ECG Basics

by | 20 May, 2020

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The 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 waves

ECG template copy Changes 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 ventricles

In 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

Interpretation Structure

  1. Rate: calculated by either:
    1. 300 ÷ number of large squares between 2 QRS complexes
    2. QRS peaks across rhythm strip x 6
  2. Rhythm: Regular or irregular? Narrow or wide QRS?
  3. 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

  1. Intervals:

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

  1. Morphology:

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

Myocyte Physiology

Ion physiology

Cardiac 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 depolarisation

Cardiac cells normally undergo cycles of depolarisation and repolarisation. These are split into 5 distinct phases:
  1. Rapid depolarisation – Na+ entry
  2. Transient repolarisation – K+ exits to produce the transient outward current (Ito­­)
  3. Plateau – K+ exit and Ca2+ entry (L-type calcium channels) occur together to prolong the action potential
  4. Terminal repolarisation – Ca2+ channels close. K+ exit completely repolarises the cell.
  5. Electrical diastole
Single Myocycte depolarisation Any change in the concentration of the above ions leads to an altered duration and/or waveform of the action potential. These electrolyte-induced changes are reflected in ECG traces.


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