1. Principles of Electrocardiography
Chapter 49

2. Learning Objectives
Define, spell, and pronounce the terms listed in the vocabulary.
Apply critical thinking skills in performing patient assessment and care.
Illustrate the electrical conduction system through the heart.
Explain the concepts of cardiac polarization, depolarization, and repolarization.
Summarize the properties of the electrocardiograph.

3. Learning Objectives
Describe the electrical views of the heart recorded by the 12-lead electrocardiograph.
Discuss the process of recording an electrocardiogram.
Perform an accurate recording of the electrical activity of the heart.
Compare and contrast electrocardiograph artifacts and the probable cause of each.
Identify a typical electrocardiograph tracing.

4. Learning Objectives Describe common electrocardiograph arrhythmias.
Summarize cardiac diagnostic tests.
Apply a Holter monitor.

5. The Cardiac Cycle
The cardiac cycle includes all of the events occurring in the heart during one single heartbeat.
Systole
Diastole
The electrocardiograph records both the intensity and the actual time it takes for each part of the cardiac cycle to occur.
Summarize the blood flow through the heart.
What is systole and asystole?

6. Heartbeat Origin
The sinoatrial (SA) node controls the rate of heart contraction by initiating electrical impulses every 60 to 100 times per minute.
The heart beats in response to an electrical signal that originates in the SA node in the right atrium, spreads over the atria, and causes atrial contraction.
This impulse continues to the atrioventricular (AV) node, through the bundle of His, and then through the right and left bundle branches, eventually causing ventricular contraction.
Summarize the electrical stimulation of the myocardium.

7. Cardiac Rhythms
Normal sinus rhythm (NSR) refers to a regular heart rate that falls within the average range of 60 to 80 beats/min
Sinus bradycardia is a heart rate less than 60 beats/min
A rate of greater than 100 beats/min is called sinus tachycardia
An irregular cardiac rhythm is called an arrhythmia. Conditions that interrupt the conduction pathway, SA node to AV node to bundle of His to right and left bundle branches, can cause arrhythmias.
Explain NSR, bradycardia, and tachycardia.
What might cause an arrhythmia?

8. Electrical Conduction System
Tie in the electrical stimulation of the myocardium with the PQRST segment.
From Hunt SA: Saunders fundamentals of medial assisting, Philadelphia, 2002, Saunders.

9. Cardiac Contraction
Polarization – resting state of the myocardial wall: no electrical activity in the heart; recorded on the ECG as a flatline
Depolarization – contraction phase; electrical system of the heart stimulates the myocardium
Repolarization – resting state after depolarization; the myocardium must return to a resting state before it can be electrically stimulated again
Discuss these terms in relation to atrial contraction, ventricular contraction and recovery, and the myocardium’s resting stage.

10. PQRST—Table 49-1
P wave – first deflection from baseline; atrial depolarization
PR segment – return to baseline after atrial contraction
PR interval – time from the beginning of atrial contraction to the beginning of ventricular contraction
QRS complex – contraction of both ventricles
ST segment – time between the end of ventricular contraction and the beginning of ventricular recovery
T wave – repolarization of the ventricles
QT interval – between the beginning of the QRS complex through the T wave
U wave – occasionally seen as a small waveform after the T wave
Refer to Table Discuss the various waves, segments, and intervals that occur during the cardiac cycle.

11. The ECG Machine
Six-channel ECG machines – Leads placed at specific anatomic locations; records all 12 leads automatically and marks each lead with identifying letters; multichannel ECG tracings take seconds to perform and can be placed into chart without mounting
Single-channel ECG machines – Older machines that record the 12 leads one at a time; strips must be cut apart and mounted onto a card before placement in the chart
Distinguish between the six-channel ECG machine and a single-channel type.
Which machine do you think is seen most frequently in the ambulatory care setting?
What is the advantage of the six-channel machine?

12. Critical Thinking Application
Martha has not been taught how to use the ECG machine in Dr. Lee’s office. What steps should she take to learn how to use this machine and feel comfortable and confident using it to obtain ECGs?
Discuss the case.

13. Sample ECG Machines
From Chester GA: Modern medical assisting, Philadelphia, 1999, Saunders.

14. Electrocardiograph Paper
The horizontal lines on the ECG paper permit the determination of the intensity of the electrical activity or the relative strength of the heartbeat.
The paper is heat and pressure sensitive; it must be handled carefully to avoid making any additional markings that would blemish the tracing.
Describe the lines on the ECG paper.
What is the purpose of the stylus?
How should ECG paper be stored and handled to prevent damage?

