Class note on ECG for Diploma Critical Care

Electrocardiography

Electrocardiography is the process of recording the electrical activity of the heart by  electrodes placed on the skin. These electrodes detect the tiny electrical changes on the skin that arise from the heart’s electrophysiological activity during each heartbeat.

The graph of voltage versus time thus printed by the ECG machine is called electrocardiogram

In a conventional 12 lead ECG, ten electrodes are connected to the patient.

The overall magnitude of the heart's electrical potential is then measured from twelve different angles ("leads") and is recorded over a period of time (usually 10 seconds).

Electrodes and leads:

An electrode is the conductive pad in contact with the body that makes an electrical circuit with the electrocardiograph.

A lead is the source of measurement of a vector.  [A vector is a geometric object that has magnitude (or length) and direction and can be added to other vectors according to vector algebra]

Leads are of two types:

1.       Bipolar leads: they limb leads and are used for comparison between two electrodes.

2.       Unipolar leads: they are Precordial leads and compared to a common lead (commonly the Wilson's central terminal)

The common lead, Wilson's central terminal V_W, is produced by averaging the measurements from the electrodes RA, LA, and LL to give an average potential across the body:

 

Leads are arranged in three sets:

1.       3 limb leads and 3 augmented limbs are arranged like spokes of a wheel in the coronal plane (vertical). They view the heart from a vertical plane.

2.       6 precordial leads that lie on the perpendicular transverse plane (horizontal). They view the heart from a horizontal plane.

Precordium is the region of the thorax immediately in front of the heart.

Limb leads: Leads I, II and III are called the limb leads. The electrodes that form these signals are located on the limbs—one on each arm and one on the left leg. The limb leads form the points of what is known as Einthoven's triangle.

Augmented limb leads: Leads aVR, aVL, and aVF are the augmented limb leads. They are derived from the same three electrodes as leads I, II, and III, but they use Goldberger's central terminal as their negative pole which is a combination of inputs from other two limb electrodes.

Precordial leads: The precordial leads lie in the transverse (horizontal) plane, perpendicular to the other six leads. The six precordial electrodes act as the positive poles for the six corresponding precordial leads: (V₁, V₂, V₃, V₄, V₅ and V₆). Wilson's central terminal is used as the negative pole.

The 12 leads in a 12-lead ECG are listed below: 

LEAD			VIEW OF HEART
Lead I			Lateral
Lead II			Inferior
Lead III			Inferior
aVR			None
aVL			Lateral
aVF			Inferior
V1			Septal
V2			Septal
V3			Anterior
V4			Anterior
V5			Lateral
V6			Lateral

Electrode name	Electrode placement
RA	On the right arm, avoiding thick muscle.
LA	On the left arm, avoiding thick muscle.
RL	On the right leg, lateral calf muscle.
LL	On the left leg, lateral calf muscle.
V1	In the fourth intercostal space (between ribs 4 and 5) just to the right of the sternum (breastbone).
V2	In the fourth intercostal space (between ribs 4 and 5) just to the left of the sternum.
V3	Between leads V2 and V4.
V4	5th Intercostal space at the midclavicular line
V5	Anterior axillary line at the same level as V4
V6	Midaxillary line at the same level as V4 and V5

Artefacts on ECG:

An ECG tracing is affected by patient’s motion. Some rhythmic motions (such as shivering or tremors) can create the illusion of arrhythmia. Artefacts are distorted signals caused by secondary internal or external sources, such as muscle movement or interference from an electrical device.

Accurately separating the ECG artefacts from the true ECG signal can have a significant impact on patient’s diagnosis, treatment, outcome of therapy and legal liabilities.

Reducing Artefacts while measuring an ECG:

Below is a list of guidelines that will help reduce artefacts when performing ECG’s.

Patient Positioning

• Place the patient in a supine or semi-Fowler’s position. If the patient cannot tolerate being flat, you can do the ECG in a more upright position.
• Instruct the patient to place their arms down by their side and to relax their shoulders.
• Make sure the patient’s legs are uncrossed.
• Move any electrical devices, such as cell phones, away from the patient as they may interfere with the machine.

Skin Preparation

• Dry the skin if it’s moist or diaphoretic.
• Shave any hair that interferes with electrode placement. This will ensure a better electrode contact with the skin.
• Rub an alcohol prep pad or benzoin tincture on the skin to remove any oils and help with electrode adhesion.

Electrode Application

• Check the electrodes to make sure the gel is still moist.
• Do not place the electrodes over bones.
• Do not place the electrodes over areas where there is a lot of muscle movement.

Electrocardiogram grid

ECG's are normally printed on a grid. The horizontal axis represents time and the vertical axis represents voltage. The standard values on this grid are shown in the image to the right:

• A small box is 1 mm x 1 mm big and represents 0.1 mV x 0.04 seconds.
• A large box is 5 mm x 5mm big and represents 0.5 mV x 0.2 seconds wide.

The "large" box is represented by a heavier line weight than the small boxes.