Physiology of the ECG

Question 1

What is P wave?

Answer 1

• atrial depolarisation

- electrical activation

Question 2

What is the PR interval?

Answer 2

• the time taken for the electrical impulse to spread from the atria to the ventricles by the AV node and bundle of His
• between 3-4 boxes

Question 3

What is the QRS interval?

Answer 3

the impulse spreading through the ventricles in ventricular contraction

Question 4

What is the ST interval?

Answer 4

• the ventricles completely activated

- usually isoelectric

Question 5

What is T wave?

Answer 5

repolarisation of the ventricular muscle to relaxed state

Question 6

What are the letters to help remember what to check in an ECG?

Answer 6

R R P W Q S T

Question 7

R (1)

Answer 7

Rate

• the closer the QRS complexes, the faster the beat
• tachycardia if >100 bpm
• bradycardia if <50 bpm

Question 8

R (2)

Answer 8

Rhythm

• regular/irregular
• determined by if the number of squares between QRS complexes are the same or not

Question 9

P

Answer 9

P wave

• sinus rhythm?
• there should be one P wave for every QRS complex
• no p-wave means no activation - abnormal rhythm

Question 10

W

Answer 10

Width

• if QRS >3 boxes then spread of electrical activity is slow
• abnormal conduction

Question 11

Q

Answer 11

• are there deep Q waves?

- deep downward deflection can be due to old MI

Question 12

S

Answer 12

ST interval

• should be level with baseline
• if elevated: MI
• if depressed: ischaemia

Question 13

T

Answer 13

T waves

- if upside down in I, II and V4-6: ischaemia/hypertrophy

Question 14

Dextrocardia

Answer 14

Heart points in the opposite way

Question 15

What direction is electrical activity going in to create the most deflection on an ECG?

Answer 15

parallel

Question 16

What is the role of the electrode on the RIGHT leg?

Answer 16

neutral lead

Question 17

What does an ECG measure?

Answer 17

• electrical activity from SA node spreading through ventricles
• not how well heart is pumping

Question 18

When would you take an ECG on patient?

Answer 18

• chest pain
• palpitations
• dizziness
• syncope episode
• stroke/TIA

Question 19

Why can’t we see atrial repolarisation on an ECG?

Answer 19

• too small

- hidden by QRS complex

Question 20

How to calculate HR from ECG?

Answer 20

• 300 big boxes = 1 minute

- 300/number of big boxes between QRS

Question 21

What characterises first degree heart block on an ECG?

Answer 21

• prolonged PR interval

- AV node delays too long

Question 22

What characterises secondary degree heart block on an ECG/

Answer 22

• 2 P waves for every QRS complex

- not all P waves resulting in QRS complex

Question 23

What characterises AF on an ECG?

Answer 23

• no P wave
• irregular QRS complexes
• arrythmia

Question 24

What characterises atrial flutter on an ECG?

Answer 24

• 4 P waves for every QRS complex

- arrythmia

Question 25

What characterises junctional (nodal) tachycardia on an ECG?

Answer 25

- normal QRS - no P waves - arrythmia

Question 26

What characterises ventricular tachycardia?

Answer 26

- after 2 sinus beats rate increases - wide QRS complexes - can return back to sinus beat

Question 27

What can abnormal P waves be a sign of on an ECG?

Answer 27

P-mitrale (left atrial hypertrophy): bifid P waves for every QRS P-pulmonale (right atrial hypertrophy): peaked P wave

Question 28

What can abnormal QRS complexes be a sign of on an ECG?

Answer 28

Wide: - ventricular conduction defects (bundle branch block) Low voltage: - hypothyroidism - CAOD - myocarditis - pericarditis/pericarditis effusion

Question 29

What characterises left and right ventricular hypertrophy?

Answer 29

- left ventricular hypertrophy: sum of S wave in V1 and R wave in V5/6 >35mm - right ventricular hypertrophy: - dominant R wave V1, T wave inversion V1-4, deep S wave V6

Question 30

How long is QT interval normally?

Answer 30

roughly 1 big box

Question 31

A prolonged QT interval is a sign of what?

Answer 31

- acute myocardial ischaemia - myocarditis - bradycardia - head injury - hypothermia - U and E imblanace - congenital - drugs

Question 32

What effect does digoxin have on ECG?

Answer 32

- T wave inversion | - ST segment sloping depression

Question 33

Left axis deviation

Answer 33

- negative QRS in II and III - LV hypertrophy - MI

Question 34

Right axis deviation

Answer 34

- negative QRS in I - RV hypertrophy - PE - MI