2. Physiology Review of the Heart’s Conduction System

Home » MEDCLIN530 » EKG » 12-Lead EKG Confidence » I. Overview of Heart Function and the EKG » 2. Physiology Review of the Heart’s Conduction System
Headshot of Chris Anderson, MD · Clinical Education Director, Pediatrics
Chris Anderson
MD · Clinical Education Director, Pediatrics
envelope icon
Headshot of Kevin Hodges, Vice Chair, Emergency Medicine
Kevin Hodges
Vice Chair, Emergency Medicine
envelope icon phone icon
Table of Contents

Identify and describe the following:

  • The six limb leads
  • The six precordial leads
  • The time lines on EKG paper
  • A 0.04 second time interval
  • A 0.20 second time interval
  • The EKG baseline
  • A positive wave
  • A negative wave
  • Measurement of wave amplitude
  • Standardization of the EKG

In 1913, Wilhelm Einthoven contributed significantly to the study of the heart by inventing the electrocardiogram (EKG). Einthoven attached wires or electrodes to the right arm, left arm, and left leg. This formed a theoretical triangle. When the electrodes were connected to a galvanometer, they measured the electrical activity generated within the heart. This activity, which was then recorded on paper, was representative of individual heartbeats.

Modern EKG machines inscribe 12 leads (or views) from differing combinations of the four limb electrodes and six chest wall electrodes.

A total of 12 leads, or views, are represented on the modern EKG. The 12 leads are
consistently arranged in a standard pattern. The first six leads represent the frontal plane (or view) of the heart. They are called the limb leads and are named I, II, III, AVR, AVL, and AVL. The next six leads represent the horizontal plane (or view) of the heart. They are called precordial (in front of the heart) leads, and are named V1, V2, V3, V4, V5, and V6.


The EKG is recorded on special standardized paper that scrolls out of the machine at a specific and controlled speed. Each large box is 5 mm wide and represents 0.20 seconds. Each large box is equivalent to 5 smaller 1-mm boxes, each representing 0.04 seconds. Measuring the width of any wave or interval from left to right, using the boxes as a scale, determines the duration of that wave in seconds. The normal EKG recording contains a P wave, a QRS complex, and a T wave.

The baseline on a 12-lead EKG is an imaginary line that connects the end of the T wave to the beginning of the P wave. All measurements of other waves are made relative to the baseline.

How to measure waves on the ekg

Each little box is 1 mm tall (vertically). A wave that goes upward from the baseline is said to be positive. A wave that goes downward from the baseline is said to be negative. Measuring the distance of a positive wave’s peak from the baseline gives the amplitude (height) of the wave in millimeters. Measuring the distance of a negative wave’s lowest point from the baseline gives the amplitude (depth) of the wave in millimeters.

Standardization

To ensure that the EKG correctly measures and records the amplitude of waves above and below the baseline, a standardized voltage is inscribed on every EKG, usually at the right side of the EKG. It should measure exactly 10 little boxes in height.