196 Cardiac Chapter 1
positive positive negative
13. In a cardiac cell, the electrical charges are provided primarily by two electrolytes: sodium (Na+) and potassium (K+). In a resting cell, the potassium is mostly on the inside, while the sodium is mostly on the outside. The sodium has a stronger charge than the potassium, making the outside of the cell more positive than the inside. In a manner of speaking, the inside of the cell is relatively negative to the outside (Figure 1A). Although both sodium and potassium carry ______________ electrical charges, they are not equally positive. This means that the resting cell will carry a ______________ charge on the outside and a relatively _____________ charge on the inside.
balanced balanced; electrical
14. This concept is really just background information for you. The important point is that the positive and negative charges are balanced in the resting state. 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.
polarized balanced flow
15. When the cell is in its resting state, it is said to be _____________ because all of the charges are _____________ and there is no electrical _____________.
electrical cardiac muscle
16. The electrical cells in the heart are unique in that they have a property called automaticity. This means that they can discharge an electrical current without an external stimulus. These cells are able to modify their membrane and pull sodium into the cell while potassium exits. This causes a difference in the electrical charges, resulting in the initiation of an electrical flow. This phenomenon is commonly referred to as the sodium pump (Figure 1). When the charges exchange places in the cell, the result is formation of an _____________ current. Once the pacemaker cells provide the stimulus, the flow is passed from cell to cell along the conduction pathway until the ___________ _________ cells are stimulated to contract.
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, and _____________ refers to the process of electrical discharge and flow of electrical activity.
original after
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.
polarization
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.
depolarization repolarization
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 _____________.
depolarization repolarization
21. If polarization is considered the ready state, and _____________ is considered the discharge state, then _____________ would be considered the recovery state.
wave
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 activity.
down the internodal and intraatrial pathways
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 means 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?_______________________
conduction Junction
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 specifically discussing the formation of impulses. Don't 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 we're talking only about _____________, but if we're specifically discussing pacemaking capabilities, we will call it the AV ______________.
Bundle of His
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.
Purkinje
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.
No, they are made up of mechanical cells, not electrical cells.
28. Are the muscle cells themselves part of the electrical conduction system?
inherent
30. Each of the three major areas of the conduction system has its own built-in 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.
SA Node 60-100 beats per minute AV Junction 40-60 beats per minute Ventricle 20-40 beats per minute
31. The inherent rate ranges of the major sites are as follows: SA Node: AV Junction: Ventricle:
rules
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 _____________.
60; 100 faster
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 EKG 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.
AV junction
34. If, however, a site becomes irritable and begins to discharge impulses at a faster-than-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.
speeds up
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.
AV junction pacemaker
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.
faster
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? _____________
initiates impulses
38. The inherent rate of different areas of the conduction system refers to the rate at which that site _____________.
60 100
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.
AV junction
40. If the rate of an EKG is between 40 and 60, the impulse for that rhythm is probably coming from the _____________.
20-40 bpm
41. What is the inherent rate of the ventricular conductive tissues? _____________
guidelines
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.
fastest
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.
SA node pacemaker
44. In the normal heart, the _____________ initiates impulses at the fastest rate and therefore becomes the _____________.
became faster than the SA node
45. If the AV junction or the ventricle became irritable, either could become the pacemaker if it were able to accelerate until it ___________________
Irratability
46. If the AV junction or the ventricle became unstable, either could become the pacemaker if it were able to accelerate until it became faster than the SA node. This process described is called _____________.
AV junction
47. If the SA node failed as pacemaker, or if its rate dropped below the normal range, the _____________ would probably take over as pacemaker.
escape
48. If the SA node failed as pacemaker, or if its rate dropped below the normal range, the AV function would probably take over as pacemaker. The safety mechanism described is called _____________.
Increased rate Increased conduction through the AV node Increased irritability
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:
sympathetic
52. The two branches of the autonomic nervous system that influence heart rate are the _____________ branch and the parasympathetic branch.
sympathetic
53. Which of these branches, when stimulated, will produce an increase in heart rate, AV conduction, and irritability? _____________
sympathetic
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? _____________
They will stay within the ranges of the normal inherent rates.
55. If both branches are exerting equal influence over the heart, what will happen to the rates?
The heart will respond to the influence of the opposing branch.
56. What will happen if one of the branches of the autonomic nervous system is blocked?
It would increase.
57. Using the reasoning described in the preceding frame, explain what would happen to the heart rate if the parasympathetic branch were blocked. _____________
parasympathetic
58. The vagus nerve is part of the _____________ branch of the autonomic nervous system.
electrical mechanical
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.
mechanical pulse
60. If the muscle cells receive an electrical stimulus, they will respond to it by contracting. Sometimes, however, the muscle itself can't 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 EKG essentially normal, but the _____________ would be absent or diminished.
arrhythmia
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 and eject insufficient blood for an adequate pulse. In all of these conditions, the erratic electrical activity will be seen on the EKG as an _____________.
