ECG Basics[1]

8/8/2019 ECG Basics[1]

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Clinical Support

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ELECTROCARDIOGRAPHY

Introduction

This article provides a basic introduction to the physiology of the human heart andthe clinical information provided by electrocardiography (ECG), with reference toECG monitoring with a Propaq vital signs monitor.

For more information about the use of the Propaq monitor, refer to the Propaq Directions For Use .

Monitoring HR/PR with the Propaq Monitor

When monitoring a patient using ECG leads, SpO2, and/or CO2

, the rate that isdisplayed is a true heart rate. If monitoring a patient using NIBP only, what is actuallybeing displayed is the patient’s pulse rate. This may be important when assessingyour patient’s cardiac status because a patient’s heart rate and pulse rate may vary ifthere is any cardiac compromise.

On the Propaq monitor you can set the HR/PR tone loudness to LOW, MEDIUM,HIGH, or OFF. This does not affect the tone of the alarm if a patient exceeds analarm limit setting.

Anatomical Structure of the Heart

The main function of the heart is to pump blood throughout the body to deliver theoxygen and nutrient demands of the body’s tissues as well as to remove carbondioxide (a byproduct of metabolism).

• The heart is approximately the size of a clenched fist.

• The heart is positioned in the mediastinum, near the midline.

• The heart is rotated and positioned on its side.



Welch Allyn Protocol Clinical Support ECG

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• 2/3 of the heart is on the left side of the chest.

• The base of the heart faces up and to the right. The apex faces down, out,and to the left. The apex actually comes into contact with the chest wall at the5th intercostal space in the mid-clavicular line. This is the PMI or Point ofMaximum Intensity. It’s easiest to hear the heart at this area.

• Of course, people are of all different shapes and sizes. The position of theheart in the chest will vary slightly with age, weight, and physical conditions.

Four Chambers of the Heart

There are four chambers of the heart – the right atrium and right ventricle, and the leftatrium and left ventricle. The wall of the left ventricle is quite a bit thicker than that ofthe right ventricle. Actually, the wall of the right atrium is approximately 3-5 mm thick,and the right ventricle is about 2-6 mm thick. The wall of the left atrium is also about 2-6 mm, slightly thicker than the wall of the right atrium, and the left ventricle is thelargest muscle mass of all at 13-15 mm thick. The wall thickness directly affects the

pressure in each of the chambers of the heart.The function of the right side of the heart is to deliver deoxygenated blood from thebody to the lungs. The function of the left side of the heart is to deliver oxygenatedblood from the lungs to the body.

Systole and Diastole

The two phases of the cardiac cycle are Systole and Diastole. Both the right and leftventricles go through each phase at the same time.

In Systole, the ventricles are full of blood and begin to contract. The mitral and

tricuspid valves (between the atria and ventricles) close. Blood is ejected throughthe pulmonic and aortic valves out to the lungs (RV) and the body (LV). The aorticand pulmonic valves then close.




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During Diastole, blood flows into theatria and then through the now openmitral and tricuspid valves into theventricles. The ventricles refill, and thecycle repeats.

NOTE Atrial systole occurs during ventricular diastole, and atrial diastole during ventricular systole.

Cardiac Conduction System

In order for the heart muscle to contract,an electrical impulse is necessary. Theelectrical conduction system of the heartis known as the Cardiac ConductionSystem.

SA Node

The SA node is often referred to as the“Pacemaker” of the heart. In a normalheart, the SA node generates theelectrical impulse and “sets the pace” ofthe heart.

The intrinsic rate of a rhythm that begins at the SA node is 60 to 100 beats perminute. Once generated, the impulse spreads out along tiny nerve fibers called“internodal tracts” and stimulates the atrial muscle.

AV Node

The AV node is located on the floor of the Right Atrium. It can be thought of as a“gateway” to the ventricles. The AV node delays the electrical impulse just longenough to allow the atria to contract and blood to enter the ventricles.

The intrinsic rate of the AV node is slower than that of the SA node, approximately40 – 60 bpm.




