Are you ready to get started learning about ECG’s and interpreting the electrocardiogram? If so, then you’re in luck, because that is what this EKG study guide is all about.

As a Respiratory Therapist, depending on where you work, you will most likely have to perform EKG’s on the job. That is why it’s important for you to learn the ins and outs of electrocardiograms, the procedure, and how to interpret the results. 

The good news it — this study guide can help you do just that. Also note: this information correlates well well Egan’s Chapter 17 on Interpreting the Electrocardiogram, so you can use it to prepare for your exams. Are you ready to get started?

What is an EKG Test?

An electrocardiogram is a noninvasive test that is used to measure the underlying heart conditions of a patient by measuring the electrical activity of the heart.

It is often abbreviated as either an EKG or an ECG. These two abbreviations can be used interchangeably.

How is an EKG Measured?

EKG’s are measured with a device called an Oscilloscope. It is a monitor that provides a continuous visual image of heart activity and generally only displays a single Lead (normally Lead II).

They can also be measured with a Holter Monitor, which is a portable version of the electrocardiograph that is used to detect cardiac arrhythmias. It is worn underneath the clothes, usually for a 24-48 hour period.

It is mainly used to detect ECG abnormalities that may occur over an extended period
of time.

Electrophysiology of the Heart

  1. The SA Node, also called the pacemaker, generates an electrical impulse.
  2. The wave of depolarization goes through the atria which causes a contraction. (P wave)
  3. The impulse is received by the AV node and then there is a short delay. (P-R interval)
  4. Then, the stimulus moves through the bundle of His, then the left and right bundle branches, then to the purkinje fibers. This produces ventricular depolarization and contraction occurs. (QRS complex)
  5. Then there is a short delay (S-T segment) and then the heart repolarizes (T wave).

What is the EKG Axis?

The EKG Axis measures the net direction of the electrical activity through the heart during a contraction.

Just remember, the impulse moves down and to the left. (Starts in the SA node/right atrium and moves towards the left ventricle.

A myocardial infarction (heart attack) most often occurs in the left ventricle. This would cause the axis to shift to the right because the electrical activity decreases on the left, RAD or right axis deviation.

EKG Leads and Electrodes


These are placed on the skin on the arms and legs (limb leads) and chest (precordial leads) to conduct a readable electric current.


They display the movement of electricity from one electrode to another.

Chest Electrodes

The 6 chest electrodes allow for a 3D projection of the heart. They are as follows:

  • V1 – 4th intercostal space on the right side of the sternum
  • V2 – 4th intercostal space on the left side of the sternum
  • V3 – Between V2 and V4 on the left side
  • V4 – 5th intercostal space on the left mid-clavicular line
  • V5 – Between V4 and V6 on the left side
  • V6 – 5th intercostal space on the mid-axillary line

Practice Questions about EKG Interpretation:

1. A P-R interval of .32 seconds would indicate what?
Disturbance in the heart’s conduction pattern.

2. A monitor that is worn for 24 hours to evaluate the performance of the heart during routine activities is known as what?
Holter monitor.

3. A patient is experiencing chest pain and the physician wishes to determine the cause of the pain during exercise. However, the patient is very feeble and cannot perform any exercise. Which of the following would you suggest for this patient?
Persantine Thallium stress test.

4. A patient is showing symptoms of a myocardial infarction, and the physician suspects that the left side (ventricle) of the heart is involved. Which of the following ECG leads will provide the physician with this information?
Leads I and aVL, and Leads V5 and V6.