15. Paper Record
The horizontal axis of the paper represents time and the vertical axis represents amplitude
Each small square measures 1 mm on each side
Every fifth line, both vertically and horizontally, is darker than the other lines creating a larger square measuring 5 mm on each side
One small 1-mm square passes the stylus every second; one large 5-mm square passes the stylus every 0.2 second; in 1 second, 5 large squares pass the stylus
Large squares represent 0.2 second; 5 of them equals 1 second; ECG paper travels past the stylus at 25 mm per second
Examine sample ECG paper. Locate the large and small boxes.
In 4 seconds how many large squares will pass through the stylus?

16. ECG Paper
Give examples of assessing patient heart rates using the large squares on the ECG paper.
From Chester GA: Modern medical assisting, Philadelphia, 1999, Saunders.

17. Electrodes and Leads
Two electrodes are placed on the arms and legs; six electrodes are placedon the chest
Electrodes must be applied to specific locations to record the heart’s electrical activity from different angles and planes
Most offices use single-use, self-stick, disposable electrodes that are packaged with conductive jelly in the center
Ten color-coded and labeled lead wires ending in a small metal clip are attached to the electrodes
The leads carry the cardiac electrical impulses into the machine.
Discuss the application of electrodes and lead wires.

18. Lead Recordings
Each lead records the electrical activity of the heart between two different electrodes, one positive and one negative
If depolarization occurs toward the positive electrode the deflection is upright; if it moves toward the negative electrode it is deflected downward
ECG records views of the heart on both a frontal and a transverse plane
Frontal leads include leads I, II, III, aVR, aVL, and aVF. Horizontal plane leads include the six precordial or chest leads
Explain how the leads work together to provide multiple views of the electrical activity of the heart.

19. Standard Leads
The first three leads recorded are called the standard or bipolar leads; they each use two limb electrodes to record the heart’s electrical activity
Lead I records tracings between the right arm and left arm
Lead II records tracings between the right arm and left leg; it is the lead recorded on a cardiac monitor or on the rhythm strip at the bottom of the 12-lead ECG
Lead III records tracings between the left arm and left leg
Why are standard leads called bipolar leads?
Explain the limbs electrodes that are used to record Leads I, II, and III.

20. Augmented Leads
Unipolar leads with a single positive electrode that uses the right leg for grounding
aVr – records the electrical activity of the atria from the right shoulder; P waves and QRS complexes are deflected below the baseline
aVl – records the electrical activity of the lateral wall of the left ventricle from the left shoulder
aVf – records the electrical activity of the inferior surface of the left ventricle from the left leg
Explain the electrical views of the heart that are recorded with the augmented leads.

21. Standard and Augmented Leads
Have students review the lead views using the diagram.
From Chester GA: Modern medical assisting, Philadelphia, 1999, Saunders. 21

22. Precordial Leads
Transverse plane view of the heart; QRS complex is a negative deflection in V1 and V2, views with each subsequent lead becoming more positive
Measure the electrical activity among six specific points on the chest wall and a point within the heart
Discuss the purpose of the precordial leads.

23. Precordial Lead Placement
V1—the electrode is placed in the fourth intercostal space, just to the right of the sternum
V2—the electrode is placed in the fourth intercostal space, just to the left of the sternum
V3—the electrode is placed midway between V2 and V4
V4—the electrode is placed in the fifth intercostal space, at the left midclavicular line
V5—the electrode is placed horizontal to V4 in the left anterior axillary line
V6—the electrode is placed horizontal to V4 in the left midaxillary line.
Students must learn the specific placement of each chest lead.

24. Chest Leads Review placement with the figure. 24
From Chester GA: Modern medical assisting, Philadelphia, 1999, Saunders. 24

25. Electrode Placement
Review limb and chest electrode placement from the figure.
Courtesy CompuMed, San Diego, Calif. 25

26. Critical Thinking Application
Dr. Lee has asked Martha to perform her first ECG on a patient who just came into the office. Martha is not confident that she knows how to properly place the chest leads in the correct location. How should she handle this situation? Should she perform the ECG procedure “the best that she can”? Why or why not?
Discuss the case.

