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18 Chapter 2

any other EKG, nor would we be able to compare several EKGs taken on one person at

different times. Similarly, if all EKG machines ran at different ,

we would not have a constant “norm” for comparing individual EKGs.

16. Since all graph paper has markings, we must learn what

these markings mean so that we will be able to interpret the EKG tracings that are

superimposed on the graph paper. Look at the sample graph paper shown in Figure 6.

You will notice that there are lines going up and down (vertical) and lines going across

(horizontal). Also notice that every fifth line is heavier than the other lighter lines. How

many light lines are there between two heavy ones?

17. The lines on the graph paper can help determine both the direction and the magnitude of deflections. When all electrical forces are equal, there is neither an upright nor a

downward deflection; an isoelectric line is created. If the electrical force is toward the positive electrode, the stylus will draw an wave. If the force travels primarily toward the negative electrode, the wave will be .

If no current is present, or if positive and negative forces are equal, the graph paper will

show a line, called an isoelectric line.

Voltage Measurements

18. It is the strength of the current, or its voltage, that will determine the magnitude of

the deflection. If it is a very strong positive wave, it will create a high spike above the

isoelectric line. If it is a very weak positive charge, the deflection will go only slightly

above the isoelectric line in response to the amplitude of the charge. Therefore, the

height of the deflection will indicate the of the electrical charge

that produced the deflection. The same principle holds for negative deflections: the

stronger the charge, the deeper the wave will go below the isoelectric line.

19. Since voltage produces either an upright or a downward deflection on the EKG,

the magnitude of the current can be measured by comparing the height of the spike

against the horizontal lines on the graph paper (Figure 7). Voltage can be measured

quantitatively (in millivolts), but you need not concern yourself with these figures for basic arrhythmia interpretation. On the graph paper, the horizontal lines

measure .

speeds

standardized

four

upright

downward (inverted)

straight

voltage (or amplitude)

voltage

Figure 6 Sample EKG Graph Paper

The three vertical lines in the upper margin are measures of time standard to all EKG graph paper. The distance between

two “tic” marks is 3 seconds; thus, this strip measures 6 seconds in duration.

Waves and Measurements 19

Time Measurements

20. The second, and more important, thing that the graph paper can provide is a determination of time. The vertical lines can tell you just how much time it took for the

electrical current within the heart to travel from one area to another. The vertical lines

are the most important markings for simple arrhythmia identification because they

can tell you about the it takes for the current to travel about

within the heart.

21. The standard rate at which the EKG machine runs paper past the stylus is 25 millimeters per second. At this standard rate, we know that it takes 0.20 second to get from

one heavy vertical line to the next heavy vertical line. Therefore, if a deflection began on

one heavy line and ended on the next heavy line, we would know that the electrical current within the heart that caused the deflection lasted second.

This is an essential figure to remember because it is the basis for many of the rates, rules,

and normal values you will learn in later sections. The distance (in time) between two

heavy vertical lines on the EKG graph paper is second.

22. If the time frame between two heavy vertical lines is 0.20 second and there are five

small squares within this same area, it would follow that each of these small squares is

equivalent to one-fifth of 0.20 second, or 0.04 second each. The distance (in time) between

two light vertical lines, or across one small square, is second.

23. You now can see that graph paper can be used to measure

and .

Cardiac Cycle

24. As you know, the heart has four chambers. The upper two are the atria and the

lower two are the ventricles. In most cases the atria function as a team and contract

together, and the ventricles also operate as a single unit. So for nearly all of our discussions we will consider the atria as a single unit and the ventricles as a single unit, even

though we realize that they are actually the separate chambers

that make up the heart.

time

0.20

0.20

0.04

voltage

time

four

Figure 7 Using Graph Paper Markings to Measure Voltage and Time

VOLTAGE is measured by comparing

the height of the spike to the horizontal

lines on the graph paper.

.20 sec .04 sec

TIME is measured by comparing

the markings to the vertical lines

on the graph paper.

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20 Chapter 2

25. The upper chambers of the heart are called the ,

and they will be considered a single . Likewise,

the are the lower chambers and will be considered

a unit.

26. In the normal heart, blood enters both atria simultaneously and then is forced into

both ventricles simultaneously as the atria contract. All of this is coordinated so that

the atria fill while the ventricles contract, and when the ventricles are filling, the atria

contract. In considering a cardiac cycle, we would expect the to

contract first.

