Recording an ECG

The contraction of any muscle is associated with electrical changes called ‘depolarization’, and these changes can be detected by electrodes attached to the surface of the body. Since all muscular contraction will be detected, the electrical changes associated with contraction of the heart muscle will only be clear if the patient is fully relaxed and no skeletal muscles are contracting. The principle behind the way the ECG records the electrical impulse is quite simple.The electrical signal that starts in the atria and travels down to the ventricle is of course moving through three dimensions. Each lead inscribes a positive deflection for that component of the net electrical vector i.e. travelling towards its positive electrode and a negative deflection for that component of the net electrical vector i.e. travelling towards its negative electrode. Thus, by knowing the position of each lead, one can determine the direction the electrical signal is travelling. When the electrical vector and direction of the lead are perpendicular, then the lead records a horizontal line, namely isoelectric line.

An actual ECG is recorded by placing electrodes on each limb and 6 electrodes on the chest. This allows the recording of 12 ECG leads. A routine ECG is recorded by connecting 12 leads.Looking at the heart with 12 leads is like looking at a sculpture, building or car from multiple angles. The more points of view one has, the more one learns about it. In clinical practice, twelve leads are usually used in the diagnostic ECG, although there is no limitation to the number of leads one may select for special purposes.

Standard Leads

There are three of these leads which are usually designated as I, II and III. They are all bipolar leads (i.e., they detect a change in electric potential between two points) and detect an electrical potential change in the frontal plane. Lead I is between the right arm and left arm electrodes, the left arm being positive. Lead II is between the right arm and left leg electrodes, the left leg being positive. Lead III is between the left arm and left leg electrodes, the left leg again being positive.A diagrammatic representation of these three leads is termed Einthoven’s triangle (shown below), after the Dutch doctor who first described the relationship. The central source of electrical potential in the triangle is the heart.

The same three leads that form the standard leads also form the three unipolar leads known as the augmented leads. These three leads are referred to as aVR (right arm), aVL (left arm) and aVF (left leg) and also record a change in electric potential in the frontal plane.

These leads are unipolar in that they measure the electric potential at one point with respect to a null point (one which doesn’t register any significant variation in electric potential during contraction of the heart). This null point is obtained for each lead by adding the potential from the other two leads. For example, in lead aVR, the electric potential of the right arm is compared to a null point which is obtained by adding together the potential of lead aVL and lead aVF.

Unipolar Leads

Precordial Leads

These six unipolar leads, each in a different position on the chest, record the electric potential changes in the heart in a cross sectional plane. Each lead records the electrical variations that occur directly under the electrode. The first chest lead is called V1 and is placed just to the right of the sternum in the fourth intercostals space. Chest lead V2 is placed just to the left of the sternum in the same space. Chest leads V3 through V6 are sequentially further to the left. These six unipolar leads, record the electrical variations that occur directly under the electrode. The first chest lead is called V1 and is placed just to the right of the sternum in the fourth intercostal space. Chest lead V2 is placed just to the left of the sternum in the same space. Chest leads V3 is placed between V2 and V4. Lead V4 is placed in the 5 th intercostal space in the mid clavicular line.Leads V5 and V6 are placed horizontal to V4 ,in the anterior and mid axillary line.

Positioning of the Chest V Leads

The relationship between the six chest leads and the heart

Each of the 6 chest leads is a positive lead. The patient’s back is considered the negative electrode for each.A standard ECG machine records leads I, II and III simultaneously, then aVR, aVL, and aVF simultaneously, then V1, V2, and V3 simultaneously and finally V4, V5, and V6 simultaneously:

Standards Conventions when Reading an ECG

The rate of paper (i.e. recording of the ECG) is 25 mm/s which results in:

• 1 mm = 0.04 second (or each individual block)

• 5 mm = 0.2 second (or between 2 dark vertical lines)

The voltage recorded from the leads is also standardized on the paper where 1 mm = 0.1 mV (or between each individual block vertically) This results in:

• 5 mm = 0.5 mV (or between 2 dark horizontal lines)

• 10 mm = 1.0 mV (this is how it is usually marked on the ECG’s)

The following picture shows ECG paper with conventional measurements.

ECG paper with its measurement