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Bundle of His

The Bundle of His is a thick bundle of nerve fibers that carries the electrical impulsevery rapidly down the interventricular septum from the AV node. The bundlebranches out to the right and left, terminating in tiny fibers called Purkinje fibers. Thesefibers bring the electrical impulse to the individual heart muscle cells, leading toventricular stimulation or depolarization. Depolarization can be simply thought of as

the electrical stimulation of the heart muscle cells.

The resting heart is polarized. Charges are balanced in and out of the cell, and noelectricity is flowing.

The cell at rest is negatively charged. When a stimulus begins, positive ions enterthe cell, changing the charge to positive.

This “depolarization” spreads from cell to cell, causing the heart muscle fibers toshorten. The shortening of the heart muscle fibers causes contraction of the heartmuscle as a whole.

Repolarization is the return of the heart muscle cells to the polarized, or resting state.The positive ions are pumped out of the cells, the cells return to their normal shape,and the heart muscle relaxes.

ECG Tracings

Each wave and interval appears on the ECG display or printout as a result of aparticular electricalfunction of the heart.

The isoelectric line,also referred to asthe “baseline”, issimply the point fromwhich each of thewaves of the ECGdeparts.




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P Wave

The P wave is the wave of atrial depolarization. As the atria depolarize, the P waveshows up on the ECG.

In a patient with normal

physiology and with theSA node acting as thepacemaker of the heart,the P wave has thesecharacteristics:

• Smooth androunded

• <= 3 mm tall

• upright in leads

I, II, aVFThe P wave at right fulfillsthe criteria.

PR Interval

The next component is the PR interval, which includes the P wave and the space upuntil the beginning of the QRS complex. The PR interval represents the time it takesthe electrical impulse to travel from the SA node to the ventricles. By the end of thePR interval, the atria are beginning to repolarize and the ventricles are beginning todepolarize or become electrically stimulated.

The PR interval is measured from the beginning of the P wave to the beginning ofthe QRS complex. The normal PR interval duration is 0.12 to 0.20 seconds or 120

– 200 ms.

QRS Complex

The QRS complex is the wave of ventricular depolarization. We generally call thewave of ventricular depolarization a “QRS complex” even if not all of thecomponents (the Q, the R, and the S) are present. Technically, the Q wave is thefirst downward stroke. An R wave is the first positive stroke, and an S wave is a

negative stroke that follows a positive upstroke.

The QRS should be at least 5 mm and not more than 20 mm tall. The width of theQRS is measured from the beginning of the Q wave to the end of the S. NormalQRS duration is 0.06 to 0.10 seconds, and does not exceed 0.12 seconds.

As discussed earlier, the left ventricular muscle is quite a bit larger than that of the rightventricle. Because there are more muscle cells to depolarize, the electrical charge of




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the left ventricle is significantly greater than that of the right. Therefore, most of whatwe see on ECG as the QRS complex is LEFT ventricular depolarization.

ST Segment

The next segment is the ST segment. The ST segment begins at the J point. The J

point is the point at which the QRS complex ends and the ST segment begins.Measure the ST segment duration from the J point up to the beginning of the Twave.

The ST segment indicates the period of time between the end of ventriculardepolarization and the beginning of ventricular repolarization. Generally the STsegment is ISOELECTRIC, or on the “baseline”. A deviation of the ST segmentfrom the baseline (either a depression or elevation) may be indicative of myocardialischemia.

T Wave

The T wave is the wave of ventricular repolarization. The T wave usually deflects inthe same direction as the QRS complex, and should be smooth and rounded.

The period from the beginning of the T wave to nearly the end is called the “relativerefractory period”. At this time, the ventricles are vulnerable. A stronger than normalstimulus could trigger depolarization. If an R wave (ventricular depolarization) shouldoccur during this time, a potentially fatal arrhythmia could result.

Summary and Review

• The P wave is the wave of atrial depolarization. The PR interval signifies theamount of time it takes the electrical impulse to travel from the SA node to the

ventricles.

• The QRS complex begins to show up as ventricular depolarization begins andthe atria repolarize. The QRS is complete when the ventricles are fullydepolarized.

• The ST segment occurs on ECG between the end of ventricular depolarizationand the beginning of ventricular repolarization.

• The T wave begins as the ventricles start to repolarize and is finally completewhen the ventricles have returned to their resting state.