5. Depressed T waves on an EKG tracing represent what?
Myocardial Ischemia.

6. Elevated ST segments are indicative of what?
Myocardial ischemia and Certain cardiac medications.

7. Every small square on the horizontal axis of the ECG grid paper represents what?
0.04 seconds.

8. How many electrophysiologic phases are there in the action potential?

9. In order to obtain a Lead I tracing, the electrode sensors provide:
Right arm (negative); left arm (positive).

10. In order to obtain a lead III tracing, the ECG sensor makes:
Left leg (positive) Left arm (negative).

11. In the normal conduction system, where does the electrical impulse of the heart originate?
SA node.

12. In which phase of repolarization is there a slow influx of calcium?
Phase 2.

13. Stimulation of the parasympathetic system causes what?
Decreased heart rate and Decreased conduction.

14. The P wave on an ECG tracing represents which of the following?
Atrial depolarization.

15. The T wave on an ECG tracing represents what?
Ventricular repolarization.

16. The ability of a cardiac cell to reach its threshold potential and respond to a stimulus or irritation is called?

17. The main purpose of a cardiopulmonary stress test is to determine what?
The criteria for an exercise program.

18. What is the normal P-R interval?
Usually between 0.12 and 0.20 seconds.

19. What is the part of the autonomic nervous system that increases the heart rate?
The Sympathetic System.

20. The type of electrodes most commonly used are disposable and should be used how many times?
Only once.

21. Two or more waveforms represents what?
A complex.

22. What does the term repolarization mean?
Cardiac cells returning to their resting membrane potential.

23. When continuous cardiac monitoring is used, what lead is often viewed?
Lead II.

24. When obtaining a 12-lead ECG, what is the correct number of electrodes to use?

25. Which of the following are characteristics of cardiac cells?
Automaticity, excitability, conductivity, and contractility.

26. Which of the following electrolytes are primarily involved in the cardiac cycle?
Sodium, Calcium, and Potassium.

27. Which of the following electrophysiologic phases represent depolarization?

28. Which of the following is NOT a common use for the 12 lead ECG?
To determine cardiac output.

29. Which of the following is counted vertically on the ECG grid paper?

30. Which of the following is the duration of each large square on ECG paper?
0.20 seconds.

31. Which of the following is the phase in which a strong stimulus may cause depolarization of cardiac cells?
Relative refractory period.

32. Which of the following leads are considered bipolar leads?
Lead I, Lead II, and Lead III.

33. Which of the following leads gives information concerning the lower surface of the heart or the inferior leads?
Lead II, Lead III, and aVF.

34. Which of the following statements are correct when the cardiac cells are in the resting or polarized state?
In this state, the inside of the cell is negatively charged with K+, and the outside is positively charged with Na+.

35. Which of the following statements are true about the U wave of an ECG complex?
U waves become visible only in the presence of electrolyte imbalances or heart disease.

36. Which of the following uses the center of the heart as the negative reference point?
V1, V3, and V6.

37. Which of the following would be a normal duration for a QRS complex?
0.08 seconds.

38. Which type of ECG testing can be used to evaluate the heart while exercised?
Treadmill stress test.

39. Which of the following leads would be required to view continuous ECG monitoring?
Lead I, V2, Lead II, and Lead III.

40. What are some good things about an EKG?
It is inexpensive, noninvasive, and easy to obtain.

41. Why do we use an EKG?
For chest pain, SOB, dyspnea, weakness, lethargy, and dizziness.

42. What does an EKG measure?
Cardiac activity, repolarization and depolarization.

43. What doesn’t an EKG measure?
It does not measure the force of contraction of the heart.

44. What is the normal pacemaker of the heart?
The SA node.

45. What is a Segment?
A portion of the baseline.

46. What is an Interval?
It contains at least one wave.

47. How do you do the rate calculation on an EKG?
Count the number of large boxes between two R waves and then divide that number by 300.

48. What does the ECG measure or record?
The cardiac electrical activity of the heart, Repolarization (relaxing), and depolarization (contracting).

49. When will an ECG be indicated?
If the patient complains of chest pain, SOB, dyspnea with palpations, weakness, lethargy, and dizziness.

50. How many seconds do the small squares represent?
0.04 seconds.

51. How many seconds do the large squares represent?
0.2 seconds (5 smaller boxes).

52. What is cardiac electrophysiology?
Basic arrhythmia interpretations (why the impulses are doing what they are doing).

53. What is the normal cardiac stimulus as it spreads through the atria and ventricles?
SA nodes, AV nodes, Bundle of HIS, R/L bundle branch, Purkinje fibers.

54. What is the normal width of duration for a QRS interval?
0.05–0.1 seconds.

55. What is the normal width of duration for aPR interval?
0.12–0.2 seconds.

56. What is the rate of the SA node?

57. What is the rate of the AV node?

58. What is the rate of the Bundle of branches?

59. What is the rate of the Purkinje fibers?

60. What are the two nerve fibers within the right atrium that alter the HR when stimulated?
Parasympathetic and sympathetic.

