the electrocardiogram

Fast & Easy ECGs, 2EFast & Easy ECGs, 2E© 2013 The McGraw-Hill Companies, Inc. All rights reserved.

The Electrocardiogram

Fast & Easy ECGs, 2nd E – A Self-Paced Learning Program

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Electrocardiogram

• An electrocardiogram is a tracing of the heart’s electrical activity

• An electrocardiograph is the machine that produces it

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Electrocardiograph

• Also called the ECG machine, it detects heart’s electrical activity through electrodes positioned on patient’s skin

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ECG Machines

• A variety of ECG machines are available – Some monitor from one

to five different leads– Some are capable of

monitoring up to twelve or more leads simultaneously

– Some are used for continuous monitoring

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The Input• Lead wires and cables

transfer the ECG signal detected through the electrodes to the ECG machine– There may be 3, 4, or 5

lead wires for monitoring purposes and up to 10 lead wires for 12-lead ECGs

– Each lead wire has a labeled clip, snap, or pin-type connector on the distal end which attaches to the electrode

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Using Lead Wires and Cables• Keep the lead wires

separated from each other to avoid tangling

• Position them loosely across the patient so they do not pull on and lift the electrodes away from the skin

• Properly clean and decontaminate then after each use

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ECG Electrodes

• Are typically disposable • Consist of a: – wet or dry electrolyte gel

(which acts to assure good signal pick-up),

– metal snap or tab (where the ECG lead wire is attached),

– self-adhesive pad that holds the electrode to the skin

Metal snap

Self-adhesive pad

Electrolyte gel

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Placing ECG Electrodes• To achieve a noise-

free recording: – Clean the skin site to

remove dead skin cells and oils

– Clip dense hair present at the sites where the electrodes are to be placed

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Placing ECG Electrodes

• Eliminate muscle tension by placing the patient’s arms and legs in a comfortable position in which the extremities are resting on a supportive surface – If artifact is still present and the patient tolerates

it, try laying him or her in a flat position

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Placing Electrodes

• Avoid removing electrodes from their sealed protective envelope before use as the conductive gel may dry

• With snap-on lead wires, attach the electrode to the lead wire before placing the electrodes onto the patient’s skin

• With clip-on type lead wires, apply the connector to the metal snap of the electrode after the electrode has been placed

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Placing Electrodes

• Place the electrodes in the correct locations – Know where each

electrode is supposed to be positioned

– Look for a flat surface and sites over soft tissues

– Avoid areas where large bones are near the skin surface and areas where there are thick muscles or skin folds

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ECG Leads

• Are a combination of electrodes that form an imaginary line in the body along which the electrical signals, detectable during the time course of the heartbeat, are measured

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ECG Leads

• Each lead provides a different view of the heart: – Electrodes are placed on chest, arms and legs– Sites vary depending on which view of the heart's

electrical activity is being assessed

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ECG Leads

• Deflection of the waveforms seen on the ECG depend on where the positive electrode is placed in comparison to the direction of the electrical forces

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ECG Leads

• ECG leads are either bipolar or unipolar

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Bipolar Leads

• Record the flow of the electrical impulse between two (one is positive, the other is negative) selected electrodes

• Includes I, II and III

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Bipolar Leads

• Record difference in electrical potential between a positive and negative electrode

• Uses a third electrode called a ground

• Include leads I, II and III

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Unipolar Leads

• Use only one positive electrode and a reference point calculated by the ECG machine

• Includes leads aVR, aVL, aVF, and V1 through V6

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Planes of the Heart• Electrodes are

placed on the extremities and chest wall to view the heart’s electrical activity from the frontal and horizontal planes– Provides a cross-

sectional view of the heart

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Frontal Plane• Leads that view this

plane are called limb leads

• Include leads I, II, III, aVR, aVL and aVF

• Provide inferior, superior, and lateral views of heart

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Horizontal Plane• Leads that view this

plane are called precordial or chest leads

• Include leads V1, V2, V3, V4, V5, and V6

• Provide inferior, superior, and lateral views of heart

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Different Leads Uses

• A given lead may be used to highlight: – A specific part of ECG complex – Electrical events of a specific cardiac cycle – Conditions such as an enlargement of heart

muscle or presence of ischemia, injury and infarction

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Limb Lead Locations• RA electrode

– positioned anywhere on the right arm or below the right clavicle in the midclavicular line

• LA electrode– positioned anywhere on the left

arm or below the left clavicle in the midclavicular line

• LL electrode– positioned anywhere on the left

leg or left midclavicular line below the last palpable rib

• RL electrode– positioned on the right leg or

right midclavicular line below the last palpable rib

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Chest Leads

• Positioned in order across the chest

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Lead II

• ECG electrode polarity– Positive electrode – LL lead wire– Negative electrode – RA lead wire– Ground electrode – LA lead wire (and RL lead wire)

• Produces positively deflected waveforms

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The Output

• ECG machine translates the electrical impulses generated in the heart into wave-like signals that are recorded on paper or displayed on a monitor

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Oscilloscope

• ECG rhythms displayed on the oscilloscope are called dynamic ECGs

• Represent real time electrical activity or what is presently occurring in the heart

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Printouts

• ECGs printed on graph paper are called static ECGs

• Show what has already occurred • Much easier to analyze a static ECG to identify

abnormalities than it is to examine an image moving across a screen

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Printouts• As the stylus moves

across the moving ECG paper it inscribes waveforms representing the heart’s electrical activity

• Vertical lines on the ECG paper represent amplitude in electrical voltage while horizontal lines represent time or duration

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ECG Paper

• Each small square equals– 0.04 sec in duration and

0.1 mV in amplitude

• Five small squares (1 large box) equals– 0.20 seconds in duration

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Isoelectric Line• Is the flat line that

occurs when no electrical activity is occurring or impulses are too weak to be detected

• Used as a baseline to identify changing electrical movement

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Calibration or Registration Mark

• Helps ensure ECG machine is properly calibrated

• Serves as reference point on ECG tracing

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Artifact

• Markings on ECG tracing that are not a product of heart’s electrical activity– Patient movement is

among its many causes

– Can mimic life-threatening dysrhythmias

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Summary

• Graphic record or tracing is called an electrocardiogram while the machine that produces it is called the electrocardiograph

• Electrodes placed on patient’s skin detect heart’s electrical activity

• Abnormalities in cardiac rate and/or rhythm are called dysrhythmias

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Summary

• Each lead provides a different view of heart. • Impulses traveling toward a positive electrode

are recorded on ECG as upward deflections. • Impulses traveling away from a positive

electrode or toward a negative electrode are recorded as downward deflections.

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Summary

• Limb leads are produced by placing electrodes on right arm (RA), left arm (LA) and left leg (LL). – Include leads I, II, III, augmented vector right

(aVR), augmented vector left (aVL) and augmented vector foot (aVF).

• Precordial leads include leads V1, V2, V3, V4, V5, and V6.

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Summary

• ECG paper consists of horizontal and vertical lines that form a grid.

• Horizontal measurements used to determine heart rate and duration of various waveforms, segments and intervals.

• Vertically on ECG paper, distance between lines, or boxes, represents amplitude in millimeters (mm) or electrical voltage in millivolts (mV).

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Summary

• Calibration• Artifact is markings on ECG tracing that have

no relationship to electrical activity of the heart.

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the electrocardiogram

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