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Electrocardiogram

The electrocardiogram, also called an ECG or EKG, is a graph of the electrical activityof the heart over time. We have been discussing the waveforms you find plotted on

the ECG.

When reading the ECG, be aware of a few basic principles:

• The standard paper speed is 25mm/ second. This is also called Sweep Speed.

• The vertical lines on the ECG measure time.

• The space between two small vertical lines (one small box) is 0.04 seconds or 4ms. The space between two larger lines (5 small boxes or 1 large box) is 0.20seconds or 20 ms.

• The horizontal lines on the ECG measure voltage.

• The space between two small horizontal lines (one small box) is 1 mm or 0.1mV.

• The space between two larger horizontal lines (5 small boxes) is 5 mm or 0.5mV.

Perhaps the best way to measure time and voltage on the ECG is with calipers, butyou can also use a ruler or a piece of paper.

We can tell something about the direction the electricity in the heart is flowing bylooking at the ECG. If the electrical flow of the heart is TOWARDS a positive

electrode, we will see a positive deflection on the ECG. If the electrical flow isAWAY FROM the positive electrode, then the wave produced on the ECG will benegative.




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Remember thatdepolarization is a wave ofPOSITIVE charges flowingthrough the heart muscle. If

that wave of depolarization isflowing towards a POSITIVEelectrode, then the result willbe a POSITIVE upstroke onthe ECG.




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Understanding Leads

Everyone has heard of 3-lead or 5-lead ECG monitoring, or ordered a 12-leadECG. But what exactly is a LEAD?

Leads, by definition, are positive and negative electrodes attached to a recorder andused to detect electrical activity of the heart.

A simple way to think of a Lead is as a picture of the electrical activity of the heart.Imagine that a camera is positioned at the location of the positive electrode in each ofthe leads we will discuss. From each individual angle, a unique view of the heart canbe captured.

3 Lead ECG

The 3 lead ECG is one of the most common. Leads I, II, and III are also known asthe Limb Leads. To obtain a 3-lead ECG, electrodes are placed on the Right andLeft arms and on the left leg.

Lead I looks from RA to LA

Lead II looks from RA to LL

Lead III looks from LA to LL




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Augmented Leads

The Augmented leads are the “Other Limb Leads”. With the augmented leads, thetwo negative electrodes are combined to form a central negative reference point.These leads offer a “mixed view”, or a single view between two of the viewsoffered by the standard limb leads.

For example, in lead AVF, our positive electrode is on the LL, and the centralnegative reference point is between the LA and RA leads. This offers a viewbetween Leads II and III.

The imaginary camera is still at the Left Leg, but it’s positioned at a different angle.Together with the standard Limb Leads, there are now six intersecting views of the

heart.

V Leads

The V leads are also called theChest leads.

These six leads offerHORIZONTAL views of the heart.This time, the camera is positionedon the chest wall, taking picturesthrough the chest and the heart itself.

Think of the V leads as spokes of aHORIZONTAL wheel, with the AVnode being the hub of the wheel.

The negative end of each lead is apoint somewhere on the patient’sback.




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V1 & V2 are positioned over theRight Ventricle, V3 & V4 over theseptum between the ventricles, andV5 and V6 over the left ventricle.

Remember the rule of electrical flow. If

the electrical flow of the heart isTOWARDS a positive electrode,there is a positive deflection on theECG.

Remember that the electrical charge ofthe Left Ventricle is greater than that ofthe right. Therefore, there is a mostlynegative deflection in V1 (over theRight Ventricle) – most of theelectricity is going down and to the left,away from V1.

As the leads get closer to the LeftVentricle, the ECG of a normal heart ina normal rhythm will demonstrate “Rwave progression” or progressivelymore positive QRS complexes.

5 Lead ECG

With 5-lead monitoring, common inmany hospitals, only one V lead isused.

The most leads used for routine ECGmonitoring are 5 leads. The commonplacement of the ECG leads is asfollows.

Proper placement of the V leads isvery important if the V leads are goingto be used for diagnostic purposes.Proper chest wall placement of the Vleads are shown below.




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Place the V1 lead just to theright of the sternum in the 4th

intercostal space.

Place V2 just to the LEFT ofthe sternum in the 4th

intercostal space.

