12 Chapter 1

Questions Referenced Frames Answers

4. How do you assess electrical activity in the heart? 5, 6, 7, 8, 10 Analyze the EKG.

5. Arrhythmias are manifestations of which type of

cardiac activity?

9, 10, 11, 12 electrical

6. What happens when the positive and negative electrical charges exchange places across the cell membrane

of a cardiac cell?

13, 14, 15, 16, 17 It initiates the flow of electrical

current.

7. Explain the polarized state. 14, 15, 16, 17, 19, 21 when electrical charges are balanced and in a state of readiness

for discharge

8. Explain depolarization. 16, 17, 20, 22 the discharge of electrical energy

that accompanies the transfer of

electrical charges across the cell

membrane

9. Is depolarization the same as contraction? 17 No, depolarization is an electrical phenomenon. Contraction is

mechanical and is expected to

follow depolarization.

10. What is repolarization? 18, 20, 21 the return of the electrical charges

to their original state of readiness

11. List the areas of the conduction system in the order in

which the impulses travel through the heart.

23, 24, 25, 26, 27, 29 1. SA Node

2. Intraatrial and Internodal

Pathways

3. AV Node

4. Bundle of His

5. Bundle Branches

6. Purkinje Fibers

12. Which site is normally the pacemaker of the heart,

and why?

24, 33, 34, 43, 44 The SA node, because it has the

fastest inherent rate.

13. Give the inherent rates for each of the following sites:

Sinus Node

AV Junction

Ventricles

30, 31, 32, 38, 39,

40, 41, 42

60–100 times per minute

40–60 times per minute

20–40 times per minute

14. What process is responsible for a site speeding up

and overriding a higher site, thus taking over as

pacemaker?

33, 34, 35, 45, 46 irritability

15. What mechanism is in play if a lower site takes over

responsibility for the pacemaking function following

failure of a higher site?

33, 36, 37, 47, 48 escape

16. Which nervous system has two branches that control

the activities of the heart?

49 autonomic

17. Name the two branches of the nervous system identified in the preceding question.

49, 52 sympathetic; parasympathetic

18. List three things that will happen to the heart if the

sympathetic branch is stimulated.

49, 50, 51, 53 increased rate, increased AV conduction, increased irritability

19. List three things that will happen to the heart if the

parasympathetic branch is stimulated.

49, 50, 51, 58 decreased rate, decreased AV

conduction, decreased irritability

Electrophysiology 13

Questions Referenced Frames Answers

20. What part of the heart does the sympathetic branch

innervate?

49, 54 the atria and ventricles

21. What part of the heart does the parasympathetic

branch innervate?

49, 54 only the atria

22. What happens if one branch is blocked? 50, 51, 55, 56, 57, 58 The influence of the opposing

branch will control the heart.

ALGrawany

14

Overview

IN THIS CHAPTER, you will learn how cardiac electrical activity is transferred to graph paper

so that it can be seen and analyzed for arrhythmia interpretation. You will learn about the equipment used for monitoring, and you will learn all the specifics of the graph paper upon which EKG

images are typically drawn. You will learn the difference between a single cardiac cycle and an

EKG rhythm strip. You will find out about the different components that make up a single cardiac

cycle on the EKG, and you will learn to identify each component and know what it suggests is

happening within the heart.

Introduction

1. In Chapter  1, you learned that arrhythmias are manifestations of the heart’s

 activity. And you learned that the study of arrhythmias is

called . To study arrhythmias, we have to transform the electrical activity into a format that can be seen.

electrical

electrocardiography

Waves and

Measurements

2

Waves and Measurements 15

Electrodes

2. The electrical patterns of the heart can be picked up from the surface of the skin by

attaching an electrode to the skin and connecting it to a machine that will display the

electrical activity on graph paper. An electrode is a small item attached to the patient’s

 and then connected by wire to a machine capable of inscribing

the patterns on graph .

3. The electrical activity is displayed best if you can ensure good contact between the

electrode and the skin. This can be done in several ways:

• By abrading the skin slightly

• By removing any obstacles, such as dirt or hair

• By using a contact medium, such as saline or a commercial gel

All of these measures are intended to improve between the

electrode and the skin.

4. An placed on the skin can pick up electrical activity from

within the heart and display it on graph paper using an EKG machine. To ensure a

good tracing, you must provide good contact between the and

the .

5. Contact between the skin and the electrode can be improved by lightly

 the skin, by wiping off excess , or

possibly by excess hair. An important way to ensure good

contact is to use some type of contact medium, such as or a

commercial .