27. Before the atria can contract, they must first receive an electrical stimulus to initiate

the muscle cells response. In fact, for any myocardial cell to contract, it must first receive

an stimulus. We know that the cells

have the ability to initiate an impulse. And we know that the same electrical impulses

that eventually produce contraction of the heart can also produce deflections on the

EKG graph paper. It is by careful scrutiny of these wave patterns that we are able

to determine the activity that is present in the heart, and

sometimes we can even speculate on the type of activity

that could be expected. But to make these determinations, we must first investigate

the patterns produced by the heart’s electrical activity.

28. During each phase of the cardiac electrical cycle, a distinct pattern is produced on

the EKG paper. By learning to recognize these wave patterns

and the cardiac activity each represents, we can study the relationships between the

different areas of the heart and begin to understand what is taking place within the

heart at any given time. For each pacemaker impulse, the electrical flow travels down

the pathways, depolarizing the atria and then the ventricles

as it goes. Following this, the pattern begins again with another impulse from the pacemaker. Each cardiac cycle includes all of the electrical activity that would normally be

expected to produce a single heart beat. The cardiac cycle begins with the initiating

impulse from the pacemaker and encompasses all phases until the ventricles are repolarized. On the EKG graph paper, the cardiac cycle includes all of the wave patterns

produced by electrical activity, beginning with the impulse

and including ventricular .

29. On the EKG, each of these phases is displayed by a specific wave pattern.

Figure  8 shows a series of cardiac electrical cycles that makes up a typical EKG

rhythm strip. In Figure 9, a single cardiac cycle has been enlarged so that we can see

each of the individual patterns more closely. A single cardiac cycle is expected to produce one beat. An EKG rhythm strip is composed of more than

one cycle.

atria

unit

ventricles

single

atria

electrical; electrical

electrical

mechanical

wave

graph

conduction

pacemaker

repolarization

heart

cardiac

Figure 8 A Typical EKG Rhythm Strip

In a healthy heart, each cardiac cycle would be expected to correlate with the patient’s

individual pulse beats.

PULSE PULSE

R

T

Q S

P

PULSE PULSE PULSE

Waves and Measurements 21

Waves, Intervals, Segments

30. In labeling the activity on the graph paper, the deflections above or below the

isoelectric line are called waves. In a single cardiac cycle there are five prominent

waves, and each is labeled with a letter. Look at Figure 9 and find the P, Q, R, S, and

T waves. An interval refers to the area between (and possibly including) waves, and

a segment identifies a straight line or area of electrical inactivity between waves.

Find the PR segment and the PR interval (PRI) on Figure 9. Does the PR segment

include any waves? Does the PR interval include any

waves?

P Wave and PRI

31. The first wave you see on the cardiac cycle is the P wave. Locate it in Figure 9.

The P wave starts with the first deflection from the isoelectric line. The

 wave is indicative of atrial depolarization.

32. When you see a P wave on the EKG, does that mean that the atria contracted?

33. As the impulse leaves the atria and travels to the AV node, it encounters a slight

delay. The tissues of the node do not conduct impulses as fast as other cardiac electrical tissues. This means that the wave of depolarization will take a longer time to get

through the AV node than it would in other parts of the heart. On the EKG, this is

translated into a short period of electrical inactivity called the PR segment. This is the

straight line between the P wave and the next wave. Locate the PR segment on Figure 9.

The PR segment is indicative of the delay in the .

No.

Yes. The PRI includes the P wave

and the PR segment.

P

No, not necessarily. It means the

atria were depolarized, but it is

possible that the muscle cells did

not contract in response. It is

impossible to tell whether or not

the atria contracted simply by

looking at the EKG.

AV node

Figure 9 The EKG Complex

PR

Segment

R

P

T

Q

S

QRS

PRI

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22 Chapter 2

34. The AV node is the area of the heart with the slowest conduction speed. That

is, the conductive tissues of the sinus node, the atria, and the ventricles all conduct

impulses faster than the AV node does. This is necessary to allow time for atrial contraction and complete filling of the ventricles. On the EKG tracing, this delay at the

AV is seen as a short isoelectric segment between

the wave and the next wave. This segment is called

the segment.

35. If you wished to refer to all of the electrical activity in the heart before the impulse

reached the ventricles, you would look at the PR interval. This includes both the P wave

and the PR segment. The P wave displays depolarization, and

the PR segment is caused by the in the AV node. Therefore, the

PR includes all atrial and nodal activity.

QRS Complex

36. By definition, the PR interval begins at the first sign of the P wave and ends at the

first deflection of the next wave, called the QRS complex. The PR interval includes

all and all activity but does not

include ventricular activity.

