ENDOCARDIUM
T
MYOCARDIUM
1. Systole/diastole
7. ST elevation
2. Action Potentials
1. Press the ‘play’ button. In this slowed down animation you can see a cross section of the ventricle contracting (in systole) which corresponds to the QRS complex on the ECG.
Contraction starts in the endocardium and radiates out through the myocardium to the epicardium, which is the last area to contract. This order of contraction makes sense when considering the thickness of the ventricle (if the epicardium contracted first it would ‘pull’ the lumen of the ventricle towards it, which would be counter-productive).
The ventricles repolarise (T) and diastole occurs as the myocardium relaxes and the ventricles fill with blood).
Cross section
Click:
3. ST segment
PLAY >
1. Ventricular systole & diastole on an ECG
4. T wave shape
VENTRICLE LUMEN
Prof MJ COFFEY (CoffeyMJ@cardiff.ac.uk)
5. T wave inversion
Dr MJ COFFEY (CofeyMJ@cardiff.ac.uk)
P
6. ST depression
Q
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R
EPICARDIUM
S
cAP durations
VENTRICLE LUMEN CROSS SECTION
2. The sequence of contraction across the ventricle is achieved because of the timing and duration of the cardiac action potentials (cAPs) seen in the endocardium, myocardium and epicardium. Click the ‘depolarisation’ button: the cAP in the endocardium is seen first. The wave of depolarisation then propagates from the endocardium to the myocardium (its cAP is of similar duration overall) and finally to the epicardium which has the shortest cAP duration of all. The combined depolarisation events seen across the ventricle constitute the QRS complex on an ECG.
Now click on the ‘repolarisation’ button: because the cAP in the epicardium is of short duration, it starts to repolarise first. Following this, the myocardium starts to repolarise and finally the endocardium. Because of this order, the net direction of repolarisation across the ventricle is from the epicardium to endocardium.
The cumulative repolarisation seen across the ventricle correpsonds to the T waves seen on the ECG.
2. Depolarisation & repolarisation
= Q R S
Direction of wave of depolarisation that causes contraction
Depolarisation
= T
Direction of wave of repolarisation as ventricle relaxes
Repolarisation
Phase 2 ‘plateau'
3. The ST segment on an ECG represents the transition period between the end of ventricular depolarisation and the start of ventricular repolarisation.
It appears as a flatline on the ECG as the ST segment is ‘isoelectric’ i.e. the transition from depolarised to repolarised has no net change in membrane potential (0 mV) and so the ST segment broadly equates to Phase 2 (the ‘plateau’ of the cAPs).
If the ST segment appears either elevated or depressed between the QRS and T, then this typically indicates some form of cardiac ischaemia; see (6) and (7) for the explanation of this.
ST segment
3. The ST segment on an ECG
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Positive depolarisation towards electrode
or...
4. Press the ‘play’ button. The T wave represents ventricular repolarisation. The orientation of the T wave on an ECG can sometimes seem counter-intuitive, but here’s why the T wave is normally upright (‘positive’) in most ECG leads:
An ECG electrode detects a wave of positive charge (depolarisation) travelling towards it as an upward ‘positive’ deflection on the graph, hence the QRS complex being seen as the large positive tracing during ventricular depolarisation. But! ECG electrodes aren’t very sophisticated which means that they also interpret a wave of repolarisation travelling away from the electrode as a ‘positive’ deflection i.e. ’negative charge’ travelling away from the electrode is technically considered the same as ‘positive charge’ travelling towards it (a sort of double negative equalling a positive). So, as the normal order of ventricular repolarisation is from epicardium to endocardium, the wave of negative charge is travelling away from the electrode, hence the T wave appears upright and ‘positive’.
ECG recording electrode
Negative repolarisation away from electrode
equals upward deflection on ECG trace
4. The orientation of the (normal) T wave
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5. Inverted T waves
Ischaemic ENDOCARDIUM
Ischaemic endocardium has short AP which repolarises first
5. Press the ‘play’ button. If the endocardium becomes ischaemic less ATP is created which reduces Na+/K+ ATP’ase pump and K+(ATP) efflux channel activity (normally, these pumps and channels would help to hyperpolarise the endocardium.) As a result, the more depolarised ischaemic endocardium has a shorter AP duration than the epicardium meaning the endocardium is the first to start repolarising. This reverses the wave of repolarisation towards the epicardium (and ECG electrode) meaning the T wave may (but not always) become inverted.
T wave inversion may also be associated with ST depression; this is because the endocardial ischaemia that causes a shortening of the endocardial AP and its early repolarisation compared to the epicardium (and hence the potential to flip the T wave recorded) may also cause an artificially elevated baseline on an ECG that will paradoxically make the ST segment ‘appear’ depressed; see (6) for full explanation of this observation.
You often see a combination of ST depression and T wave inversion in NSTEMI heart attacks.
Note: not all T wave inversion is pathological (e.g. in paediatric patients or in Lead aVR.)
Apparent depression
True baseline 0mV (& normal ECG overlaid)
Elevated baseline due to background endocardial partial depolarisation travelling towards ECG electrode at rest
6. ST depression (e.g. seen in angina)
Apparent baseline
Ischaemic ENDOCARDIUM
6. Demand driven ischaemia (e.g. as in stable angina) makes the endocardium partially depolarised prior to systole which pushes the ‘apparent’ baseline on the ECG upwards (red arrows) compared to a non-ischaemic ECG. This is because the partial depolarisation in the ischaemic endocardium is travelling towards the ECG electrode (yellow arrow). But full ventricular depolarisation happens as normal meaning the resulting isoelectric ST segment represents the true baseline (0mV), so appears depressed (blue arrow) compared to the apparent baseline. Only when the ventricles repolarise does the endocardial ischaemia again push the ‘apparent’ baseline back upwards due to the partial depolarisation at rest. So, ST depression is only relative to the ‘apparent’ ischaemic baseline of the ECG.
Note: ST depression may also be associated with T wave inversion; see (5).
Depressed baseline due to transmural ischaemic depolarisation travelling away from electrode at rest
7. Coronary artery occlusion can lead to an entire region of ventricle (from endocardium to epicardium) becoming acutely ischaemic. Transmural ischaemic tissue is partially depolarised at rest but crucially, the wave of partial depolarisation cannot travel towards the ECG electrode and instead radiates away into other parts of the ventricle (the yellow arrows). Because this partial depolarisation is travelling away from the electrode, the ‘apparent’ baseline appears depressed (the blue arrows). But full ventricular depolarisation occurs as normal and the resulting isoelectric ST segment represents the true baseline (0mV), so appears elevated compared to the ‘apparent’ baseline. Only when the ventricles repolarise does the transmural background ischaemia depress the ‘apparent’ baseline again on the ECG. So, ST elevation is only relative to the ischaemic baseline.
7. ST elevation (e.g. seen in MI)
Apparent elevation
Coronary occlusion causing a transmural ischaemia across the ventricle.