7. You might occasionally encounter a situation in which the heart muscle is not able

to contract in response to the electrical stimulus. In this case, you could have electrical

activity but no response. If you had a functioning electrical

system but a failing heart muscle, you could very likely see a viable tracing, but the

patient might not have palpable or blood pressure.

8. To evaluate a patient’s cardiac function, you must assess the mechanical function

by examining and and evaluate the

electrical function by analyzing the tracing.

9. An EKG tracing is designed to give a graphic display of the electrical activity in the

heart. The pattern displayed on the EKG is called the heart rhythm. Technically, the word

arrhythmia refers to an abnormal heart rhythm, although the term is also used more

generally to refer to all cardiac electrical patterns. The term dysrhythmia is synonymous

with arrhythmia; both are used to refer to patterns of activity

within the heart. All three terms are used loosely (and often interchangeably) to refer

to the heart’s activity.

10. An EKG can’t tell you about the heart’s mechanical activity—you have to assess the

patient’s pulse and blood pressure to determine that. But an EKG can tell you about the

 activity, which can be a vital part of your patient assessment.

This data is provided in the form of recognizable patterns, called arrhythmias. Arrhythmias are graphic representations of the heart’s activity.

11. To understand and interpret arrhythmias, it is necessary to understand the

electrical activity that is occurring within the heart. This is because all arrhythmias

are actually graphic displays of electrical activity. The term electrocardiography is

given to the study of arrhythmias because arrhythmias are manifestations of

 activity within the heart.

12. To help you understand and eventually be able to interpret individual arrhythmia

patterns, you might want to know a little bit about the electrical processes that take

place in the heart to produce the arrhythmia. To do this, we’ll consider the electrical

mechanical

electrical

contracting

electrical

electrical; mechanical

electrical

pulses

blood pressure

mechanical

pulses

pulses; blood pressure

EKG

electrical

electrical

electrical

electrical

electrical

Electrophysiology 3

component independent of the mechanical component. For now, we are discussing only

the activity in the heart.

Impulse Formation

13. The electrical (pacemaking) cells in the heart are distinctive in that they can create their own electrical impulses without an outside stimulus. On the cellular level,

they create a change in electrical balance in the cell, causing an electrical current to

form. This ability of cardiac cells to initiate electrical impulses on their own is called

automaticity. Automaticity is the ability of cardiac cells to create their own impulses

 an outside stimulus. It is not the whole heart that creates

the charge, it’s the individual pacemaking within the heart’s

electrical system.

14. The creation of an electrical impulse is a function of electrolytes within cardiac

cells, or more accurately, the way those electrolytes move across cell walls. The primary

electrolytes involved in creating the heart’s electrical stimulus are sodium (Na+) and

potassium (K+). Sodium and are the primary electrolytes that

allow the heart to initiate impulses. Both carry a positive electrical charge, but they

are not present in equal quantities. The sodium “outweighs” the potassium, making

the potassium relatively negative to the sodium. It is the difference in that potential that

allows electrolytes to move through cell membranes. Movement of electrolytes through

the cell is what creates the electrical impulse.

electrical

without

cells

potassium

membrane

Figure 1 The Sodium Pump: Chemical Basis for Impulse Formation

+

+ +

+

+ – – +

– –

– –

Na+

Na

Na

K+

POLARIZATION

(the ready state)

DEPOLARIZATION

(the discharge state)

REPOLARIZATION

(the recovery state)

K

K

A

B

C

ALGrawany

4 Chapter 1

15. In a resting cell, the potassium is on the inside and the sodium is on the outside.

The outside of the cell is positive, and the inside is relatively negative. The charges are

balanced so no electricity flows (Figure 1A). As the sodium enters the cell, and the

potassium leaves, an electrical charge is created (Figure 1B). The sodium then returns

to the outside of the cell and the potassium goes back in (Figure 1C). This phenomenon is commonly referred to as the sodium pump. The cycle is repeated for every

heartbeat. The term refers to the movement of electrolytes

in and out of the cell to create an electrical stimulus. When the positive and negative

charges are , no electricity flows. When the positive and negative charges exchange places, an impulse is formed.

16. For an electrical current to form, there must be a difference between the electrical

charges. In the resting cell, the charges are balanced; hence no electricity flows. This is called

the polarized state; the cell charges are and ready for action.

Polarization refers to a ready state where the electrical charges are

and no current flows. When the cell is in its ready state, it is said

to be . When the charges exchange places in the cell, the result

is formation of an current. Once the pacemaker cells provide

the stimulation, the flow is passed from cell to cell along the conduction pathways until

the cardiac cells are stimulated to contract.

