Physiology chapters
The ECG and cardiac axis
What each wave and interval of the normal ECG represents, how they match the phases of the cardiac action potential, the 12 leads, and how to work out the cardiac axis and its deviation.
- P = atrial depolarisation; QRS = ventricular depolarisation (phase 0); ST segment = plateau (phase 2); T = ventricular repolarisation (phase 3).
- PR interval (0.12–0.20 s) shows AV conduction; QT interval (about 0.35–0.44 s) shows the whole ventricular action potential.
- The normal cardiac axis is about −30° to +90°; left axis deviation beyond −30°, right beyond +90°.
On this page
Overview
The electrocardiogram (ECG) records the electrical activity of the heart from electrodes on the skin. It shows the sum of the action potentials of many cells, seen from different angles (leads).
The ECG shows electrical events only. It does not directly measure how strongly the heart pumps.
ECG paper
- Standard speed: 25 mm per second.
- One small square (1 mm) = 0.04 s; one large square (5 mm) = 0.2 s.
- Standard voltage: 10 mm = 1 mV.
Heart rate (regular rhythm) = 300 ÷ number of large squares between two R waves (or 1,500 ÷ small squares).
The normal waves and intervals
| Part | What it shows | Normal duration |
|---|---|---|
| P wave | Atrial depolarisation | Up to 0.12 s (under 3 small squares) |
| PR interval | Start of P to start of QRS: time for the impulse to pass from atria, through the AV node, to the ventricles | 0.12–0.20 s |
| QRS complex | Ventricular depolarisation | Under 0.12 s (usually 0.06–0.10 s) |
| ST segment | Ventricles fully depolarised (plateau) | Normally on the baseline (isoelectric) |
| T wave | Ventricular repolarisation | - |
| QT interval | Start of QRS to end of T: whole ventricular depolarisation and repolarisation | About 0.35–0.44 s (varies with heart rate) |
| U wave | Late repolarisation (possibly papillary muscles or Purkinje fibres) | Small; prominent in hypokalaemia |
Atrial repolarisation happens at the same time as the QRS complex and is hidden by it. That is why it is not seen.
The T wave is upright in most leads even though it represents repolarisation. This is because the ventricles repolarise from the outside (epicardium) inwards, the opposite direction to depolarisation.
Matching the ECG to the ventricular action potential
| ECG | Action potential phase |
|---|---|
| QRS complex | Phase 0 (rapid depolarisation) |
| ST segment | Phase 2 (plateau) |
| T wave | Phase 3 (repolarisation) |
| TP segment (between beats) | Phase 4 (rest) |
| QT interval | Duration of the whole ventricular action potential |
The ST segment matches the plateau, when all ventricular cells are depolarised and no current flows between them, so the line is flat.
The leads
A lead is a view of the heart's electrical activity from a particular direction. A wave moving towards the positive electrode gives an upward deflection.
Standard (bipolar) limb leads
These measure the difference between two limbs. Together they form Einthoven's triangle.
| Lead | Negative electrode | Positive electrode | Angle |
|---|---|---|---|
| I | Right arm | Left arm | 0° |
| II | Right arm | Left leg | +60° |
| III | Left arm | Left leg | +120° |
Einthoven's law: the voltage in lead II equals the sum of leads I and III (II = I + III).
Augmented (unipolar) limb leads
These compare one limb with the average of the other two.
| Lead | Positive electrode | Angle |
|---|---|---|
| aVR | Right arm | −150° |
| aVL | Left arm | −30° |
| aVF | Left leg (foot) | +90° |
In aVR most waves are negative in a normal ECG, because depolarisation moves away from the right shoulder.
Chest (precordial) leads
V1 to V6 lie across the chest from the right of the sternum to the left mid-axillary line. They view the heart in the horizontal plane.
- V1–V2: septum and right ventricle.
- V3–V4: anterior wall.
- V5–V6: lateral wall of the left ventricle.
In V1 the QRS is mainly negative; by V6 it is mainly positive.
The cardiac axis
The mean electrical axis is the average direction of ventricular depolarisation in the frontal plane. It normally points downwards and to the left, because the left ventricle is the larger mass.
| Axis | Range |
|---|---|
| Normal | About −30° to +90° (Guyton gives a usual mean of about +59°) |
| Left axis deviation | More negative than −30° |
| Right axis deviation | More than +90° |
A quick method
Look at the QRS in lead I and aVF (or lead II):
| Lead I | aVF / lead II | Axis |
|---|---|---|
| Positive | Positive | Normal |
| Positive | Negative (and negative in lead II) | Left axis deviation |
| Negative | Positive | Right axis deviation |
The axis shifts towards hypertrophy and away from infarction.
Causes of axis deviation
| Left axis deviation | Right axis deviation |
|---|---|
| Left ventricular hypertrophy (hypertension, aortic stenosis) | Right ventricular hypertrophy (pulmonary hypertension, pulmonary stenosis) |
| Left anterior fascicular block | Left posterior fascicular block |
| Short, stocky person, or late pregnancy (heart pushed up) | Tall, thin person; normal in newborns |
| Inferior myocardial infarction | Lateral myocardial infarction; pulmonary embolism |
Clinical relevance
Heart block
What you see: first degree: PR interval longer than 0.20 s. Complete block: P waves and QRS complexes with no relationship.
Bundle branch block
What you see: a wide QRS (0.12 s or more), because one ventricle is activated slowly through muscle instead of the Purkinje system.
Myocardial infarction
What you see: ST elevation in leads facing the damaged area, then Q waves and T wave inversion.
Potassium
What you see: hyperkalaemia: tall peaked T waves, then wide QRS. Hypokalaemia: flat T waves and U waves.
Common questions: what each wave and interval represents, normal PR and QRS durations, the hidden atrial repolarisation, matching ST and T to phases 2 and 3, bipolar vs unipolar leads, Einthoven's law, the normal axis range, and the causes of left and right axis deviation.
Quick revision
- P: atrial depolarisation. QRS: ventricular depolarisation. T: ventricular repolarisation.
- PR 0.12–0.20 s; QRS under 0.12 s; QT about 0.35–0.44 s.
- Atrial repolarisation hidden in QRS.
- QRS = phase 0; ST = phase 2; T = phase 3.
- I, II, III bipolar (Einthoven, II = I + III); aVR, aVL, aVF unipolar.
- Normal axis −30° to +90°. LAD: LVH, left anterior fascicular block. RAD: RVH.
Written by The House Job from standard physiology references (Guyton and Hall Textbook of Medical Physiology; BRS Physiology).
FAQ
What does the QRS complex represent?
Depolarisation of the ventricles.
Why is atrial repolarisation not seen on the ECG?
It occurs at the same time as the QRS complex, which hides it.
What is the normal PR interval?
0.12 to 0.20 seconds.
What causes left axis deviation?
Left ventricular hypertrophy and left anterior fascicular block are common causes.