Electrical
After the _____________ cells initiate the impulse and conduct it through the heart, the mechanical cells respond by contracting and pumping blood.
contracting
After the electrical cells have discharged their stimuli, the mechanical cells are expected to respond by _______________.
mechanical
After the electrical cells initiate the impulse and conduct it through the heart, the _____________ cells respond by contracting and pumping blood.
electrical electrical
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.
electrical
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 often 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.
electrical
Arrhythmias are graphic representations of ________ activity.
Electrical
Arrhythmias are manifestations of which type of cardiac activity?
Electrical & Pulses and blood pressure
As part of our assessment of mechanical function, we use blood pressure, pulses, and other perfusion parameters to determine whether or not the heart is pumping adequately. We must also look for external evidence to evaluate the heart's electrical activity. The best way to do this is to monitor the electrocardiogram (EKG). An EKG tracing is used to evaluate the _____________ activity of the heart, while the mechanical activity is evaluated by assessing _____________ and _____________.
increase
Blocking of the vagus nerve would cause the heart rate to _____________.
electrical
Depolarization differs from contraction in that depolarization is an ________ phenomenon, whereas contraction is mechanical and is expected to follow depolarization.
discharge
Depolarization is the ________ of energy that accompanies the transfer of electrical charges across the cell membrane.
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.
Describe the path that electrical current takes through the heart:
mechanical
Electrical activity can occur without ________ response (pulse).
mechanical
Electrical activity precedes ________ activity.
SA Node
Electrical flow in the normal heart originates in the:
the discharge of electrical energy that accompanies the transfer of electrical charges across the cell membrane
Explain depolarization.
when electrical charges are balanced and in a state of readiness for discharge
Explain the polarized state.
Electrical cells stimulate muscle cells to contract.
How do these two types of cells work together to produce cardiac activity?
Analyze the EKG.
How do you assess electrical activity in the heart?
block the parasympathetic branch
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
ventricle AV junction SA node
If an EKG 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.
autonomic
If one branch of the _________ nervous system is blocked, the effects of the opposing branch will prevail.
mechanical
If the electrical impulse stimulates the ________ cells to contract, the heart is expected to contract and pump blood, thus producing a pulse.
decrease
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.
You would get a response similar to stimulation of the sympathetic branch: heart rate would increase as well as irritability and AV conduction.
If the vagus nerve (which is part of the parasympathetic branch) is stimulated, you would expect the heart rate to decrease. On the other hand, if both the sympathetic and the parasympathetic branches are balanced, the heart rate would remain normal.
No, depolarization is an electrical phenomenon. Contraction is mechanical and is expected to follow depolarization.
Is depolarization the same as contraction?
electrical: conduction; mechanical: contraction
Name the two types of cardiac cells and tell what type of activity each is responsible for.
down the left and right bundle branches and then to the Purkinje fibers
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?
electrical and mechanical
Since it is not practical to see inside a living patient's heart, we must rely on external evidence to evaluate the status of both electrical and mechanical cardiac function. For a complete assessment of cardiac status, we must evaluate both ______________ and _____________ functions.
decrease
Stimulation of the vagus nerve would cause the heart rate to _____________,
SA
The ____ Node is the normal pacemaker of the heart.
sympathetic
The _________ branch influences both the atria (i.e., the SA node, the intraatrial and internodal pathways, and the AV junction) and the ventricles;
parasympathetic
The __________ branch influences only the atria.
Electrical & Mechanical
The heart has two types of cells:
electrical
The heart will respond with contraction only if it is stimulated by electrical activity. Thus, you cannot have a mechanical response if there is no ______________ stimulus.
1. The electrical impulse that tells the heart to beat 2. The mechanical beating of the heart in response to the electrical stimulation, resulting in pumping of blood
The human heart is intended to pump blood to the rest of the body. This process has two distinct components: What are the two components?
Decreased heart rate Decreased AV conduction Decreased irritability
The influence of the autonomic nervous system can also affect the heart: Parasympathetic stimulation causes:
Increased heart rate Increased AV conduction Increased irritability
The influence of the autonomic nervous system can also affect the heart: • Sympathetic stimulation causes:
40-60
The inherent rates of the AV Node
60-100
The inherent rates of the SA Node
20-40
The inherent rates of the ventricles
increased heart rate; increased AV conduction; 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?
pacemaker.
The site with the fastest rate will be the ________
parasympathetic
The two branches of the autonomic nervous system that influence heart rate are the sympathetic branch and the ___________ branch.
pulses; blood pressure EKG
To evaluate a patient's cardiac function, you must assess the mechanical function by examining ______________ and _____________ and evaluate electrical function by analyzing the _____________ tracing.
electrical
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.
It initiates the flow of electrical current.
What happens when the positive and negative electrical charges exchange places across the cell membrane of a cardiac cell?
pulses, blood pressure, other perfusion parameters
What physical signs are used to reflect the mechanical function of the heart?
electrical
Without _____________ stimulus, the mechanical cells can't be expected to contract.
mechanical pulses
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 EKG tracing but the patient might not have palpable _____________ or blood pressure.
Repolarization
________ is the return of electrical charges to their original state of readiness.
Polarization
________ is when the electrical charges are balanced and ready for discharge.
Escape
________ is when the normal pacemaker slows down or fails and a lower site assumes pacemaking responsibility.
Irritability
_________ is when a site speeds up and takes over as pacemaker.
