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Flashcards in ECG Interpretation Deck (49)
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What is the physiological basis of ECGs?

- Contraction of muscles associated with electrical changes (depolarisation)
- Changes detected by electrodes


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

Otherwise skeletal muscle activity may obscure the heart muscle activity


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

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


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

The muscle mass of the atria is relatively smaller


What produces a P ECG wave?

Contraction of the atria


Where does the P wave arise from?

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


What does the QRS complex represent?

Ventricular contraction


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

Isoelectric line - where there is no depolarisation or repolarisation occurring


What does the T wave represent?

Repolarisation: Return of ventricular mass to its resting electrical state


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

Hidden by the QRS


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

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


Why is the electrical current delayed in the AV node?

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


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

- 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


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

- 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


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

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


What is the intrinsic firing rate of the SA node?

60-100 depolarisations/min


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

The AV node acts as a backup


What is the intrinsic firing rate of the AV node?

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


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

Purkinje fibres act as a backup for depolarisation


What is the intrinsic firing rate of the Purkinje fibres?



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

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


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

Large: 0.2 seconds
Small: 0.04 seconds


What does the duration of the QRS complex represent?

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


What does the PR interval represent?

- 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)


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


What are some of the conduction abnormalities?

- Ventricular ectopic beats
- First, second, third (complete) degree heart block
- Bundle branch block


What is an example of ventricular ectopic beats?

Ventricular extrasystoles
- Extra beat not arising from SA node
- Looks abnormal & wide
- Depolarisation & repolarisation mixing together


What is first degree heart block?

- Slowing of electrical current moving through conducting system
- Prolongation of PR interval


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


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