27. Preparation of Room and Patient
Pick the quietest location in the office, as far away as possible from all electrical equipment
Patient should empty the bladder and rest for at least 10 minutes
Patient should disrobe to the waist; gown open in front; pantyhose removed and limbs exposed
Supine position with pillows to support head, back, and under knees
If patient is in a seated position the feet must rest comfortably on the floor or on a footstool; record any alternative position
Why should ECGs be conducted in a quiet room that is not located next to electrical equipment?
Why would you have the patient rest for 10 minutes before performing the ECG?
Does a woman have to remove her bra and pantyhose to have an ECG done?
Why would you place a small pillow under the knees and lower back before doing the ECG?
Can a patient be in a semi-Fowler’s position?

28. Preparation of Patient
Record vital signs and current medications on the medical record or ECG
Explain the procedure and purpose of the ECG
Make the patient as comfortable as possible
Stress importance of not moving during entire procedure; observe that he or she is breathing normally
Why should the patient’s vital signs be recorded before the ECG?
Why does the patient need to remain still during the procedure?

29. Apply Electrodes and Leads
Applying leads to the patient
Why might it be necessary to rub the electrode site with an alcohol wipe before application?

30. Recording an ECG—Procedure 49-1
Enter specific information about the patient including age, sex, prescriptions, etc.
After the ECG is programmed, remind the patient to lie still and press the appropriate key to run the ECG strip
Six-channel machines will print and label all 12 leads with a rhythm strip across the bottom of the paper in lead II in a matter of seconds
Review the printout; if acceptable give it to the physician
Review Procedure 49-1.
What data should be recorded on the ECG strip?

31. Standardization, Sensitivity, and Speed
Stylus should deflect exactly 10 mm when the standardization button is depressed with a quick pecking motion
Most machines have three sensitivity standards that can be selected
If QRS complex is too tall the STD should be set to 1/2 STD; if QRS complex is too short the STD should be set to 2 STD
Usual speed for an ECG recording is 25 mm/sec
Explain the standardization process. Why is it important?
How should the sensitivity of the machine be adjusted if the QRS complexes are very tall?

32. Sensitivity Standards32

33. Mounting an ECG Tracing
The facility will select the mount that is best suited to the type of ECG equipment used in that particular office
Paper clips and staples are never used; they scratch and mark a tracing; tape should not be used because it yellows with age
A photocopy can be made of the tracing and placed in the medical record
With electronic medical records the tracing is scanned into the patient’s record
Is mounting the ECG always necessary?
How are ECG strips typically stored? How is this accomplished in the electronic medical record?

34. Labeling ECG Strip Patient’s full name, sex, age Date and time of ECG
List of all medications and/or supplements
Variations from normal sensitivity and normal speed
A very nervous or anxious patient
Lack of rest before test
Smoking immediately before test
Failure to follow any pretest instructions
Summarize the information that should be included when documenting an ECG procedure.
Why might anxiety and lack of rest interfere with a good strip?

35. Telephone Transmission
An ECG machine can transmit a recording over a telephone to an interpretation center
Recording is interpreted by a computer at the data center and verified by a cardiologist
Patient information that is important to interpretation such as medications and vital signs sent with the ECG
Printout with interpretations returned to sender by fax or
Explain how an ECG strip can be transmitted for interpretation.
Should the results be included in the patient's medical record?

36. Artifacts
Erratic movement of stylus on ECG paper from outside interference
ECG is extremely sensitive to any kind of nearby electrical activity
Medical assistant should have thorough understanding of the causes of and remedies for artifacts
Explain the meaning of an artifact in an ECG strip.
It is the medical assistant’s responsibility to make sure the ECG strip is clean and free of artifacts so the physician can interpret the strip without interference.

37. Wandering Baseline Stylus gradually shifts away from center of paper
Caused by patient movement or poor electrode attachment
Remind patient to remain as still as possible; make patient more comfortable
What might be the cause of a wandering baseline artifact?
How can it be resolved?

38. Somatic Tremor
Any muscle movement produces a measurable electrical impulse causing stylus movement during the tracing
Shows up as jagged peaks of irregular height and spacing with a shifting baseline
Causes: patient discomfort, apprehension, movement, talking, or having a condition that causes uncontrollable body tremors.
What might be the cause of a somatic tremor artifact?
How can it be resolved?

39. Alternating Current (AC) Interference
Appears as series of uniform small spikes
Electrical currents in nearby equipment or wiring can leak small amounts of electrical energy that the ECG picks up
Management – use three-pronged grounded outlet; do not cross lead wires; unplug other electrical appliances in room; move table away from wall; and turn off fluorescent lights
What might be the cause of an AC interference artifact?
How can it be resolved?
From Aehlert B: ECGs made easy, ed 3, St Louis, 2006, Mosby.

40. Interrupted Baseline Electrical connection is interrupted
Stylus moves erratically up and down across the paper, or it may record a straight line across the top or the bottom of the paper
Patient movement that dislodges electrodes causes most baseline interruption; also from broken lead wires or detached leads
What might be the cause of an interrupted baseline?
How can it be resolved?

41. Critical Thinking Application
Dr. Lee asks Martha to explain to her the causes of artifacts and the methods for correcting ECG recordings that show outside interference. Based on what you have learned about ECG artifacts, what are the typical causes, and how would you recommend correcting each?
Discuss the case.

42. Normal ECG Appearance Table 49-2
NSR – each beat of the heart is initiated with an impulse from the SA node that then travels, without interruption, along the normal conduction pathway of the heart
Each beat of the heart shows a P wave followed by a QRS complex
Refer to Table 49-2.
Describe the PQRST segment with an NSR.

43. Rate and Rhythm
To calculate heart rate count the number of P waves in a 6-second strip (30 large squares) and multiply by 10
Ventricular contraction rate – count the number of complete QRS complexes in 6 seconds and multiply that number by 10 to get the number of contractions in 1 minute
If the patient’s heart is in a regular rhythm each cardiac cycle occurs within the same time frame and each cardiac cycles occur exactly the same length of time apart
What is the difference between rate and rhythm?
How can the patient’s heart rate be determined from an ECG strip?

44. Typical Rhythm Abnormalities
Sinus rhythm – heart’s electrical activity begins in the SA node and follows through the electrical system, ending in atrial and ventricular depolarization
Sinus arrhythmias – pathway of the electrical charge is normal but the rate or rhythm of the heartbeat is altered
Sinus bradycardia – heart rate less than 60 beats/min
Sinus tachycardia – heart rate more than 100 beats/min
What is the difference between a sinus rhythm and a sinus arrhythmia?
Explain the heart rates in sinus bradycardia and tachycardia.

45. ECG Rhythm Abnormalities
Atrial arrhythmias
Premature atrial contraction (PAC) – atria contract before they should; show up as abnormally shaped P wave or an extra P
Atrial flutter – extremely rapid atrial rate, up to 300 beats/min
Ventricular arrhythmias
Premature ventricular contraction (PVC) – ventricles contract before they should; QRS complex appears before P wave
Ventricular tachycardia – (V-tach) 101 to 250 beats/min
Ventricular fibrillation – (V-fib) life-threatening arrhythmia; heart muscle quivering uncontrollably; unable to pump blood; there is no pulse
Asystole – no heartbeat; flatline on ECG
Explain the difference between a PAC and atrial flutter. How would each look in an ECG strip? Which is more serious?
Can PVCs be thrown occasionally without symptoms and disease?
What is the difference between V-tach and V-fib?
Why is there no pulse with V-fib?

46. Rhythm Abnormalities Discuss the abnormal recordings.
From Chester GA: Modern medical assisting, Philadelphia, 1999, Saunders.

47. Biochemical Arrhythmias
Digitalis – causes scooping of ST segment in V5 and V6
Potassium – hyperkalemia or hypokalemia can both cause life-threatening arrhythmias
What is the most common ECG abnormality for patients on digitalis medications?
How can potassium levels affect the cardiac cycle?
From Aehlert B: ECGs made easy, ed 3, St Louis, 2006, Mosby.

48. Pacemaker Rhythm
Pacemakers stimulate the electrical activity of the heart
Implanted under the skin; it is a small metal pulse generator with a battery and electronic leads that extend from the generator to the myocardium
Programmed to fire according to individual patient needs
Readings transmitted over phone
Explain a pacemaker, its purpose, and how it works.
What does a pacer rhythm look like?
From Lewis S et al: Medical-surgical nursing, ed 7, St Louis, 2007, Mosby.

