to the right sternal border, 4th intercostal space

to the left sternal border, 4th intercostal space

between V2 and V4

midclavicular line, 5th intercostal space

between V4 and V6

midaxillary line, 5th intercostal space

• The atrial depolarisation is towards the electrode, so makes a positive deflection [1], the P wave. Followed by a small negative deflection [2] where the left atrium depolarises.

• Next the AV node conducts the signal to the septum, the delay in crossing the node causing the ‘PR interval’. Depolarisation then travels down the septum, then to the right ventricle.

• Depolarisation of the right ventricle is the R wave [3]. In V1 this is towards the electrode so is positive. Any negative deflection before the R wave is called the Q wave and represents depolarisation in the septum.

• The next conduction very shortly after is the depolarisation of the large left ventricle, causing a large signal but this time away from the electrode [4], so there is a negative deflection.

• The negative deflection after an R wave is called the S wave.

• Finally the ventricles repolarise, causing the T wave [6]

This is the same principle as the trace in V1:

• The P wave has a similar morphology but is slightly flatter [1].

• Then the Q wave [3] is a small negative deflection as the signal in the septum is now moving away from the electrode.

• The R wave is a large positive spike on the trace as the signal is towards the electrode [4].

• As before the S wave is a small negative deflection after the R wave [5].

• Lastly the T wave represents ventricular repolarization. [6]

V1 to V4 are known as ‘anterior’ leads as they show the signal predominantly from the anterior of the heart.

V1 and V2 are also known as ‘septal’ leads as they show conduction in the septum.

V4 is the approximate ‘transition’ point where the R wave becomes bigger than the S wave.

V5 and V6 are ‘lateral’ leads showing the signal predominantly in the posterior part of the heart.