ECG

• records the amplitude and direction of the wave of depolarisation as spreads through heart

• Electrodes attached to both arms and legs, chest leads on 6 places on chest (specific places)

• Potential difference between electrodes measured

• Signals reliably detected (of more than 1mV)

Chest leads:

• V1 - 4th ICS, right of sternum

• V2 - 4th ICS, left of sternum

• V3 - between V2 and V4

• V4 - 5th ICS, mid clavicular line

• V5 - anterior axillary line, level with V4

• V6 - mid axillar line, level with V4 and V5

Clinical practice:

• 12 different directions to produce 12-lead ECG

• Bipolar and unipolar

• Leads I, II, III bipolar limb leads - potential difference between 2 limb leads

• Augmented unipolar limb leads - aVL, aVR, aVF, same electrodes, record with respect to zero

• I - left arm (to right arm)

• II - left leg (to right arm)

• III - left leg (to left arm)

• aVR - right arm

• aVL - left arm

• aVF - left leg

Einthoven’s triangle:

• limb leads interrogate heart in vertical (coronal) plane

• Chest leads interrogate heart in transverse plane

• Leads I-III electrical activity of heart parallel to sides of triangle

• Unipolar leads look at heart from points of triangle

• Chest leads look at heart from 6 positions around chest wall

Normal ECG:

• electrical activity related to different phases of contraction and relaxation of muscle

• Depolarisation moving towards electrode produces ECG with positive deflection

• Repolarisation moving towards electrode produces negative deflection

• P wave produced by depolarisation of atria, positive deflection as wave of depolarisation moves towards positive electrode

• QRS complex represents depolarisation of ventricles

• Q - depolarisation of interventricular septum from L to R

• R - depolarisation of main mass of ventricles

• S - depolarisation of area of heart near base

Determinants of ECG deflections:

Timing of normal ECG:

T wave produced- ventricular repolarisation

Normal ECG: