Electrocardiogram (7) Flashcards
What is an electrocardiogram?
- Electrodes on the body’s surface that are able to detect the changing electrical field in the myocardium.
What is meant by the myocardium?
- Large mass of muscle undergoing electrical changes at the same time which generates a large changing electrical field.
What does a depolarisation and repolarisation look like on an ECG?
- Depolarisation: upwards signal
- Repolarisation: downwards signal
What are the 4 main rules regarding ECGs?
- Depolarisation towards an electrode: upwards going signal
- Depolarisation away from electrode: downwards going signal
- Repolarisation towards an electrode: downwards going signal
- Repolarisation away from electrode: upwards going signal
What affects the amplitude of signals?
- How much the muscle’s depolarising
- How directly towards the electrode the excitation is moving
What is the P wave?
- Atrial depolarisation: small upwards deflection
- Small as little muscle involved
- Upwards as moving towards electrode
What is meant by the Q wave?
- Excitation spreads halfway down the septum then out across the axis of the heart: small downwards deflection
- Downwards as moving away
- Small as not moving directly away
What is meant by the R wave?
- Depolarisation spreads through the ventricular muscle along an axis slightly to the left of the septum: produces a large upward deflection
- Upward because moving towards electrode
- Large as lots of muscle
What is a S wave?
- Depolarisation spreads upwards to the base of the ventricles: small downwards deflection
- Downwards as mainly away
- Small as not directly away
What is meant by T wave?
- Ventricular repolarisation spreads through the ventricular myocardium in the opposite way to depolarisation: medium upwards deflection
- Upwards because moving away
- Medium as turning in different cells dispersed
What has happened to the atrial repolarisation deflection?
- Lost in QRS complex
What happens to the signals as you move the electrodes to different positions?
- Directions and amplitude of waves change predictably
Why do we need to put electrodes in different positions?
- Allows detection and localisation of abnormal patterns of electrical activity.
How are signals amplified?
- Invert negative input
- Adds to positive input
- Then amplifies total
What are the positions of the positive and negative electrode, equivalent positive, and the total for lead 1
- Positive electrode top left
- Negative electrode top right
- Equivalent positive of negative bottom left
- 2 together = looking from left side.
What are the positions of the positive, negative, equivalent positive and total for lead 2?
- Positive electrode bottom left
- Negative electrode top right
- Equivalent positive of negative bottom left
- Total: 2 views from bottom left (apex)
What are the positions of the positive, negative and equivalent positive and total for lead 3?
- Positive bottom left
- Negative top left
- Equivalent positive bottom right
- Total: straight up/from bottom
What is the best way of getting a picture of the ECG?
- 12 leads
- 12 different views
What is meant by an augmented lead?
- 2 negatives connected
- Convert 2 negatives to one first
- Then convert to a positive and add to a real positive to give one view
What are the 6 different limb leads in order starting from top right.
- AVL
- Lead 1
- Lead 2
- AVF
- Lead 3
- AVR
What are the chest leads and how are they arranged?
- V1 to V6
- V1: right 4th rib
- V2: left 4th rib
- V3: slightly below V2
- V4-V6 across rest of chest to left axilla
What are the main things that you look for in an ECG?
- Rate
- Rhythm
- Axis
- P wave
- P-R segment
- QRS complex
- Q-T interval
- T wave
At what rate does an ECG run at?
- Each large square is equivalent to 0.2s
- 300 squares per minute
What is the rhythm related to?
What are the different components that should be looked out for in an ECG?
- Relates to the timing of components of ECG
- Rhythm is regular/irregular
- Relationship between atrial and ventricular depolarisation
When may the P wave be absent in an ECG?
- Atrial fibrillation
What is the P-R interval and how long should it last?
- Time taken for impulse to reach ventricles
- (0.12-0.2s)
How is the P-R interval affected in 1st/2nd/3rd degree heart block?
- 1st: prolonged
- 2nd: erratic (sometimes absent, sometimes not)
- 3rd: no relationship between p wave and QRS complex
What does the QRS complex tell us?
- Tells us about the axis of heart and pattern of conduction through ventricles
What is the electrical axis of the heart?
- Relates to main spread of depolarisation through ventricle wall: R wave
If there’s hypertrophy in a ventricle wall what may this cause?
- More muscle in left: left shift
- More muscle in right: right shift
How do you find the electrical axis?
- Lead with smallest and most equiphase deflection
- So net deflection’s 0 indicating that electrical axis must run at right angles to the view.
- Normally lead 2
Why may the QRS complex change shape?
- Determined by route of spread of depolarisation
- Damage to conducting pathways alters route of spread
- Example: Bundle branch block
Why may perfusion problems occur?
- Partial occlusion of coronary arteries
- Acute occlusion by thrombus
What may a partial occlusion of coronary arteries lead to?
- Intermittent poor myocardial perfusion
- Angina - pain during exercise
What is acute occlusion of thrombus?
- Death of part of myocardium
- Myocardial Infarction
What may show that an MI is in process on an ECG?
- S-T elevation
- Pathological Q waves
- Inverted T waves
What would a Q wave look like in an MI?
- More than 0.04s (1 small square) wide
- x>2mm
- Remains after other changes resolve
