Week 7 - ECG Flashcards
Describe the spread of excitation over the normal heart
- Activity starts at the SA node
- Depolarisation spreads over the atria to the AV node
- There is a delay of 120ms at the AV node
- After the delay, activity spreads down the septum
- It then spreads out over the ventricular myocardium
- From the inside (endocardial) surface to the outside (epicardial) surface
- Until all the ventricular cells are depolarised
Describe the spread of repolarisation over the normal heart
- After about 280 ms, cells begin to repolarise
- Repolarisation spreads in the opposite direction over the ventricle to depolarisation
- – Epicardial surface repolarises first, then the endocardial surface
What is an electrocardiogram?
A record of a person’s heartbeat
- The myocardium is a large mass of muscle undergoing electrical changes all more or less at the same time
- This generates a large changing electrical field which can be detected by electrodes on the body surface
What do the skin electrodes detect?
Changes in membrane potential
- So they ‘see’ 2 signals with each systole
- – One on depolarisation, one on depolarisation
- The spread of excitation over the myocardium also generates a changing signal which electrodes detect
- So the ECG is explained by a combination of the effects of depolarisation and repolarisation and their spread over the heart
What are the rules for an electrode view?
- Depolarisation moving towards an electrode = upward signal
- Depolarisation moving away from an electrode = downward signal
- Repolarisation moving towards an electrode = downward signal
- Repolarisation moving away from an electrode = upward signal
- The amplitude of the signal depends on:
- – How much muscle is depolarising
- – How directly towards the electrode the excitation is moving
What type of signal will be produced by atrial depolarisation if there is a single electrode ‘viewing’ the heart from the apex?
Small, upward deflection
- Small because little muscle is involved
- But moving towards the electrode
What type of signal will be produced by spread of depolarisation from the septum if there is a single electrode ‘viewing’ the heart from the apex?
Small, downward deflection
- Downward because moving away from electrode
- Small because not moving directly away
- Excitation spreads about halfway down the septum, then out across the axis of the heart
What type of signal will be produced by spread of depolarisation through the ventricular myocardium if there is a single electrode ‘viewing’ the heart from the apex?
Large, upward deflection
- Large because there is lots of muscle and it is moving directly towards the electrode
- Upwards because it is moving towards the electrode
- Depolarisation spreads through the ventricular muscle along an axis slightly left of the septum
What type of signal will be produced by the end of the spread of depolarisation if there is a single electrode ‘viewing’ the heart from the apex?
Small, downward deflection
- Downward because moving away from electrode
- Small because not moving directly away
- Depolarisation finally spreads upwards to the base o the ventricles
What type of signal will be produced by ventricular repolarisation if there is a single electrode ‘viewing’ the heart from the apex?
Gap between deflection due to 280 ms delay
Medium, upward deflection
- Upward because moving away
- Medium because timing in different cells is dispersed
What do the P, Q, R, S and T waves mean?
- P = atrial depolarisation
- Q = septal depolarisation spreading to ventricle
- R = main ventricular depolarisation
- S = end ventricular depolarisation
- T = ventricular repolarisation
Why is there no ECG signal for atrial repolarisation?
It is lost within the QRS complex
Describe how the R wave signal changes when you view it differently
- Head on = a large upward deflection
- Sideways = no signal (see nothing at a right angle)
- End on = large downward deflection
Define ‘lead’
An electrical view of the heart
What are augmented leads?
Leads that have 2 negatives connected - First convert the 2 negatives to 1 - Then convert that to a positive - And combine it with the actual positive to give 1 view Gives aVR, aVL and aVF
What are amplifiers?
Have 1 positive and 1 negative electrode
- Take the signal coming in on the negative input, invert it and add it to the signal from the positive input
- These are the limb leads: gives lead I (left side), lead II (apex) and lead III (bottom)
Describe where the limb leads are placed in an ECG
- Right upper = red
- Left upper = yellow
- Left lower = green
- Right lower = black
(ride your green bike)
How are the chest leads positioned?