Place V4 in the leftmidclavicular line in the 5th

intercostal space.

Place V3 between V2 andV4.

Place V5 in the anterioraxillary line in the 5th

intercostal space.

Place V6 in the mid-axillaryline in the 5th intercostal space.

ECG Lead Skin Preparation

Good lead preparation is very important also. The ECG can tell us many things, butartifact can hinder the accuracy of the ECG. To avoid artifact, be sure to prepare theskin properly.

Before applying electrodes, skin should be free of hair, clean, and dry. For bestresults, attach electrodes to the leads before placing the leads on the patient.Electrodes should have plenty of gel and should be replaced if they become soiled

or wet.

Place electrodes as close as possible to the recommended areas, but make aneffort to keep them out of the way of areas of large muscle movement. Flat bonysurfaces are the best location.




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Rhythm Analysis

Normal sinus rhythm is the rhythm that most of you are probably in right now. Therhythm is regular.

Sinus tachycardia is really a fast normal sinus rhythm. The SA node still generates theimpulse, but it will be generated at a higher rate.

The rhythm should still be regular.

However, the very high rate can cause strain on the heart, especially in a patient withCAD. Possible causes include caffeine, stress, nicotine, alcohol, pain, fever,congestive heart failure, hypovolemia, hyperthyroidism, dig toxicity, and somemedications.




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Sinus bradycardia is a slow sinus rhythm. Again, this rhythm is initiated by the SAnode. The rhythm is regular.

Sinus bradycardia can be normal in some people, especially the very athletic. It can

also be caused by sedation, increased intracranial pressure, medications such asbeta blockers, vagal stimulation as with straining or vomiting, hypothyroidism, andhyperkalemia.

Treatment is based on symptoms. Some of the symptoms may include decreasedurine output, dizziness, weakness, and hypotension. Treatment may includeadministration of Atropine or Dopamine, or placement of an external pacemaker.

NOTE For additional lessons on ECG Rhythm analysis, contact Welch Allyn Protocol Clinical Support to find out about our AACN-approved CEU offering: ECG Interpretation and Basic Arrhythmia Analysis.




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References

1. Barash, P., Cullen, B., and Stoelting, R. (1992). Clinical Anesthesia. 2nd Edition.J.B. Lippincott Company. Philadelphia, PA.

2. Guyton, A. (1991). Textbook of Medical Physiology. W.B. Saunders Co.Philadelphia, PA.

3. Loeb, S. (1993). Monitoring Clinical Functions. Advanced Skills. SpringhouseCorp. Springhouse, Pennsylvania.

4. Thomas, C. Taber’s Cyclopedic Medical Dictionary. 16th Edition. F. A. Davis Co.Philadelphia, PA. 1989.

5. Dubin D. Rapid Interpretation of EKG’s. Tampa, FL: Cover; 1993.

6. Flynn JM, Bruce NP. Introduction to Critical Care Skills. St. Louis, MO: Mosby;

1993.

7. Grauer K, Cavallaro D. Arrhythmia Interpretation: ACLS Preparation and ClinicalApproach. St. Louis, MO: Mosby; 1997.

8. Lipman B, Cascio T. ECG Assessment and Interpretation. Philadelphia, PA:FA Davis Co.; 1994.

9. Huszar RJ. Basic Dysrhythmias: Interpretation and Management. St.Louis, MO:Mosby; 1994.

10. Marler CA. Introduction to ECG. Dallas, TX: Parkland Health & HospitalSystems; 1993.

11. Marriott H, Conover M. Advanced Concepts in Arrhythmia. St. Louis, MO:Mosby, 1983.

12. Metzgar ED, Polfus PM. A Study and Learning Tool: Health Assessment (Second Edition). Springhouse PA: Springhouse Corporation; 1994.

13. Smith- Huddleston S, Ferguson SG. Critical Care and Emergency Nursing: 2 nd

Edition. Springhouse, PA: Springhouse Corporation; 1994.

14. Cummins, RO. Textbook of Advanced Cardiac Life Support. American HeartAssociation, 1994.

15. Walraven G. Basic Arrhythmias (3rd. Edition). Englewood Cliffs, NY: Brady;1992.

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