Electrophysiology 7

This information would give you a clue that if an rate was between 20 and 40 beats per

minute (bpm), the electrical impulse that stimulated the rhythm probably originated

in the . If the rate was between 40 and 60 bpm, the impulse

probably came from the , and it most likely came from the

 if the rate was between 60 and 100 bpm.

32. These rates are often helpful clues to be used in interpreting arrhythmias, but

they can be misleading unless they are understood to be mere guidelines and not

concrete .

33. Generally speaking, the fastest inherent rate will become the pacemaker of the

heart and override all other stimuli. The inherent rate of the SA node is the fastest and therefore keeps the heart at a rate between and

 bpm. Thus, the normal is “sinus” in origin. The SA

node is the normal pacemaker for the heart because the rate of the SA node is

 than the other conduction sites.

Irritability and Escape

34. If, however, a site becomes irritable and begins to discharge impulses at a fasterthan-normal rate, it can override the SA node and take over the pacemaking function

for the heart. If the SA node is discharging at a rate of 72 and the AV junction begins to

fire at a rate of 95, the will become the pacemaker.

35. This mechanism of an irritable site speeding up and taking over as pacemaker is

called irritability. It is usually an undesirable occurrence, since it overrides the normal pacemaker and causes the heart to beat faster than it otherwise would. Irritability

occurs when a site below the SA node and takes over the

pacemaking role.

36. Something very different happens if the normal pacemaker slows down for

some reason. If the SA node drops below its inherent rate, or if it fails entirely,

the site with the next highest inherent rate will usually take over the pacemaking

role. The next highest site is within the , so that site would

become the pacemaker if the SA node should fail. This mechanism is called escape

and is a safety feature that is built into the heart to protect it in case the normal

 fails.

37. Escape mechanism, unlike irritability, is a safety feature to protect the heart.

Would you expect an irritable rhythm to be faster or slower than an escape rhythm?

38. The inherent rate of different areas of the conduction system refers to the rate at

which that site .

39. The SA node has an inherent rate of to

 bpm. This means that the normal rate of the heart will usually

be within that range.

40. If the rate of an EKG is between 40 and 60, the impulse for that rhythm is probably

coming from the .

41. What is the inherent rate of the ventricular conductive tissues?

ventricle

AV junction

SA node

rules

60

100

faster

AV junction

speeds up

AV junction

pacemaker

faster

initiates impulses

60

100

AV junction

20–40 bpm

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8 Chapter 1

42. Because these rates cannot be relied upon as firm rules, they should be viewed

only as . If they are used as clues, the rates will be helpful in

interpreting arrhythmias, but if they are considered inflexible, they will simply confuse

the learner.

43. A rule regarding the pacemaker function of the heart states that the site that initiates impulses at the rate will usually become the pacemaker.

44. In the normal heart, the initiates impulses at the fastest

rate and therefore becomes the .

45. If the AV junction or the ventricle became irritable, either could become the pacemaker if it were able to accelerate until it

46. The process described in the preceding frame is called .

47. If the SA node failed as pacemaker, or if its rate dropped below its normal range,

the would probably take over as pacemaker.

48. The safety mechanism described in the preceding frame is called .

Nervous System Influence

49. In addition to the inherent rates, the heart can be influenced by the autonomic

nervous system. The two branches of this nervous system oppose each other and thus

keep the heart in a relative state of balance. The sympathetic branch influences both the

atria (i.e., the SA node, the intraatrial and internodal pathways, and the AV junction)

and the ventricles. If the sympathetic branch is stimulated, it will cause both the atria

and ventricles to react in these ways:

• Increased rate

• Increased conduction through the AV node

• Increased irritability

The parasympathetic branch has the opposite effects, but it influences only the atria; it has

little or no effect on the ventricles. While stimulation of the parasympathetic branch

causes the atria to slow down, as well as decreasing irritability and slowing conduction

through the AV node, stimulation of the sympathetic branch would cause what three

effects on the atria and ventricles?

These nervous influences are outlined in Figure 3.

50. If the vagus nerve (which is part of the parasympathetic branch) is stimulated,

you would expect the heart rate to . On the other hand, if both

the sympathetic and the parasympathetic branches are balanced, the heart rate would

remain normal. What would you expect if you blocked the normal influence of the

vagus nerve?

guidelines

fastest

SA node

pacemaker

became faster than the SA node

irritability

AV junction

escape

increased heart rate;

increased AV conduction;

increased irritability

heart rate would decrease

You would get a response similar

to stimulation of the sympathetic

branch: heart rate would increase

as well as irritability and AV

conduction.

Electrophysiology 9

51. If a patient had a heart rate that was too slow, you might try to speed it up by giving

a drug that would either stimulate the sympathetic branch or

.

52. The two branches of the autonomic nervous system that influence heart rate are the

 branch and the branch.

53. Which of these branches, when stimulated, will produce an increase in heart rate,

AV conduction, and irritability?

54. One of the branches has control over the atria and the ventricles, while the

other influences only the atria. Which one affects both the atria and the ventricles?

