IntroductionThe electrocardiogram (ECG) measures the total extracellular signals from the heart, allowing arrhythmia to be detected. The 12-lead ECG uses 10 electrodes placed on the chest and limbs. These are used to create 12 leads, which are measures of electrical activity between a pair of electrodes.
ECG wavesChanges in extracellular voltage produce waves on the ECG.Depolarisation and repolarisation of myocytes in different directions produces the characteristic ECG pattern:
P wave: Depolarisation of atria
QRS complex: Depolarisation of ventricles
T wave: Repolarisation of ventriclesIn addition, the intervals between these waves can be examined.
PR interval: time from start of P wave to start of QRS complex
QT interval: time from beginning of QRS complex to end of T wave
ST interval: time from end of QRS complex to start of T wave
- Rate: calculated by either:
- 300 ÷ number of large squares between 2 QRS complexes
- QRS peaks across rhythm strip x 6
- Rhythm: Regular or irregular? Narrow or wide QRS?
- Axis: Examine the QRS complexes in leads I, II and aVF and check if they are predominantly positive or negative deflecting.
Normal: Dominant R wave in I and II
Right axis deviation: Dominant S wave in lead I
Left axis deviation: Dominant S wave in leads aVF and II
PR interval: 120-200ms (3-5 small squares)
QRS complex: <120ms (<3 small squares)
QT interval: As a rough guide, it should be less than half the R-R interval
P waves: Peaked = P-pulmonale. Broad, bifid = P-mitrale
QRS complex: Narrow QRS = supraventricular rhythm. Wide QRS = ventricular rhythm
Bundle branch block: WiLLiaM = Left bundle branch block. MaRRoW = Right bundle branch block
ST segment: Elevation = Infarction. Depression = Ischaemia
Ion physiologyCardiac myocytes are kept at a resting potential of -90mV via the Na-K-ATPase pump. This maintains a high intracellular concentration of potassium and a low intracellular concentration of sodium. Ca2+ is kept at a low intracellular concentration, and is used for depolarisation and maintaining a plateau phase. Mg2+ is also involved in regulating the Na-K-ATPase.
Myocyte depolarisationCardiac cells normally undergo cycles of depolarisation and repolarisation. These are split into 5 distinct phases:
- Rapid depolarisation – Na+ entry
- Transient repolarisation – K+ exits to produce the transient outward current (Ito)
- Plateau – K+ exit and Ca2+ entry (L-type calcium channels) occur together to prolong the action potential
- Terminal repolarisation – Ca2+ channels close. K+ exit completely repolarises the cell.
- Electrical diastole