- V1 = 4th intercostal space to the right of the sternum
- V2 = 4th intercostal space to the left of the sternum
- V3 = directly between the leads V2 and V4
- V4 = 5th intercostal space at the midclavicular line
- V5 = level with V4 at left anterior axillary line
- V6 = level with V5 at left midaxillary line (directly under the midpoint of the armpit)
How many leads and electrodes does an ECG have?
12 leads
- Only 10 electrodes
- Only 9 are recording (right lower limb is neutral)
How can you calculate the heart rate from an ECG?
- All ECG machines are run at a standard rate of 300 squares per minute
- So to calculate the heart rate, you divide 300 by the number of squares in the R-R interval
What should you look at in an ECG?
- Rate
- Rhythm
- Axis
- P wave
- P-R
- QRS complex
- Q-T interval
- T wave
Describe the ECG that will be seen in atrial fibrillation
- The P wave reflects atrial depolarisation
- So if the muscles are not contracting in a coordinated way (atrial fibrillation) the P wave will be absent
- In its place are irregular fibrillation waves
- There is no stimulus reaching the AV node, so other pacemakers must generate rhythm
What is a heart block?
A communication problem between the atria and ventricles
What does the P-R interval show?
The time taken for an impulse to reach the ventricles
What is ‘first-degree heart block’?
There is a conduction delay through the AV node, but all electrical signals reach the ventricles
- P-R interval is elongated from its normal 200 ms
What is ‘2nd-degree (type 1) heart block’?
Some, but not all, atrial beats get through to the ventricles
- The P-R interval is erratic
- It follows a pattern of the P-R elongating, until eventually a QRS complex is dropped
- The system is then reset
What is ‘2nd-degree (type 2) heart block’?
Electrical excitation sometimes fails to pass through the AV node or bundle of His
- Not all atrial contractions are followed by ventricular contraction
- Atrial rate is usually faster than ventricular rate
- More P waves than QRS complexes
What is ‘complete 3rd-degree heart block’?
Atrial contractions are normal, but no electrical conduction is converted to the ventricles
- The ventricles generate their own signal through an ectopic pacemaker
- These beats are usually slow
- Hence there is no relationship between the atrial and ventricle depolarisation
What is the effect of a bundle branch heart block on an ECG?
Lengthens and changes the shape of the QRS complex
- There are many variations depending on the location of the block
What is ventricular fibrillation?
Uncoordinated contraction of the ventricles
- Myocardium causes it to quite rather than contract properly
- Hence ECG is simply a quiver (there are no identifiable P waves, QRS complexes or T waves)
What are ventricular ectopic beats?
Ventricular cells gain pacemaker activity, causing ventricular contraction before the underlying rhythm would normally depolarise the ventricles
- The resulting ECG is often wider and taller than that seen with the underlying rhythm
- The ventricular ectopic beats may occur every other beat, very 3rd beat, every 4th beat, etc.
- Or they may occur in groups such as couplets, triplets, etc
What is the effect of damage to the myocardium on electrical activity?
- Affects the spread of electrical activity
- Generates current flows during systole
- – This produces extra signals in the ST segment
What is the effect of temporary shortage of oxygen on the heart?
Get angina
- ST depression in most cases
What is the effect of a lack of blood flow to part of the myocardium?
Causes myocardial infarction
- Dying tissues generate injury currents which produces ST elevation usually
What are the features of a MI on an ECG?
- S-T elevation
- Pathological Q waves
- Inverted T waves
What are pathological Q waves?
Q waves that are: - More than 0.04s (1 small square) wide - >2mm deep - Present in full thickness MI They remain after changes resolve
What is the electrical axis of the heart?
Relates to the main spread of depolarisation through the wall of the ventricle
- The combination of the depolarisation of the right and left ventricles generates a single vector normally pointing slightly left
How can the electrical axis of the heart be altered?
By changes in the relative amount of muscle in the right and left heart
- More muscle in left ventricle = ‘left shift’
- More muscle in right ventricle = ‘right shift’
How can you detect electrical axis deviation on an ECG?
Look at limb leads to estimate axis
- Find the lead with the smallest and most equiphasic deflection
- The net deflection is 0 indicating that the electrical axis must run at right angles to that view
