S6: the electrocardiogram

Question 1

State rules governing the sign of the signal recorded by a positive recording electrode when depolarisation and repolarisation spreads towards and away from that electrode

Answer 1

Depolarisation spreading towards a positive recording electrode yields an upward deflection
Depolarisation spreading away from a positive recording electrode yields a downward deflection
Repolarisation spreading towards a positive recording electrode yields a downward deflection
Repolarisation spreading away from a positive recording electrode yields an upwards deflection

Question 2

Identify the waves associated with atrial depolarisation, ventricular depolarisation, and ventricular repolarisation

Answer 2

P wave = atrial depolarisation
Isoelectric segment after p wave = delay at AV node
Q wave = depolarisation of the interventricular septum
R wave = depolarisation of apex and free ventricular walls
S wave = depolarisation spreading upwards
T wave = ventricular repolarisation

Question 3

Which ECG leads face which parts of the ventricle?

Answer 3

Inferior surface of ventricles = II, III and aVF
Lateral surface of ventricles = I, aVL, V5 and V6
Septal = V1 and V2
Anterior = V3 and V4

Question 4

Determine if an ECG is in sinus rhythm

Answer 4

P waves seen
Normal PR interval
Every P wave followed by a QRS
Every QRS preceded by a P wave
Normal QRS width

Question 5

List the normal values for the PR, QRS & QT intervals

Answer 5

PR = 0.12 – 0.2 seconds
QRS = < 0.12 seconds
QT = 0.4 – 0.48 seconds

Question 6

Describe the ECG changes in ventricular ectopic beats

Answer 6

Wide QRS due to slower depolarisation
Premature = occurs earlier than would be expect for the next sinus impulse
Ventricular tachycardia = run of > 3 consecutive PVCs (high risk of ventricular fib)
Ventricular fib = no coordinated contraction, no cardiac output and if sustained results in cardiac arrest

Question 7

Describe the ECG changes in atrial fibrillation

Answer 7

Arises from multiple atrial foci
No P waves, wavy baseline
Irregular R-R interval
Normal QRS

Question 8

Describe the ECG changes in first degree heart block

Answer 8

Regular rhythm
PR interval prolonged
QRS normal
Conduction is slowed without skipped beats

Question 9

Describe the ECG changes in second degree heart block

Answer 9

Mobitz type 1 = successively longer PR intervals until one QRS is dropped. Then cycle starts again
Mobitz type 2 = PR intervals do not lengthen, suddenly dropped QRS complex, ventricular rhythm is irregular

Question 10

Describe the ECG changes in third degree heart block

Answer 10

Atria and ventricles are depolarising independently
Ventricular pacemaker takes over -> 20-40 bpm
Wide QRS complex, urgent pacemaker required

Question 11

Describe the ECG changes in bundle branch block

Answer 11

Delayed conduction within the bundle branches
P wave and PR intervals are normal
Wide QRS complex
Typical ‘w’ in V1 or ‘m’ in V6 seen on ECG

Question 12

Describe the ECG changes seen in STEMI

Answer 12

Due to complete occlusion of coronary artery
Within minutes: hyperacute T waves, ST elevations
Within hours: T wave inversions, pathologic Q waves

Question 13

Describe the ECG changes seen in NSTEMI

Answer 13

Subendocardial injury

ST depression and T wave inversion

Question 14

Describe the ECG changes seen in stable angina

Answer 14

ST depression during exercise

ECG changes will reverse at rest

Question 15

Describe the ECG changes seen in unstable angina

Answer 15

Can be normal

Inverted T waves & ST depression

Question 16

Describe the ECG changes seen in hypokalaemia

Answer 16

Potassium level < 3.5mmol/L
Increased amplitude & width of P wave
Prolongation of the PR interval
T wave flattening & inversion
ST depression
Prominent U waves (best seen in precordial leads)

Question 17

Describe the ECG changes seen in hyperkalaemia

Answer 17

Potassium level > 5mmol/L
Tall tented T waves
Loss of P wave
Widening QRS
QRS continues to widen, approaching to sine wave

Question 18

What are the different colours for the ECG leads?

Answer 18

Right arm = red 
Left arm = yellow 
Left leg = green 
Right leg = black 
V1 = red 
V2 = yellow 
V3 = green 
V4 = brown 
V5 = black 
V6 = purple