Leads I - III
Leads I - III
Embedded Files
Overview
Overview
Depolarisation of the atria (P wave) and ventricles (QRS complex) occurs in the direction of down and to the left.
Where the ECG leads are placed will determine how this signal is displayed in the ECG trace.
ECG traces can be taken with either 1, 3 or 12 leads. 1 and 3 leads can be taken quickly, for example by ambulance crew. 3 lead ECG monitoring is often used to monitor patients under general anaesthesia in theatre.
A 12 lead ECG will give you the most information.
Understanding the placement of the leads is important to understand and interpret the ECG trace.
Basic 3 Lead
Basic 3 Lead
A basic ‘3-lead’ ECG is taken using 4 leads attached to the patient. The right arm (RA), left arm (LA) and left leg (LL) all contribute to forming the shape of the trace. The right leg acts as a ground.
The measurements between these leads for the first 3 traces on an ECG:
RA to LA is ‘Lead I’
RA to LL is ‘Lead II’
LA to LL is ‘Lead III’
These 3 leads form Einthoven’s triangle.
A 12 lead ECG uses 6 further leads to give 12 traces. (Yes, a 12 lead ECG only uses 10 leads! The principle is that these 10 leads are used to capture 12 separate pieces of information about electrical conduction in the heart).
Lead 1
Lead 1
The trace from right arm to left arm is known as ‘Lead I’ and is the first trace on the 12 lead ECG.
This is displayed as a positive deflection in the ECG trace. This is because the vector (direction) of the electrical signal in the heart is similar to the orientation of the electrodes (right to left).
Lead 2
Lead 2
The trace from right arm to left leg is known as ‘Lead II’.
This lead is also in a similar orientation to the cardiac electrical signal. Thus the deflection in the ECG trace will be positive. It will be more exaggerated than lead I as the vector measured in Lead II is even more similar to the cardiac signal.
Lead 3
Lead 3
The trace from left arm to left leg is known as ‘Lead III’.
This lead is orientated across the cardiac electrical signal so shows a smaller deflection (remember Fig 5-8 on page 6).
The positive and negative deflections are more equal as the cardiac signal passes between the 2 electrodes more perpendicularly, rather than heading from one electrode to another (such as in Leads I and II where the cardiac signal is travelling in the direction of the positive electrode).
Example
Example
Here is an example of a normal ECG showing Leads I-III.
Page updated
Google Sites
Report abuse