thaler chapter 1

1   The Basics

18

Time and Voltage

The waves that appear on an EKG primarily reflect the electrical activity of the myocardial cells , which make up the vast bulk of the heart. Pacemaker activity and transmission by the conducting system are generally not seen on the EKG; these events simply do not generate sufficient voltage to be recorded by surface electrodes. The waves produced by myocardial depolarization and repolarization are recorded on EKG paper and, like any wave, have three chief characteristics: 1. Duration , measured in fractions of a second 2. Amplitude , measured in millivolts (mV) 3. Configuration , a more subjective criterion referring to the shape and appearance of a wave

Amplitude (millivolts)

Duration (fractions of a second)

A typical wave that might be seen on any EKG. It is two large squares (or 10 small squares) in amplitude, three large squares (or 15 small squares) in duration, and slightly asymmetric in configuration.

EKG Paper EKG paper is a long, continuous roll of graph paper, usually pink (but any color will do), with light and dark lines running vertically and horizontally. The light lines circumscribe small squares of 1 × 1 mm; the dark lines delineate large squares of 5 × 5 mm. The horizontal axis measures time. The distance across one small square represents 0.04 seconds. The distance across one large square is five times greater, or 0.2 seconds.