49. Implanted Cardioverter Defibrillator (ICD)
Monitors heart rhythm and delivers shock to the heart if it detects a dangerous tachycardia or fibrillation
A small, battery-operated device that is implanted under the skin in the chest or abdomen
Can reverse V-tach and V-fib, especially after the patient has had an MI
Generator programmed to treat the patient’s particular or potential cardiac arrhythmia
Patient can telephone in periodic readings
Discuss the purpose of ICDs.
What patient education is needed for ICD operation?

50. Myocardial Infarction
Causes specific ECG changes based on the phase that the patient is in when the ECG is recorded (Table 49-4).
Three most common changes – elevated ST segments, inverted (upside-down) T waves, and abnormal (pathological) Q waves.
Explain the effect of an MI on the ECG strip.
From Aehlert B: ECGs made easy, ed 3, St Louis, 2006, Mosby.

51. MI Treatment
Sooner treatment is initiated the more effective and the better the patient outcome
Immediate treatment – nasal oxygen, sublingual nitroglycerin (to dilate coronary arteries), narcotic analgesic (to eliminate pain), aspirin (to reduce inflammation and decrease clotting time), and possibly a thrombolytic agent to dissolve the clot in the coronary arteries
After discharge – quit smoking, modify diet, enter cardiac rehabilitation program to improve cardiac strength and recovery by exercise
Why is it so important for a patient having an MI to seek help immediately?
Why are thrombolytic medications given IV for a patient having an MI?
Discuss the treatment of an MI and postrecovery follow up.

52. Cardiac Stress Test
Conducted to observe and record the patient’s cardiovascular response to measured exercise challenges.
Discuss the reasons for stress testing.
How is the procedure performed?
Why must a physician be present during the test?
Courtesy Cardiac Science

53. Critical Thinking Application
Mr. Sonderford actually had an MI when he was previously at Dr. Lee’s office. He has now completed cardiac rehabilitation and is at the office for a checkup. Dr. Lee wants him to be scheduled for a stress test. Mr. Sonderford has never had one before. He confides to Martha that he is afraid if he takes the test, he will die from another heart attack. How should Martha handle this situation?
Discuss the case.

54. Holter Monitor
A portable system for recording the cardiac activity of a patient over a 24-hour period or longer.
The patient must keep a journal of all stressful events and activities during the entire time the monitor is worn and press the event button if symptoms occur.
A medical assistant is often responsible for instructing the patient in applying and removing the monitor.
Courtesy Welch Allyn, Skaneateles Falls, NY.
Why might a Holter monitor be ordered?
What patient education is needed to make sure the test is conducted properly?

55. Critical Thinking Application
Mrs. Jamison was fitted with a Holter monitor at the office yesterday at 4 pm. When Martha arrived at the office at 8 o’clock this morning, she found Mrs. Jamison had left a message with the answering service to call her as soon as possible. When Martha returned the call, she told her she took a shower last night and she noticed when she got up to go to the bathroom that the “light is not on” on the monitor. How should Martha handle this situation?
Discuss the case.

56. Cardiac Event Monitor
Recording device that can be worn up to 30 days to catch events that are difficult to record in a 24-hour Holter monitor period
Patients trigger the recording when feel any symptoms
Remove during bathing – patient must be taught how to remove and reapply the electrodes
Patient education – do not alter lifestyle; record activities when events occur; change electrodes daily and batteries at the same time each day; instructions on how to transmit recordings by phone
What is the difference between an event monitor and a Holter monitor?
What patient education is needed to make sure the test is conducted properly?

57. Electron Beam Tomography Heart Scan (EBT)
Noninvasive method of assessing possible cardiac risk
Takes less than 5 minutes and does not require any needles or injections
Records amount of plaque present in coronary arteries by showing the presence of calcium deposits
Calcium makes up approximately 20% of arterial plaque deposits
The patient’s calcium score can predict future cardiac problems
Explain the connection between calcium deposits in the arteries and the EBT test.
Why is the arterial calcium score an indicator of future cardiac disease?

58. Patient Education
Heart disease and stroke account for more than one-third of all deaths.
Talk to the patient about factors that could be modified, and give him or her encouragement for any attempt at complying with these suggestions.
Include visual aids, posters, and brochures.
Summarize patient education factors discussed in the chapter.