ECG Interpretation Flashcards

1
Q

What is the physiological basis of ECGs?

A
  • Contraction of muscles associated with electrical changes (depolarisation)
  • Changes detected by electrodes
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2
Q

Why must the patient be fully relaxed when an ECG recording is made?

A

Otherwise skeletal muscle activity may obscure the heart muscle activity

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3
Q

How is the heart viewed from an electrical point of view?

A

As only having two sections/chambers, i.e. the two atria contract together & the two ventricles contract together

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4
Q

Why is the electrical activity of the atria smaller than the ventricles?

A

The muscle mass of the atria is relatively smaller

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5
Q

What produces a P ECG wave?

A

Contraction of the atria

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6
Q

Where does the P wave arise from?

A

The sino atrial node (SA node) of the right atrium, spreads across the atria

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7
Q

What does the QRS complex represent?

A

Ventricular contraction

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8
Q

What is the line between the ECG waves called and what does it represent?

A

Isoelectric line - where there is no depolarisation or repolarisation occurring

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9
Q

What does the T wave represent?

A

Repolarisation: Return of ventricular mass to its resting electrical state

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10
Q

Where does the repolarisation of the atria lie on an ECG?

A

Hidden by the QRS

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11
Q

What is the pathway of the electrical current to the ventricles?

A
  • Starts in SA node
  • Depolarisation spreads across atrial muscle fibres & internal pathways
  • Spreads to ventricles via AV node
  • Delayed in the AV node
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12
Q

Why is the electrical current delayed in the AV node?

A

To allow time for the blood to move into the ventricles so it can be pumped out

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13
Q

What happens to the electrical current once it reaches the ventricles?

A
  • Passed by specialised conducting tissue
  • Bundle of His (single pathway)
  • Then divides into L & R bundle branches in inter ventricular septum
  • L bundle branch divides in 2
  • Current spreads across mass of the ventricle via Purkinje fibres
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14
Q

What is the intrinsic rhythm of the heart usually set by?

A
  • SA node, normally has the highest frequency of discharge of all possible sites
  • But any other conducting part can also have its own intrinsic rhythm
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15
Q

Under normal conditions, what keeps the heart working as an effective pump?

A

Intrinsic firing rates of pacemaker cells in 3 critical areas of the heart

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16
Q

What is the intrinsic firing rate of the SA node?

A

60-100 depolarisations/min

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17
Q

What happens if there is a failure of the SA node to depolarise?

A

The AV node acts as a backup

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18
Q

What is the intrinsic firing rate of the AV node?

A

40-60/minute (only happens if SA node fails)

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19
Q

What happens if there is a failure of both the SA & AV nodes?

A

Purkinje fibres act as a backup for depolarisation

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20
Q

What is the intrinsic firing rate of the Purkinje fibres?

A

20-40/minute

21
Q

What does it mean when the waves on an ECG are upside down?

A

The wave is moving away from the electrode, i.e. the electrode is in a different position

22
Q

On ECG paper, what does each large & small square represent?

A

Large: 0.2 seconds
Small: 0.04 seconds

23
Q

What does the duration of the QRS complex represent?

A
  • How long it takes for excitation to spread through the ventricle
  • Normal QRS < 0.12 seconds (3 small squares)
24
Q

What does the PR interval represent?

A
  • How long it takes for excitation to spread from SA node through atrial muscle & AV node, down Bundle of His & into ventricular muscle
  • Normal PR interval 0.12-0.2 seconds (3-5 small squares)
25
What are the characteristics of a normal sinus rhythm?
- HR 60-100bpm - Rhythm regular - P wave before each QRS, identical - PR interval 0.12-0.2 seconds - QRS < 0.12 seconds
26
What are some of the conduction abnormalities?
- Ventricular ectopic beats - First, second, third (complete) degree heart block - Bundle branch block
27
What is an example of ventricular ectopic beats?
Ventricular extrasystoles - Extra beat not arising from SA node - Looks abnormal & wide - Depolarisation & repolarisation mixing together
28
What is first degree heart block?
- Slowing of electrical current moving through conducting system - Prolongation of PR interval
29
What is second degree heart block?
- Excitation fails to pass through AV node or bundle of His - I.e. P waves not being conducted through to ventricles - No QRS complex - 3 variations
30
What are the 3 variations of second degree heart block?
1. Mobitz type 2 phenomena: Normal PR intervals, then sudden atrial contraction without subsequent ventricular contraction 2. Wenkebach phenomena: Progressive lengthening of PR interval then failure to conduct atrial beat, followed by shorter PR interval (cyclic) 3. Alternate conducted/non-conducted atrial beats
31
What is third degree (complete) heart block?
- Atrial contraction is normal - No beats conducted to ventricles - Atria & ventricles contracting independently
32
What is a bundle branch block?
- Depolarisation wave doesn't spread via bundle branches - QRS complex wider due to delayed conduction in ventricles - Can be right or left
33
How are heart rhythms classified?
According to their point of origin
34
What is the difference between supraventricular rhythms & ventricular rhythms?
- Supraventricular: Narrow (normal) QRS complex, arise from AV node or above - Ventricular: Wide complex (Exception: SV rhythm with R/L bundle branch block)
35
What are the 4 ways abnormal rhythms are classified?
1. Cardiac slowing (escape beats/rhythms, bradycardia) 2. Early single beats (extrasystoles) 3. Sustained & fast rhythm (tachycardias) 4. Total disorganisation of atrial or ventricular firing (fibrillation)
36
What is a type of cardiac slowing?
Sinus bradycardia - May be normal (e.g. in fit people) - Normal sinus pattern of conduction - Slow rate <60
37
What are examples of early single beats?
- Ventricular ectopic beats (VEBs) | - Premature ventricular contraction (PVCs)
38
What is the R-on-T phenomenon?
- VEB that occurs on top of preceding T wave | - Can cause ventricular tachycardia
39
What are paired VEBs?
- 2 VEBs occurring in a row - Can cause ventricular tachycardia - Can be normal in patients with heart conditions (i.e. check if it's normal for them)
40
What are multifocal VEBs?
- VEBs look different from each other | - I.e. arising from different areas of the heart (more than one abnormal focus)
41
What is bigeminy and trigeminy?
- Bigeminy: Continuous rhythm of 1 normal beat, then 1 VEB | - Trigeminy: 2 normal beats, 1 VEB
42
What are types of tachycardia?
- Supraventricular (e.g. atrial flutter or fibrillation) | - Ventricular
43
What is atrial flutter?
Atrial tachycardia with supra ventricular rhythm, e.g. multiple P waves for every QRS complex
44
What are the risks of atrial flutter?
- Stroke - Clotting - Blood pooling - Can degenerate into atrial fibrillation
45
What is atrial fibrillation?
- Multifocal tachycardia | - Disorganised baseline
46
What is ventricular tachycardia?
- Rapid QRS complex - All wide - Over 100bpm - Can degenerate into ventricular fibrillation
47
What clinical features can be correlated with abnormal ECGs?
- Total clinical picture (disease, meds, electrolytes, refer to prior tracings) - LOC - Hypotension - Palpitations - Abnormal pulse - Chest pain - Shortness of breath
48
Where should the ST segment lie?
Isoelectric, i.e. should be at the same level as the line between T and the next P
49
What are examples of ST segment abnormalities?
- Horizontal depression with upright T wave: Sign of ischaemia - Elevation: Sign of acute myocardial injury (pericarditis or recent infarction)