55. If both branches are exerting equal influence over the heart, what will happen to

the rates?

56. What will happen if one of the branches of the autonomic nervous system is

blocked?

block the parasympathetic branch

sympathetic

parasympathetic

sympathetic

sympathetic

They will stay within the ranges of

the normal inherent rates.

The heart will respond to the influence of the opposing branch.

Figure 3 Innervation of the Heart by the Autonomic Nervous System

SYMPATHETIC

• Affects the atria

 and the ventricles

• Increases:

—heart rate

—conduction

—irritability

PARASYMPATHETIC

• Affects only the atria

• Decreases:

—heart rate

—conduction

—irritability

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10 Chapter 1

57. Using the reasoning described in the preceding frame, explain what would happen

to the heart rate if the parasympathetic branch were blocked.

58. The vagus nerve is part of the branch of the autonomic

nervous system. Therefore, stimulation of the vagus nerve would cause the heart rate

to , and blocking of the vagus nerve would cause the heart

rate to .

59. All of this discussion is about activity and does not yet connect with mechanical activity. In order to discuss the heart contracting and producing

a pulse, we must connect the electrical activity with activity.

60. If the muscle cells receive an electrical stimulus, they will respond to it by contracting. Sometimes, however, the muscle itself contract because it is injured or

chemically imbalanced. In these cases, the electrical component is all right, but the

 component needs attention. In such a patient you would

expect to find the essentially normal, but the would be absent

or diminished.

61. The opposite situation is more common and is the reason you are reading this book.

This is when the heart muscle is able to respond but the electrical activity is erratic.

Sometimes the electrical stimuli will make the ventricles contract before the atria do,

or maybe there will just be too many electrical stimuli, so that the heart is not able to

respond effectively to any of them. And sometimes the electrical impulse will discharge

before the ventricles have time to fill with blood, thereby causing the ventricles to contract early, and eject insufficient blood for an adequate pulse. In all of these conditions,

the erratic electrical activity will be seen on the as an .

It would increase.

parasympathetic

decrease

increase

electrical

mechanical

mechanical

pulse

arrhythmia

11

KEY POINTS

■ The heart has two types of cells:

• Electrical cells, which initiate and conduct impulses

• Mechanical cells, which contract in response to

stimulation

■ Arrhythmias are graphic representations of electrical

activity.

■ Electrical activity precedes mechanical activity.

■ Electrical activity can occur without mechanical response

(pulse).

■ If the electrical impulse stimulates the mechanical cells

to contract, the heart is expected to contract and pump

blood, thus producing a pulse.

■ Polarization is when the electrical charges are balanced

and ready for discharge.

■ Depolarization is the discharge of energy that accompanies the transfer of electrical charges across the cell

membrane.

■ Repolarization is the return of electrical charges to their

original state of readiness.

■ Depolarization differs from contraction in that depolarization is an electrical phenomenon, whereas contraction

is mechanical and is expected to follow depolarization.

■ As shown in Figure 2, electrical flow in the normal heart

originates in the SA node, then travels via the intraatrial

and internodal pathways to the AV node, then through

the Bundle of His to the left and right bundle branches,

and finally to the Purkinje fibers, where the mechanical

cells are stimulated.

■ The inherent rates of the conduction system are as

follows:

SA Node 60–100 bpm

AV Junction 40–60 bpm

Ventricles 20–40 bpm

■ The site with the fastest rate will be the pacemaker.

■ The SA node is the normal pacemaker of the heart.

■ Irritability is when a site speeds up and takes over as

pacemaker.

■ Escape is when the normal pacemaker slows down or

fails and a lower site assumes pacemaking responsibility.

■ The influence of the autonomic nervous system can also

affect the heart:

• Sympathetic stimulation causes:

Increased heart rate

Increased AV conduction

Increased irritability

• Parasympathetic stimulation causes:

Decreased heart rate

Decreased AV conduction

Decreased irritability

■ The sympathetic branch influences both the atria (i.e., the

SA node, the intraatrial and internodal pathways, and

the AV junction) and the ventricles; the parasympathetic

branch influences only the atria.

■ If one branch of the autonomic nervous system is

blocked, the effects of the opposing branch will prevail.

SELF-TEST

Directions: Complete this self-evaluation of the information you have learned from this chapter. If your answers are

all correct and you feel comfortable with your understanding of the material, proceed to the next chapter. However,

if you miss any of the questions, you should review the

referenced frames before proceeding. If you feel unsure of

any of the underlying principles, invest the time now to go

back over the entire chapter. Do not proceed with the next

chapter until you are very comfortable with the material

in this chapter.

Questions Referenced Frames Answers

1. Name the two types of cardiac cells and tell what type

of activity each is responsible for.

1, 2, 3, 4 electrical: conduction;

mechanical: contraction

2. How do these two types of cells work together to produce cardiac activity?

1, 2, 3, 4 Electrical cells stimulate

muscle cells to contract.

3. What physical signs are used to reflect the mechanical

function of the heart?

5, 6, 7, 8 pulses, blood pressure, and other

perfusion parameters

ALGrawany