37. Ventricular depolarization is shown on the EKG by a large complex of three waves:

the Q, the R, and the S. Collectively, these are called the QRS complex. This complex

is significantly larger than the P wave because ventricular depolarization involves

a greater muscle mass than atrial depolarization. The QRS complex starts with the

Q wave. The Q wave is defined as the first negative deflection following the P wave

but before the R wave. Locate the Q wave on Figure 9. The Q wave flows immediately into the R wave, which is the first positive deflection following the P wave. Next

comes the S wave, which is defined as the second negative deflection following the

P wave, or the first negative deflection after the R wave. Collectively, the QRS complex

signifies depolarization.

38. The QRS complex is larger and more complicated than the P wave, primarily

because it involves a larger part of the heart. Very often, the QRS complex looks different from the complex shown in Figure 9, but it is still called the QRS complex. Several

different configurations of the QRS complex are shown in Figure 10. Regardless of

appearance, these still indicate depolarization of the .

39. After the ventricles depolarize, they begin their repolarization phase, which results

in another wave on the EKG. The T wave is indicative of ventricular repolarization. The

atria also repolarize, but their repolarization is not significant enough to show up on the

EKG, so you do not see an atrial equivalent of the T wave. Ventricular repolarization

is much more prominent and is seen on the EKG as the wave.

40. Now that you have learned the definitions of all of the waves on the EKG and what

each one means, turn to the Practice Strips at the end of this chapter and label each wave

on each practice strip in Part I (strips 2.1–2.12). Be sure to recall what each wave means

as you mark it on the EKG. When you finish marking the waves, go back and identify

the PR interval, the PR segment, and the QRS complex for each strip.

41. To interpret arrhythmias you must be able to measure the duration of both the

PR interval and the QRS complex. The grid markings on the graph paper are used

to determine just how many seconds it took for the impulse to create those intervals.

node

P

PR

atrial

delay

interval

atrial; nodal

ventricular

ventricles

T

Practice Strips (Part I)

Waves and Measurements 23

To make these measurements, you will use EKG calipers. Let’s measure the PRI first.

You can use Figure 9 for practice. Place one point of the calipers on the very first deflection that marks the beginning of the P wave. Then place the other point of the calipers

on the final point of the PR interval, which you will recall is actually the very beginning

of the complex. Make sure you don’t have any part of the

QRS complex included in your measurement. Now, count the number of small boxes

within your caliper points, and multiply that number by second, which is the amount of time allotted to each small box. What is your measurement? second.

42. For the PR interval to be considered normal, it must be between 0.12 and 0.20

second. If it is less than 0.12 second, it is considered a short PRI, and if it is greater

than 0.20 second, it is said to be prolonged. The P wave itself does not contribute to

a long PRI; it is actually the delay in the AV node, or the PR ,

that varies according to how long the node held the impulse before transmitting it.

The normal PRI should be second; a long PRI would suggest

a in the .

43. Is the PRI measurement you determined for the complex shown in Figure  9

considered to be normal?

QRS

0.04

0.16

segment

0.12–0.20

delay; AV node

Yes. It is 0.16 second, which

is within the normal range of

0.12–0.20 second.

Figure 10 Various QRS Configurations

R

R

Q S Q S

R

R R

Q

Q

Q

S

R

R R

Q

S

S

R R R

Q Q S S

24 Chapter 2

ST Segment and T Wave

44. You measure the QRS complex in the same way as the PR interval. Just make sure

your caliper points are exactly where the definitions tell you they should be. Starting

with the Q wave, measure where the deflection first begins to go below the isoelectric

line. This part usually isn’t so hard. The S wave is more difficult. Between the S wave

and the T wave is a section called the ST segment. Although segments are supposed

to be straight lines, the ST segment often gets caught up in the transition between

the QRS complex and the T wave and is very rarely a cut-and-dried configuration.

So you must look for some clue that indicates to you where the S wave stops and

the wave begins. If such an indication is present, it will usually

be a very small notch or other movement suggesting an alteration of electrical flow. Use

this point as the outside measurement of the QRS complex. Include in your measurement the entire S wave, but don’t let it overlap into the ST segment or the T wave. The

QRS measurement should include the beginning of the wave

and the end of the wave.

Measurements

45. For practice, measure the QRS complex shown in Figure 9. What is your measurement? second.

46. People very rarely agree on what a normal time range is for the QRS measurement. It is usually considered to be between 0.06 and 0.11 second. For simplicity, we’ll

define the normal QRS complex measurement as anything less than 0.12 second. This

means that the ventricles took a normal amount of time to depolarize if they did it in

less than second.

