Session 7 - The ECG

Question 1

What two things can cause an upward signal on an ECG?

Answer 1

• Depolarisation towards an electrode
  Or
• Repolarisation away from an electrode

Question 2

What effects the amplitude of a trace on an ECG?

Answer 2

• The amount of muscle being de/repolarised

- how directly towards the electrode the signal is moving

Question 3

What causes the P-wave on an ECG?

Answer 3

Atrial depolarisation

Question 4

What causes the plateau between the P and Q wave?

Answer 4

The delay at the AVN for 120ms

Question 5

What causes the Q-wave on the ECG?

Answer 5

• As the left bundle branch travels a little bit faster excitation spreads from the left to the right bundle branch
• This produces a small downward trace

Question 6

What causes the R-wave on an ECG trace?

Answer 6

Depolarisation spreading through ventricular muscle

Question 7

What causes the S-wave on an ECG?

Answer 7

Depolarisation travelling up the ventricle walls

Question 8

What causes the T-wave on an ECG?

Answer 8

Repolarisation of the ventricles from the epicardial -> endocardial surface

Question 9

If an electrode sits side on to a signal what kind of trace and amplitude will it show?

Answer 9

None

Question 10

Where hold each of the limb leads be placed?

Answer 10

Red - Right upper limb
Yellow - Left upper limb
Green - Left lower limb
Blue - right lower limb

Question 11

How do we calculate HR from an ECG with a regular rhythm?

Answer 11

300 / Number of big squares in R-R interval

Question 12

Describe the pattern of the spread of excitation over the heart?

Answer 12

• AP generated by SAN, spreads over surface of heart to AVN
• Short pause for around 120ms
• spreads down septum via left +right septum bundle branches
• then over ventricular myocardium from endocardial to epicardial surfaces

Question 13

What is a ventricular ectopic beat? And what would an ECG trace of a sufferer look like?

Answer 13

• Ventricular cells gain pacemaker activity

- Some of ECG complexes will be wider and taller than the underlying rhythm

Question 14

What is an atrial fibrillation? And what would an ECG trace of a sufferer look like?

Answer 14

• Many AP’s are fired in the atria and not all can be conducted by the AVN
• P wave will be up and down continuously, before a normal trace will appear

Question 15

What is a ventricular fibrillation? And what would an ECG trace of a sufferer look like?

Answer 15

• Uncontrolled contractions of ventricles but not all as one
• Constant quivering on ECG

Question 16

What is a first degree heart block? And what would an ECG trace of a sufferer look like?

Answer 16

• Conduction delay through the AVN, but all signals still reach ventricles
• P-R interval elongated

Question 17

What is a type 1 second degree heart block? And what would an ECG trace of a sufferer look like?

Answer 17

• Communication problem between atria and ventricles

- P-R interval elongates as the trace progresses, then QRS suddenly drops off and the system will reset

Question 18

What is a type 2 second degree heart block? And what would an ECG trace of a sufferer look like?

Answer 18

• Communication problem between atria and ventricles

- Not all P-R intervals are followed by a QRS complex, P-R intervals are usually constant

Question 19

What is a third degree heart block? And what would an ECG trace of a sufferer look like?

Answer 19

• The atrial contraction is usual, but there is no electrical conduction to ventricles, must have an ectopic ventricular beat for life to continue
• P-waves usually at normal rate, few QRS complexes that are wide

Question 20

What is different about bundle branch block ECGs?

Answer 20

• lengthening and changing in shape of QRS complex

- many variations