Flashcards in ECG Interpretation Deck (49)
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?
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?
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?
- 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