Module title = Tutorial: Basic Electric Stuff

Lesson title = ECG Deflections

This is lesson 2 of 7 in this module

We understand that each cell depolarizes during a heartbeat and that this depolarization changes the voltage of the cell. We also understand that this voltage change gets transmitted from cell to cell. This voltage change in DEpolarization is positive. When the cells REpolarize, the voltage change is negative.

Let's begin our discussion with only thinking about a positive electric charge.

As the cells next to each other depolarize, the electrical charge "moves" across the heart. This movement is very important because when we measure the electrical signal of the heart from the electrodes on the skin, the electrodes can only detect charges that are moving.

An electrode is a conducting wire that is placed on the skin. It can detect changes in voltage. 2 electrodes make an ECG lead. The voltage in one electrode is compared to the voltage in the other electrode. By convention, one of the electrodes is designated positive and the other is designated negative. The electrodes can only detect a change in voltage when the electrical charge in the heart is moving toward or away from one of the electrodes.

There are some very important principles:

If a positive charge is moving toward the positive electrode, an upward deflection will result.
If a positive charge is moving away from the positive electrode, a downward deflection will result.
If a charge is not moving no deflection will result.
If a charge is moving toward but at an angle to the positive electrode, an upward but smaller deflection will result (the opposite is true for away and downward).

When we observe the movement of a negative charge (which is what happens in REpolarization, the opposite deflection will occur. For example, if a negative charge is moving toward the positive electrode, a downward deflection will result.

Examples using a circle (instead of a heart) demonstrate these principles. The 2 black dots are the 2 ECG electrodes which together make up one ECG lead. The electrode with the + sign is the "positive" electrode and by convention, an electrical charge travelling toward that electrode will create an upward deflection.

For those perfectionists out there ... with a perfectly perpendicular movement of charge that is travelling exactly 90 degrees to the leads, and is perfectly centered between the 2 leads, in a perfectly symmetical object (like a circle), there would actually be no deflections on the ECG. In real life however, this never happens and what we see are an upgoing deflection and downgoing deflection that are both very small and equal in size to each other.

Angled depolarization results in a lower amplitude deflection.

Lesson 2 of 7

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Module title = Tutorial: Basic Electric Stuff

Lesson title = ECG Deflections