6. When an EKG machine is turned on but isn’t yet connected to the patient’s electrodes, the writing point (stylus) of the machine will simply produce a straight line

on the paper. This line is called the isoelectric line because all of the electrical forces

are equal; no current is flowing. Once the machine is connected to the patient’s electrodes, the needle will move up or down on the paper (above or below the isoelectric

line) in response to the electrical forces it receives. If no current is flowing, or if the

forces balance each other out, the graph paper will show a .

If the machine receives a flow of electricity, the needle will move

 or in response to the current.

Rule of Electrical Flow

7. A very basic rule of electrocardiography refers to the flow of electricity through

the heart and out to the electrodes. This rule states that if the electricity flows toward

the positive electrode, the patterns produced on the graph paper will be upright. The

converse of this rule is also true: if the electricity flows away from the positive electrode (or toward the negative electrode), the pattern will be a downward deflection.

If the flow of electricity is toward the positive electrode, the machine will produce

an deflection on the graph paper (Figure 4).

8. Look at Figure 4. If the electrical flow is toward the negative electrode, would you

expect the graph paper to show a positive or a negative deflection?

9. If the graph paper shows a positive deflection, you would assume that the electrical

activity is flowing primarily toward the electrode.

skin

paper

contact

electrode

skin; electrode

abrading; dirt

shaving

saline

gel

straight line

up

down

upright

negative

positive ALGrawany

16 Chapter 2

10. If the deflection on the graph paper is negative, you would assume that the

electrical flow is toward the electrode and away from

the electrode.

11. Thus, we can determine the direction of electrical flow by the type of deflection

made on the EKG paper. But to draw any conclusions based on this information,

we must be sure that the electrodes are always in the same place on the patient so

that the information is not misleading. The placement of the electrodes on patients is

always to avoid confusion or misinterpretation of information.

Monitoring Leads

12. The positioning of electrodes for monitoring the EKG allows you to see a single view

of the heart’s electrical pattern. By rearranging electrodes, many such views are possible.

(This concept can be compared to a camera that can photograph the heart from many

angles, each one giving additional depth to the overall visualization of the heart itself.)

Each view of the heart is called a lead. Leads can be changed by a knob on the machine

that diverts the flow of electricity through different electrodes. For sophisticated EKG

interpretation, many leads are inspected to visualize the entire heart. However, for basic

arrhythmia interpretation, it is necessary to monitor only a single lead. A monitoring

lead shows only one of the heart’s electrical activity.

negative

positive

the same (standardized)

view

Figure 4 Rule of Electrical Flow

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Waves and Measurements 17

13. When monitoring a patient for patterns of electrical activity such as arrhythmias,

a lead is selected to give a clear picture of the basic wave forms. Single leads that give

good pictures of the basic waves are called monitoring leads because they are used

to patterns such as arrhythmias. The first widely used monitoring lead was Lead II, but now it is common to use other leads as well, especially

variations of the chest leads (such as MCL1). The modified chest leads often give a

better view of the heart’s atrial activity, which is sometimes needed to differentiate

complex arrhythmias. The examples in this book all happen to be Lead II. This does

not mean that Lead II is better than MCL1, nor does it mean that you must always

use Lead II for monitoring arrhythmias. The same patterns apply whether you view

them in Lead II, MCL1, or any other monitoring lead. Regardless of which lead is used,

the remain the same, so it doesn’t matter which lead you use

to learn basic arrhythmias. Just because the examples in this book are Lead II doesn’t

mean that it is the only monitoring lead, or even the best. You will encounter other

monitoring leads as you learn more about EKGs, but for now you can assume that the

information in this book refers to unless otherwise specified.

14. Figure 5 shows the placement of electrodes to monitor Lead II. Note that the positive electrode is at the apex of the heart, and the negative electrode is below the right

clavicle. The third electrode is a ground electrode and does not measure electrical flow

in this lead. Since the pacemaker is normally in the and the

electrical current flows toward the ventricles, the primary thrust of electrical flow in

the heart will be toward the positive electrode in Lead II. Thus, the primary deflections

in Lead II will be .

Graph Paper

15. All EKG interpretation relies on the use of standardized, uniform graph paper.

The size of the graph on the paper and the speed at which the paper travels through

the EKG machine are both kept constant; all EKG paper is the same, and all EKG

machines operate at the same speed. By keeping the paper and the speed standardized, we can look at the patterns created by an individual’s heart activity and compare them to what has been established as “normal” activity. If the graph paper was

not , we would not be able to compare one person’s EKG to

monitor

patterns

Lead II

SA node

upright

standardized

Figure 5 Electrode Placement for Monitoring Lead II

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