Flashcards in ECG Theory and Interpretation Deck (37):
Why must relaxation of the heart occur from out to in?
The muscle fibres are in a figure of 8 and rotated as they contracted. If they relaxed the same way they contracted the endocardium would be twisted.
If depolarisation occurs toward an electrode, which way will deflection on the ECG occur?
If depolarisation occurs away from an electrode, which way will deflection on the ECG occur?
If repolarisation occurs toward an electrode, which way will deflection on the ECG occur?
If repolarisation occurs away from an electrode, which way will deflection on the ECG occur?
What does the amplitude of an ECG deflection depend on?
Amount of muscle depolarising
How directly towards an electrode the excitation is moving.
Describe what causes the P wave when using lead II
Depolarisation of the atria towards the lead. Small because little muscle is moving.
Describe what causes the flat space between the P and Q waves when using lead II of an ECG
There is a pause at the AVN.
Describe what causes the Q deviation of an ECG when using lead II
The current spreads around away from the view which causes the downward signal. It's small because the signal is away and relatively little muscle is contracting.
Describe what causes the R deviation of an ECG when using lead II
Most of the muscle is depolarising with the majority directly towards the view causing a large positive deflection.
Describe what causes the S deviation of an ECG when using lead II
Final depolarisation away from the viewpoint so is negative and only a small amount of muscle is involved.
Describe what causes the T wave of an ECG when using lead II
Repolarisation away from the view causing the positive deflection. There is the same amount of muscle involved as in the QRS complex but less height as the timing in different cells is dispersed.
State the processes happening in the heart that cause P, Q, R, S, T.
P - Atrial depolarisation
Q - depolarisation of endocardium in the right and left ventricle
R - Main depolarisation of ventricles
S - end of ventricular depolarisation
T - repolarisation of the ventricles
Why can't you see atrial repolarisation in an ECG reading?
Lost in the QRS complex
Where do each of the limb leads view the heart from?
I - from the left
II - from the apex
III - from the bottom
Describe augmented leads and state what they are.
Two negatives are connected with one positive. The two negatives are added to form one, then inverted to be positive. Combine with the positive to give one view.
All three electrodes are being used.
- aVL - augmented from the left
- aVR - augmented from the right
- aVF - augmented from below
Give the places where you would place each of the chest electrodes.
V1 - 4th intercostal space, right sternal line (Red)
V2 - 4th intercostal space, left sternal line (Yellow)
V3 - Between V2 and V3 (Green)
V4 - 5th intercostal space, mid-clavicular line (Black)
V5 - 5th intercostal space, below start of axilla (Brown)
V6 - 5th intercostal space, below centre of axilla (Purple)
Give the places you would place the limb electrodes with their colour
Right arm - Red
Left arm - Yellow
Left leg - Green
Right leg (neutral) - Black
Are the leads and electrodes the same thing?
What lead is generally included as a repeat on an ECG and why?
Shows repeats which allow you to see irregularities in the heart beat.
Give each of the things you need to check for when reading an ECG
What is a normal heart rate?
How do you calculate heart rate from an ECG?
300/Number of large squares R-R
What is the length of a normal PR segment?
What is the length of a normal QT interval?
Describe the cause of atrial fibrillation and the change to an ECG trace.
Damage to a pacemaker so those in the AVN take over, which is slightly slower than the SAN.
R-R will not be equal, slower heart rate, loss of P wave.
The heart can tolerate it but blood may pool in the atria, causing clots to form.
Describe the cause of heart block.
AVN damage that causes prolonged delay.
There is something affecting the transmission of action potentials between the atria and ventricles.
Describe the ECG changes in first degree heart block
Prolonged PR segment, low heart rate
Describe the ECG changes in second degree heart block
Prolonged PR segment, low heart rate, erratic pulse
Describe the ECG changes in third degree heart block
No relationship with P wave and QRS complex
Fewer QRS than P wave as ventricles contracting slowly
Slow heart rate
What factor causes a shift in the electrical axis of the heart?
Hypertrophy of muscle in the right or left ventricle independent of the other
Describe the ECG changes in left axis deviation and give a condition that may cause it
Lead II small
Lead III negative
Lead I large
Caused by aortic stenosis
Describe the ECG changes in right axis deviation and give a condition that may cause it.
Lead II small
Lead III large
Lead I negative
Caused by pulmonary hypertension
What can cause bundle branch block and what effect will this have on the ECG?
Damage to the conducting pathways
Route of spread is changed which alters the shape of QRS and tends to lengthen it.
There are many variations
Describe the ECG changes in someone having a myocardial infarction.
ST elevation (only in STEMI)
Pathological Q waves
Inverted T waves
If a patient survives a myocardial infarction, what changes will there be to the ECG and why?
The dead or damaged tissue is replaced by non-conducting scar tissue.
Permanent changes to the QRS complex
Pathological Q waves which are large