The McGill Physiology Virtual Lab

Cardiovascular Laboratory

 ECG> Setup
  Practical Points
To ensure that the recording technique is uniform and to reduce the number of variables influencing the tracing, the following points must be kept in mind:
  • there must be effective contact between electrode and skin
  • the subject should lie  flat on their back; lying on side or sitting up alters the heart's electrical axis.
 
Equipment
  • The ECG signal will be recorded through the PowerLab data acquisition system, which contains an isolated input amplifier (BioAmp).  A five-lead patient cable is connected to the input of the BioAmp. Here, only three leads will be used.
  • Four disposable electrodes and alcohol pads are also required.

Procedure
  • The subject should lie flat on his/her back on a bench or cot, with hands at their sides.
  • The inner aspect of the right forearm just above the wrist is rubbed briskly with alcohol. 

  • The disposable electrode is then placed onto the cleansed area.

 

  • The electrodes are placed in the same way for the left arm, and then for both legs, attaching the electrodes a few inches above the subject's ankle.
Depending on the lead orientation, the appropriate leads of the patient cable are snapped on the electrodes. For example, the following connections are made in order to record from Lead II:
  • NEG (white plug) to the right arm
  • POS (black plug) to the left leg
  • Earth (green plug, Common or reference) to the right leg
 
The Three Standard Bipolar Limb Leads
The LL electrode is taken to the + terminal of the BioAmp, and the RA electrode is taken to the - terminal of the BioAmp.  Thus the output called Lead II is the difference in the potentials appearing on the left leg and the right arm:
Lead II = (VLL - VRL) - (VRA - VRL) = VLL - VRA

Remember that the potential measured at the right leg (VRL) is used as the reference potential. 

The connections for all three standard limb leads are shown to the left below.  The figure to the right shows a diagrammatic representation of the Einthoven Triangle Hypothesis. Willem Einthoven (1860-1927) attempted to explain the principles of the ECG in scientific terms.  In Einthoven's triangle, the heart may be considered to lie at the centre of an equilateral triangle and the corners of the triangles are the effective sensing points - the right arm, left arm and left leg electrodes.

Einthoven's Triangle
Lead I = (VLA - VRL) - (VRA - VRL) = VLA - VRA
Lead II = (VLL - VRL) - (VRA - VRL) = VLL - VRA
Lead III = (VLL - VRL) - (VLA - VRL) = VLL - VLA
To continue with the next section: ECG Basics, click here