EngradeWikisCardiology EMT 1825 › The ECG

The ECG

Electrocardiogram

1. The Electrocardiogram records the electrical activity of a large mass of atrial and ventricular cells a specific waveforms and complexes
2. The electrical activity within the heart can be observed by means of electrodes connected by cables to an ECG machine
3. The basic function of the ECG is to detect current flow as measured on the patient's skin
4. ECG monitoring may be used for the following purposes:
a. monitor a patient's heart rate
b. evaluate the effects of disease or injury on heart function
c. evaluate pacemaker function
d. evaluate the response of medications (such as antiarrhythmics)
e. obtain a baseline recording before, during, and after a medical procedure
5. The ECG can provide information about the following:
a. the orientation of the heart in the chest
b. conduction disturbances
c. electrical effects of medications and electrolytes
d. the mass of cardiac muscle
e. the presence of ischemic damage
6. the ECG does not provide information about the mechanical (contractile) condition of the myocardium
7. To evaluate the effectiveness of the heart's mechanical activity, the patient's pulse and blood pressure are assessed

NOTES:
A standard ECG does not directly record the activity of the heart's electrical system.These structures are too small to produce detectable voltage on the body surface. What you see on the ECG is the activation and recovery of the working cells of the heart (Mirvis & Goldberger, 2004). It is possible to record signals from the heart's electrical system. However, this requires the use of specialized equipment, signal-averaging techniques, or the use of recording electrodes placed in the heart.

ELECTRODES

1. Electrode refers to the paper, plastic, or metal device that contains conductive media and is applied to the patient's skin
2. Electrodes are applied at specific locations on the patient's chest wall and extremities to view the heart's electrical activity from different angles and planes
3. Three types of electrodes used for surface (skin) electrocardiography are the metal disk, metal suction cup and the disposable disk
4. Disposable disk electrodes consist of an adhesive ring with a conductive substance in the center
5. The conductive media of the electrode conducts skin surface voltage changes through wires to a cardiac monitor.
6. To minimize distortion (artifact) be sure the conductive jelly in the center of the electrode is not dry, and avoid placing the electrodes directly over bony areas
7. One end of a monitoring cable is attached to the electrode and the other end to an ECG machine
8. The cable is a wire that attaches to the electrode and conducts current back to the cardiac monitor
9. Do not rely on the color-coding of ECG cables. Colors are not standard and often vary.

ECG Monitoring

STEP ONE

1. Explain the procedure to the patient while checking your equipment
2. Make sure there are no loose pins in the end of the ECG cable and no frayed or broken cable or lead wires
3. Make sure the monitor has an adequate paper supply
4. Connect the ECG cable to the machine
5. Connect the lead wires to the ECG cable (if not already connected)
6. Turn the power on to the monitor
7. Adjust the contrast on the screen if necessary

STEP TWO

1. Open a package of ECG electrodes
2. Make sure the electrode gel int he electrodes to be used is moist
3. Attach an electrode to each lead wire

STEP THREE

1. Prepare the patient's skin to minimize distortion of the ECG tracing
2. Do this by briskly rubbing the skin with a dry gauze pad
3. Do not use alcohol, tincture of benzoin, or antiperspirant when prepping the skin
4.If electrodes will be applied to the patient's chest instead of limbs, shave small amounts of chest hair if needed before applying electrodes to ensure good contact

STEP FOUR

1. One at a time, remove the backing from each electrode and apply them to the patient
2. Limb lead electrodes usually are placed on the wrists and ankles but may be positioned anywhere on the appropriate limb
3. To reduce muscle tension, make sure the patient's limbs are resting on a supportive surface
4. Do not apply electrodes over bony areas, broken skin, joints, skin creases, scar tissue, burns or rashes
5. Connect the lead wires to the electrodes

STEP FIVE

1. Coach patient to relax. Select desired lead on the cardiac monitor.
2. Adjust the ECG size if necessary.If the ECG size is set too low, the monitor will not detect QRS complexes and the heart rate display will be incorrect
3.Feel the patient's pulse and compare it with the heart rate indicator on the monitor
4. If not already preset, set the heart rate alarms on the monitor according to your agency's policy

STEP SIX

1. Select the print or record button to obtain a copy of the patient's ECG. Intepret the ECG rhythm
2. Assess the patient to find out how he is tolerating the rate and rhythm. Attach the rhythm strip to the prehopsital care report. Continue patient care

Leads

1. A lead is a record (tracing) of electrical activity between two electrodes
2.Each lead records the average current flow of a specific time in a portion of the heart
3. Leads allow viewing the heart's electrical activity in two different planes; frontal (coronal) and horizontal (transverse)
4. A 12-lead ECG provides views of the heart in both the frontal and horizontal planes and views the surfaces of the left ventricle from 12 different angles

NOTE: The word "lead" is used in two ways. Lead refers to both the actual tracing obtained and the position of the electrode.

