Outline the principles of the ECG
- The ECG involves measuring the electrical activity of the heart using electrodes placed on the skin.
- This is an extracellular recording of the combined spread of electrical activity across the whole heart.
- Because the heart is a relatively large mass of muscle and depolarizes and repolarizes in a coordinated fashion it is possible to detect this electrical activity using electrodes placed on the skin.
What composes the specialised conducting system of the heart?
- Sinoatrial node
- Atrioventricular node
- Bundle of His
- Right and Left bundle branches
- Fibres of the Purkinje system
Briefly outline how the action potentials spread over the heart in a precise pattern.
What does the nature of the signal depend on?
The nature of the signal depends upon the direction of spread of the electric field relative to the position of the recording electrode.
The nature of the signal may be predicted by the following rules:
- Depolarisation spreading towards a positive recording electrode yields an upward deflection.
- Depolarisation spreading away from a positive recording electrode yields a downward deflection.
- Repolarisation spreading towards a positive recording electrode yields a downward deflection.
- Repolarisation spreading away from a positive recording electrode yields an upward deflection
Explain the factors affecting the amplitude of the deflection.
- It depends of the size of the muscle changing potential and how fast
- It depends on how directly the wave of activity is traveling towards the electrode:
I. Directly towards /away yields a large signal
II. Obliquely towards/away yields a smaller signal
III. Spread at right angles yields no signal
ECG component: R-R interval
- Where to measure: From peak to peak of R-waves
- What does this indicate: Shorter interval = faster heart rate
ECG component: QRS complex
- Where to measure: Start of the Q-wave to the end of the S-wave
- What does this indicate: Wider QRS complexes are associated with ventricular depolarisations that are not initiated by the normal conductance mechanism
ECG component: P-R interval
- Where to measure: Start of the P-wave to start of the Q-wave
- What does this indicate: Longer P-R intervals indicate slow conduction from the atria to the ventricle (first degree heart block)
ECG component: ST segment
- Where to measure: End of S-wave to start of T-wave
- What does this indicate: The ST segment should be isoelectric. If it is raised or depressed this indicates myocardial infarction or ischaemia.
ECG component: Q-T interval
- Where to measure: Start of Q-wave to end of T-wave
- What does this indicate: A prolonged Q-T interval suggests prolonged repolarisation of the ventricles. This can lead to arrhythmia as occur in long QT syndrome.
- Like the R-R interval, the Q-T interval is dependent on the heart rate in an obvious way (i.e. The fast the heart rate, the shorter the R-R and Q-T interval) and may be adjusted to improve the detection of patients with increased risk of ventricular arrhythmia.
- Modern computer based ECG machines can easily calculate a corrected QTc, but this correction may not aid the detection of patients at an increased risk of arrhythmia, as there are a number of different correction formulas.
Outline the structures of the conducting system of the heart.
- SAN: sets the rhythm = sinus rhythm
- AVN is continuous with the Bundle of His: the only conducting path from the atria to the ventricles
- Right and left bundle branches: lie sub-endocardially in the IV septum.
- Purkinje fibres: fine branches of the bundles of His. Rapid spread of depolarisation throughout the ventricular myocardium
Distinguish between repolarisation and depolarisation of the ventricles.
- Depolarisation of the ventricles happen in reverse order i.e. the last part of the ventricles to depolarise is the first part to repolarisation
What is the P wave?
P wave: atrial depolarisation spreads along both atrial fibres & internodal pathways towards the AV node
What is the Q segment?
Q: The initial downward deflection after the P wave as the muscle in the interventricular septum depolarises from left to right.
What is the QRS segment?
What is the R segment?
R: initial upward deflection after p wave (upward because it is moving directly towards the electrode, large because there is greater muscle mass i.e. more electrical activity)
What is the S segment?
S: Downward deflection after the R (depolarisation finally spreads upwards to the base of the ventricles, small because it does not move directly away)
What is the T segment?
T: ventricular repolarisation (begins on the epicardial surface – spreads in the opposite direction to depolarisation) produces medium upward deflection (moves away from electrode)
Limb leads: II, III and aVF
They look at the inferior surface of the heart
Limb leads: I and aVL
They look at the left side of the heart
Outline the process of recording the ECG
- There are 10 electrodes
I. 4 on the limbs
II. 6 on the chest
- They are connected by cables to the ECG machine
- 12 views of the heart are given
Chest leads: V1 and V2
- They face the right ventricle & septum
- Aka 'septal leads'
- V1: 4th intercostal space (right sternal border)
- V2: 4th intercostal space (left sternal border)
Chest leads: V3 and V4
- They face the anterior wall of the ventricles
- Aka 'anterior leads'
- V3: Between V2 and V4
- V4: 5th intercostal space (mid-clavicular line)
Chest leads: V5 and V6
- They face the left ventricle
- Aka 'lateral leads'
- V5: Between V4 and V6
- V6: 6th intercostal space (mid-axillary line)
How does one determine normal sinus rhythm?
- Is the rhythm regular?
- Is the heart rate normal (60 - 100 bpm)?
- Are there P waves?
- Is every P wave followed by QRS?
- Is every QRS preceded by a P wave?
- Are the P waves upright in leads I, II?
- Is the PR interval normal (3-5 small boxes)?
- Is the QRS width normal (3-5 small boxes)?
Distinguish between sinus bradycardia and sinus tachycardia.
- Sinus rhythm with rate < 60/minute = sinus bradycardia
- Sinus rhythm with rate > 100/minute = sinus tachycardia
How does one calculate heart rate from an ECG?
How does one determine if the heart beat is irregular?
What does each block in an ECG represent?
With reference to the electrical activity of the heart, describe what is happening at each of the numbered points.
1 - SAN depolarisation
2 - Atrial depolarisation
3 - AVN delay
4 - Ventricular depolarisation