Solution:

ECG (Electrocardiography) is the electrical measurement of heartbeat activity. This continuous electrical activity—occurring an average 60 to 80 beats per minute and repeated some two and a half billion times over an average human lifespan—can be non-invasively detected and recorded by attaching leads and electrodes to the skin.

The flow of electricity associated with heart activity can then be accurately displayed as a waveform on a computer screen or a chart recorder.

The ECG waveform can be used to reliably determine the normality or abnormality of a heartbeat by analyzing various “components,” or noted electrical and mechanical events generated during each cardiac cycle.

How does ECG work?

In order to understand ECG, it’s helpful to have a basic understanding of heart activity.

The main function of the heart is to pump blood through two circuits:

1. Pulmonary circuit: through the lungs to oxygenate the blood and remove carbon dioxide; and

2. Systemic circuit: to deliver oxygen and nutrients to tissues and remove carbon dioxide. Because the heart moves blood through two separate circuits, it is sometimes described as a dual pump.

In order to beat, the heart needs three types of cells:

1. Rhythm generators, which produce an electrical signal (SA node or normal pacemaker);

2. Conductors to spread the pacemaker signal; and

3. Contractile cells (myocardium) to mechanically pump blood.

Leads:

The particular bipolar arrangement of two electrodes (one positive, one negative) with respect to a third electrode (the ground) is called a lead. The electrode positions for the different leads have been standardized.

Normal factors include body size (BSA) and distribution of body fat, heart size (ventricular mass,) position of the heart in the chest relative to lead locations, metabolic rate, and others.

The dominant ECG component in any normal standard lead record is the QRS complex.

For example, in a person who has a high diaphragm, the apex of the heart may be shifted slightly upward and to the person’s left.

This change in the position of the heart alters the “electrical picture” of ventricular depolarization seen by the Lead II electrodes, resulting in decreased positivity of the R wave and increased negativity of the S wave.

In other words, the positive amplitude of the R wave decreases and the negative amplitude of the S wave increases.