis deviation, what could this imply?

Answer 23

Right ventricular hypertrophy

Question 24

Where do the ECG leads face in terms of the heart itself?

Answer 24

Lateral: I, AVL, V5 + V6

Inferior: II, III + AVF

Anterior: V3 + V4

Septal: V1 + F2

AVR = anomaly

Question 25

Which 3 things do you interpret from an ECG?

Answer 25

1. Heart rate
2. Heart rhythm
3. QRS axis

Question 26

If T waves are inverted, what can this mean?

Answer 26

Cardiac ischaemia

Previous infarct

Question 27

If the ST segment is elevated or depressed, what can this mean?

Answer 27

Elevation: suggestion of infarction, high take off?

Depression: suggestive of cardiac ischaemia

Question 28

How long is a normal PR interval?

Answer 28

0.12-0.2 secs (3-5 small squares)

Question 29

How long is a normal QRS segment?

Answer 29

< 0.12 secs (< 3 small squares)

Question 30

What does the Q-T interval represent?

Answer 30

Time for ventricular depolarization + repolarization; roughly estimates duration of ventricular AP - ranges from 0.2-0.4 secs

Question 31

How will an increase in heart rate affect the Q-T interval?

Answer 31

Increased HR -> shortened ventricular APs -> decreased Q-T internal

Question 32

Why is a decreased Q-T interval clinically important?

Answer 32

Need corrected QT = QTc

Question 33

How is electrical current conducted through the heart?

Answer 33

Sinus node -> atrial muscle -> A-V node -> common bundle -> bundle branches -> purkinje fibres -> ventricular muscle