Frontal Plane Leads

1. Frontal plan leads view the heart from the front of the body as if it were flat
2. Directions in the frontal plane are superior, inferior, right and left
3. Six leads view the heart in the frontal plane: three bipolar leads and three unipolar leads
4. A bipolar lead is an ECG lead that has a positive and negative electrode (polar refers to poles such as on a magnet=positive and negative. Bi=two, uni=one)
5. Each lead records the difference in electrical potential between two selected electrodes
6. Leads I, II, and III are called standard limb leads or bipolar leads
7. A lead that consists of a single positive electrode and a reference point is called a unipolar lead
8. These leads are also called unipolar limb leads or augmented limb leads
9. The reference point (with zero electrical potential) lies in the center of the heart's electrical field (left of the interventricular septum and below the AV junction)

Standard Limb Leads

1. Leads I,II,and III make up the standard limb leads

a. if an electrode is placed on the right arm, left arm and left leg, three leads are formed
b. Since each of these three leads has a distinct negative pole and a distinct positive pole, they are considered ______?_______.
c. The positive electrode is located at the left wrist in lead I, while leads II and III both have their positive electrode located at the left foot
d. the difference in electrical potential between the positive pole and its corresponding negative pole is measured by each lead

2. An imaginary line joining the positive and negative electrodes of a lead is called the axis of the lead
a. the axes of these three limb leads form a equilateral triangle with the heart at the center (Einthoven's triangle).
b. Although placement of the left leg electrode may appear to make the triangle out of balance, it is an equilateral triangle because all electrodes are about equidistant from the electrical field of the heart (Conover,1996)
c. Einthoven's triangle is a way of showing that the two arms and the left leg form apices of a triangle surrounding the heart
d. the two apices at the upper part of the triangle represent the points at which the two arms connect electrically with the fluids around the heart
e. the lower apex is the point at which the left leg connects with the fluids (Guyton,1996)

NOTE: Over the years, electrode placement for leads I, II, and III has been altered and moved to the patient's chest. This has been done to allow for patient movement and to minimize distortion on the ECG tracing. However, proper electrode positioning for these leads includes placement on the patient's extremities. Where the electrodes are placed on the extremity does not matter as long as bong areas are avoided.

Lead I

1. Lead I records the difference in electrical potential between the left arm (+) and right arm (-) electrode
2. The positive electrode is placed on the left arm and the negative electrode is placed on the right arm
3. The third electrode is a ground that minimizes electrical activity from other sources
4. Lead I views the lateral surface of the left ventricle

Lead II

1. Records difference in electrical potential between the left leg (+) and right arm(-) electrode
2. Positive electrode is placed on the left leg and the negative electrode is placed on the right arm
3. Lead II views the inferior surface of the left ventricle
4. Commonly used for cardiac monitoring

Lead III

1. Lead III records the difference in electrical potential between the left leg(+) and the left arm(-) electrodes
2. The positive electrode is placed on the left leg and the negative electrode is placed on the left arm
3. Lead III views the inferior surface of the left ventricle

(See chart and diagrams on pages 44 and 45)

Augmented Limb Leads



1. Leads aVR,aVL and aVF are augmented limb leads
2. The electrical potential produced by the augmented leads is normally relatively small
3. The ECG augments (magnifies) the amplitude of the electrical potentials detected at each extremity by about 50% over those recorded at the bipolar leads
4. The "a" in aVR,aVL and aVF refers to augmented
5. The "v" in these leads refers to voltage
6. The "r" refers to right arm, the "L" to left arm and the "F" to left foot
7. The position of the positive electrode corresponds to the last letter in each of these leads
8. Since they have only one true pole, they are referred to as unipolar leads
9. Theoretically, this makes the heart the negative electrode (Phalen & Aehlert, 2006)

Lead aVR

1. Lead aVR views the heart from the right shoulder (the positive electrode) and views the base of the heart (primarily the atria and the great vessels)
2. This lead does not view any wall of the heart

Lead aVL

1. Lead aVL views the heart from the left shoulder (positive electrode) and is oriented to the lateral wall of the left ventricle