Polarization and Depolarization

17. The polarized state is considered a “ready for action” phase. When the two chemical charges (sodium and potassium) trade places, the electricity flows in a wave-like

motion throughout the heart. This wave of electrical flow is called depolarization and

is how the electrical stimulus travels through the heart (Figure 1B). Polarization refers

to the “ready” state, while refers to the process of electrical

discharge and flow of electrical activity. Depolarization does not mean that the heart

muscle contracted. Depolarization is an function. Contraction

is , and is expected to follow depolarization.

18. After the cell depolarizes, the positive and negative electrical charges will again

return to their original positions around the cell, and the cell will prepare itself for

another discharge (Figure 1C). The process that follows depolarization, when the cell

charges are returning to their original state, is called repolarization. Repolarization

refers to the return of the electrical charges to their position.

Repolarization occurs depolarization.

19. If each of the positive charges on the outside of the cell is balanced by a negative

charge on the inside of the cell, the electrical charges will be balanced, and there will

be no movement of electricity. This state is called and can be

considered a “ready” state.

20. The wave of electrical activity that takes place when the electrical charges surrounding the cell trade places is called , and the return of the electrical

charges to their original state is called .

21. If polarization is considered the ready state, and is considered the discharge state, then would be considered the

recovery state.

22. Now let’s relate this cellular activity to what is actually happening in the heart. All

of the sequences described in the preceding frames are happening to single cells within

the heart, but they do it in a -like movement, resulting in the

entire heart responding electrically to the same stimuli.

sodium pump

balanced

electrical

balanced

balanced

electrical

polarized

electrical

muscle

depolarization

electrical

mechanical

original

after

polarization

depolarization

repolarization

depolarization

repolarization

wave

Electrophysiology 5

Conduction System

23. The electrical cells in the heart are all arranged in a system of pathways called

the conduction system. The physical layout of the conduction system is shown in

Figure 2. This information is an essential part of arrhythmia interpretation and should

therefore be memorized now. Normally, the electrical impulse originates in the SA

node and travels to the ventricles by way of the AV node. Look at Figure 2 and trace

a normal electrical impulse. Where would the impulse go after it left the AV node and

the Bundle of His?

24. In the normal heart, the first impulse that starts the flow of electrical current

through the heart comes from the SA node. The impulse travels through the atria by

way of the intraatrial pathways and to the AV node by way of the internodal pathways.

If you look microscopically at the cells along these pathways, you would not see any

physical difference between them and the cells in other areas of the atria, so researchers have questioned whether they actually exist. However, current electrophysiologic

studies support the concept that these pathways do exist, if only as a preferred route

by which impulses travel to the AV node. As it leaves the SA node, where does the

current go?

down the left and right bundle

branches and then to the Purkinje

fibers

down the internodal and intraatrial

pathways

Figure 2 Conduction System

ELECTRICAL CONDUCTION THROUGH THE HEART

Sinoatrial (SA) node

Internodal pathways

Right bundle branch

Purkinje fibers

Purkinje fibers

Intraatrial pathway

Atrioventricular

(AV) junction

Bundle of His

Left bundle branch

ALGrawany

6 Chapter 1

25. The next area of conductive tissue along the conduction pathway is at the site

of the AV node. The AV node is unique in that it does have conductive tissue, but

it does not have any pacemaker cells like other areas of the conduction system.

The pacemaking cells are actually located at the junction between the AV node

and the atria, in an area called the AV junction. Thus, the term AV node can be

used when talking about conduction, but the term AV junction is more accurate

if you are referring to formation of impulses. Let this confuse you. It is simply an

explanation of what might otherwise appear to be indiscriminate use of the two

phrases. We will use the term AV node if talking only about ,

but if we’re specifically discussing pacemaking capabilities, we will call it the AV

 .

26. After leaving the area of the AV node, the impulses go through the

 to reach the right and left bundle branches. These branches

are located within the right and left ventricles, respectively.

27. At the terminal ends of the bundle branches, smaller fibers distribute the electrical

impulses to the muscle cells to stimulate contraction. These terminal fibers are called

 fibers.

28. Are the muscle cells themselves part of the electrical conduction system?

29. Rearrange the following parts of the conduction system to place them in the actual

order of conduction:

(1) (a) Bundle of His

(2) (b) SA node

(3) (c) Purkinje fibers

(4) (d) left and right bundle branches

(5) (e) AV node

(6) (f) intraatrial pathways

Inherent Rates

30. Each of the three major areas of the conduction system has its own preferred rate,

called an inherent rate, at which it initiates impulses. An inherent rate means simply that each site has a rate range at which it usually produces impulses. A site can

exceed or fall below its inherent rate, indicating that these rates are not concrete rules.

But generally speaking, the sites will produce impulses at a rate within their own

 rate ranges.

31. The inherent rate ranges of the major sites are as follows:

SA node 60–100 beats per minute

AV junction 40–60 beats per minute

Ventricles 20–40 beats per minute

conduction

junction

Bundle of His

Purkinje

No, they are made up of mechanical cells, not electrical cells.

1. b

2. f

3. e

4. a

5. d

6. c

inherent