The Electrocardiogram

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

Electrocardiogram • An electrocardiogram is a tracing of the heart’s electrical activity • An electrocardiograph is the machine that produces it

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

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 I

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 I

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 I

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 Self-adhesive pad Electrolyte gel Metal snap I

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 I

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

Placing Electrodes • Avoid removing electrodes from their sealed protective envelope before use as the conductive gel may dry • With snap-onlead 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 I

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

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 I

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

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 I

ECG Leads • ECG leads are either bipolar or unipolar

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

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 I

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

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 I

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

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

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 I

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

Chest Leads • Positioned in order across the chest

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

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

Oscilloscope • ECG rhythms displayed on the oscilloscope are called dynamic ECGs • Represent real time electrical activity or what is presently occurring in the heart

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 I

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 I

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 I

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

Calibration or Registration Mark • Helps ensure ECG machine is properly calibrated • Serves as reference point on ECG tracing I

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 I

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

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.

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.

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).

Summary • Calibration • Artifact is markings on ECG tracing that have no relationship to electrical activity of the heart.

The Electrocardiogram