Lead aVF

1. Lead aVF views the heart from the left foot (leg) (positive electrode) and views the inferior surface of the left ventricle

(See chart on page 46 in textbook on Augmented leads)

Horizontal Plane Leads



1. Horizontal plane leads view the heart as if the body were sliced in half horizontally
2. Directions in the horizontal plane are anterior, posterior, right and left
3. Six chest (precordial or "V") leads view the heart in the horizontal plane
4. Allows a view of the front and left side of the heart
5. The chest leads are identified as V1, V2, V3, V4, V5and V6
6. Each electrode placed in a "V" position is a positive electrode
7. The negative electrode is found at the electrical center of the chest
8.Thus the chest leads are also unipolar leads

(See Textbook on page 47 for chart explaining placement of each lead)

Other Lead Placement

Right Chest Leads

1. Other chest leads that are not part of a standard 12-lead ECG may be used to view specific surfaces of the heart
2. When a right ventricular myocardial infarction is suspected, right chest leads are used
3. If time does not permit obtaining all of the right chest leads, the lead of choice is V4R

(See page 50 chart for correct placement of Right Chest Leads)

Posterior Chest Leads

1. On a standard 12-lead ECG, no leads look directly at the posterior surface of the heart. Additional chest leads may be used for this purpose
2. The leads are placed further left and toward the back
3. All of the leads are placed on the same horizontal line as V4 to V6
4. Lead V7 is placed at the posterior axillary line
5. Lead v8 is placed at the angle of the scapula (posterior scapular line) and lead V9 is placed over the left border of the spine

(See skill 2-4 in textbook for the placement of these leads)

Modified Chest Leads

1. The modified chest leads (MCL) are bipolar chest leads that are variations of the unipolar chest leads
2. Each modified chest lead consists of a positive and negative electrode applied to a specific location on the chest
3. Accurate placement of the positive electrode is important
4. The modified chest leads are useful in detecting bundle branch blocks, differentiating right and left premature beats, and differentiating supraventricular tachycardia from ventricular tachycardia

MCL1

1. Lead MCL1 is a variation of the chest lead V1 and views the ventricular septum
2. The negative electrode is placed below the left clavicle toward the left shoulder, and the positive electrode is placed to the right of the sternum in the fourth intercostal space
3. In this lead the positive electrode is in a position to the right of the left ventricle
4. Because the primary wave of depolarization is directed toward the left ventricle, the QRS complex recorded in this lead will normally appear negative

MCL6

1. Lead MCL6 is a variation of the chest lead V6 and views the low lateral wall of the left ventricle
2. The negative electrode is placed below the left clavicle toward the left shoulder and the positive electrode is placed at the fifth intercostal space, left midaxillary line

What Each Lead

1. Think of the positive eletrode as an eye looking in at the heart
2 The part of the heart that each lead "sees" is determined by two factors
3. The first factor is the dominance of the left ventricle on the ECG and the second is the position of the positie electrode on the body.
a. Because the ECG does not directly measure the heart's electrial activity, it does not "see" all of the current flowing through the heart
b. What the ECG does see is the net result of countless individual currents competing in a tug-of-war.
c. For example, the QRS complex, which represents ventricular depolarization, is not a display of all the electrical activity occuring in the right and left ventricles. It is the net result of a tug-of-war produced by the many individual currents in both the right and left ventricles.
d. Since the left ventricle is much larger than the right, the left overpowers it.
e. What is seen in the qRS complex is the remaining electrical activity of the left ventricle, i.e. the portion not used to cancel out the right ventricle
f. Therefore, in a normally conducted beat, the QRS complex represents the electrical activity occurring in the left ventricle (Pjhalen &Aehlert, 2006)

4. the second factor, position of the positive electrode on the body, determines which portion of the left ventricle is seen by each lead
5. You can commit the view of each lead to memory, or you can reason it easily by remembering where the positive electrode is located.

Leads II,III, and aVF

1. Positive electrode on left leg
2. Each lead "sees" the inferior wall of the left ventricle

Leads I and aVL

1. Positive electrode on left arm
2. Each lead "sees" the lateral wall of the left ventricle

Leads V5and V6

1. Positive electrode on axillary area of the left chest
2. Each lead "sees" the lateral wall of the left ventricle

Leads V3 and V4


1. Leads V3 and V4 are positioned in the area of the anterior chest
2. From this perspective, these leads "see" the anterior wall of the left ventricle

Leads v1 and V2

The septal wall is "seen" by leads V1 and V2and which are positioned next to the sternum
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