47. Is the QRS measurement shown in Figure 9 considered to be normal?

Practice

48. Now that you know how to measure PRIs and QRSs, the rest is up to you. All it takes

is practice, practice, and more practice. It is particularly helpful if you can get someone

to check your measurements in the beginning so you don’t develop bad habits. You can

start by measuring PRI and QRS intervals on each of the strips in Part II of the Practice

Strips at the end of the chapter (strips 2.13–2.30). The answer key shows you where the

calipers were placed to obtain the answers, so if your measurements differ from those

given, look to see where the complex was measured to arrive at the answer shown.

Artifact, Interference

49. The complexes on an EKG tracing are created by electrical activity within the heart.

But it is possible for things other than cardiac activity to interfere with the tracing you

are trying to analyze. Some common causes of interference, or artifact, are:

• Muscle tremors, shivering

• Patient movement

T

Q

S

0.08

0.12

Yes. It measures 0.08 second,

which is less than 0.12 second.

Practice Strips (Part II)

Waves and Measurements 25

• Loose electrodes

• The effect of other electrical equipment in the room (called 60-cycle interference)

Each of these situations can cause on the EKG tracing, which

may interfere with your interpretation of the arrhythmia. When such external factors

cause deflections on an EKG strip, those deflections are considered to be artifact and are

important to recognize because they can with your interpretation of the arrhythmia.

50. Figure 11 shows you what each of these types of interference can look like on

an EKG tracing. As you can see, can often confuse you and

lead you to believe that the deflection was caused by cardiac activity when it was

not. As you practice identifying the P waves and QRS complexes, you will become

more and more familiar with the normal configurations of these wave forms

and will be more apt to distinguish them from artifact. When trying to determine whether or not a deflection was caused by artifact, you should try to identify

the waves and complexes of

the underlying rhythm and compare these configurations with the questionable

deflections.

artifact

interfere

artifact

P; QRS

Figure 11 Types of Interference

Muscle Tremors

Patient Movement

26 Chapter 2

Refractory Periods

51. Let’s go back to electrophysiology to make one final point. Since depolarization

takes place when the electrical charges begin their wave of movement by exchanging

places across the cell membrane, it would follow that this process cannot take place

unless the charges are in their original position. This means that the cell cannot depolarize until the process is complete. For depolarization to take

place, repolarization must be .

52. When the charges are depolarized and have not yet returned to their polarized

state, the cell is said to be electrically “refractory” because it cannot yet accept another

impulse. If a cell is , it cannot accept an impulse because it isn’t

yet .

53. On the EKG, the refractory period of the ventricles is when they are depolarizing or repolarizing. Thus, the QRS and the T wave on the EKG would be considered

the period of the cardiac cycle, since it signifies a period when

the heart would be unable to respond to an impulse.

54. Sometimes an electrical impulse will try to discharge the cell before repolarization is fully complete. In most cases nothing will happen because the cells aren’t back

to their original position and therefore can’t . But once in a

while, if the stimulus is strong enough, an impulse might find several of the charges

repolarization

complete

refractory

repolarized

refractory

depolarize

Figure 11 (Continued)

Loose Electrode

60-Cycle Interference

Waves and Measurements 27

in the right position and thus discharge them before the rest of the cell is ready. This

results in abnormal depolarization and hence is an undesirable occurrence. This

premature depolarization can occur only if most of the cell charges are back to

their positions. Thus, there is a small part of the refractory period that is not absolutely refractory. This small section is called the

relative refractory period because some of the charges are polarized and thus can

be if the impulse is strong enough.

55. So there are actually two refractory periods: an absolute refractory period, when

no impulse can cause depolarization, and a relative refractory period, when a strong

impulse can cause a premature, abnormal discharge. The refractory period would allow depolarization if the impulse were strong enough, while

the refractory period would not allow any response at all.

56. Figure 12 shows you where these refractory periods are located on the EKG. Notice

that while all of the T wave is considered a refractory period, the downslope of the

T wave is only relatively refractory. This means that if a strong impulse fell on the

downslope of the T wave, it could result in ventricular . This

fact will become more important to you when we begin to look at specific arrhythmias.

57. You now have all of the information you need to begin analyzing EKG rhythm

strips. You can identify all of the different waves that make up a cardiac cycle, and you

can measure the PRI and the QRS complex. You are now ready to turn to Chapter 3

and learn how to apply this knowledge as you develop a technique for analyzing EKG

rhythm strips.

original

depolarized

relative

absolute

depolarization

Figure 12 Refractory Periods

Absolute Refractory Period Relative Refractory Period

28