Electrocardiogram (ECG)

An ECG is a series of waves and deflections recording the heart’s electrical activity from a certain “view.”
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■ Many views, each called a lead, monitor voltage changes

between electrodes placed in different positions on the body.

■ Leads I, II, and III are bipolar leads, which consist of two

electrodes of opposite polarity (positive and negative). The

third (ground) electrode minimizes electrical activity from

other sources.

■ Leads aVR, aVL, and aVF are unipolar leads and consist of a

single positive electrode and a reference point (with zero

electrical potential) that lies in the center of the heart’s

electrical field.

■ Leads V1–V6 are unipolar leads and consist of a single positive

electrode with a negative reference point found at the

electrical center of the heart.

■ Voltage changes are amplified and visually displayed on an

oscilloscope and graph paper.

■ An ECG tracing looks different in each lead because the

recorded angle of electrical activity changes with each lead.

■ Several different angles allow a more accurate perspective

than a single one would.

■ The ECG machine can be adjusted to make any skin electrode

positive or negative. The polarity depends on which lead the

machine is recording.

■ A cable attached to the patient is divided into several

different-colored wires: three, four, or five for monitoring

purposes, or ten for a 12-lead ECG.

■ Incorrect placement of electrodes may turn a normal ECG

tracing into an abnormal one.

♥ Clinical Tip: Patients should be treated according to their

symptoms, not merely their ECG.

♥ Clinical Tip: To obtain a 12-lead ECG, four wires are attached

to each limb and six wires are attached at different locations on

the chest. The total of ten wires provides twelve views (12

leads).

Placement:

Chest Leads

Components of ECG tracing 

Normal Sinus Rhythm

SINUS TACHYCARDIA

SINUS BRADYCARDIA

PREMATURE ATRIAL CONTRACTION

ATRIAL FLUTTER

VENTRICULAR TACHYCARDIA

ATRIAL FIBRILLATION

PREMATURE VENTRICULAR CONTRACTION