61. What does the P wave represent?
It represents atrial depolarization (contraction).

62. What does the QRS complex represent?
It represents ventricular depolarization and atrial repolarization.

63. What does the T wave represent?
It represents ventricular repolarization.

64. What does the Q wave represent?
It represents the conduction of an impulse down the interventricular septum.

65. What does the U wave represent?
It represents repolarization of the bundle of HIS and Purkinje fibers.

66. What is the PR interval?
The beginning of the P wave to the beginning of the QRS.

67. What does it mean if the PR interval is increased?
This would indicate a 1st-degree heart block.

68. What is the S1 heart sound?
The sound of the AV valves closing.

69. What is a murmur?
An abnormal heart sound.

70. What is the S2 heart sound?
The sound of the semilunar valves closing.

71. Which rhythm would you likely see with acute hypoxia?
Sinus tachycardia.

72. Which rhythm will you see with chronic hypoxia?
Sinus bradycardia.

73. Which of the following criteria applies to a third-degree block?
There is no relationship between the P waves and the QRS complexes.

74. What is implied by an abnormally prolonged PR interval?
Atrioventricular block.

75. Why is the electrical impulse temporarily delayed at the atrioventricular (AV) node?
To allow better filling of the ventricles..

76. Which of the following medications is used to treat sinus bradycardia?

77. What term is used to define the ability of certain cardiac cells to depolarize without stimulation?

78. Which of the following ECG abnormalities is most life-threatening?
Elevated ST segment.

79. On an ECG rhythm strip from an adult patient, you notice the following: regular sawtooth-like waves occurring at a rate of 280/min and a regular ventricular rhythm occurring at a rate of about 140/min. What is the most likely interpretation?
Atrial flutter.

80. What is suggested by inverted T waves on the ECG?
Myocardial Ischemia.

81. What structure normally paces a healthy heart?
Sinoatrial (SA) node.

82. Is atrial flutter considered to be a life-threatening arrhythmia?
No, no it is not.

83. Which of the following is NOT a common characteristic of a premature ventricular complex (PVC)?
A narrow QRS.

84. Which of the following is NOT a common cause of ventricular tachycardia?

85. At what part of the cardiac conduction system does the electrical impulse travel most rapidly?
Purkinje fibers.

86. What medication is most useful for the treatment of premature ventricular contractions?

87. The ECG you are looking at has one P wave for every QRS complex and the PR interval is 0.30 second. What is your interpretation?
First-degree heart block.

88. Which of the following waves represents depolarization of the ventricles?
QRS wave.

89. What is the width of the normal QRS complex?
Not wider than 3 mm.

90. Which of the following waves represents repolarization of the ventricles?
T waves.

91. For which of the following arrhythmias would an electronic pacemaker be indicated?
Third-degree block.

92. Your patient has a normal ECG reading. What does this finding tell you about the patient’s likelihood of having a myocardial infarction in the immediate future?
No predictive value.

93. What is the normal maximum length of the P wave?
3 mm

94. Which of the following clinical conditions is not associated with tachycardia?

95. The electrocardiogram (ECG) is primarily used to what?
To evaluate the patient with symptoms suggestive of acute myocardial disease.

96. What is a possible serious complication associated with atrial fibrillation?
Atrial thrombi.

97. What is the normal period of time for the PR interval?
Not longer than 0.20 seconds.

98. What structure serves as the backup pacemaker for the heart?
The atrioventricular (AV) node.

99. What parameter is measured on the vertical axis of the ECG paper?

100. Is an occasional premature ventricular complex (PVC) a major concern?
No, no it is not.

101. What condition is often associated with right-axis deviation?
Cor pulmonale.

102. The QRS of an ECG falls on a dark vertical line of the ECG paper. Subsequent QRS complexes fall on every other dark line (10 mm apart). What is the ventricular rate?

103. What type of medications may lead to a first-degree heart block?

104. What is Atrial Fibrillation?
Atrial muscle quivers in an erratic pattern that does not result in a coordination contraction. There are no true P waves. The ventricular rate may be abnormal, resulting in an abnormal R-R interval.

105. What is Atrial Flutter?
Rapid depolarization of the atria resulting from ectopic focus that depolarizes at a rate of 250-350 times per minute. The P waves appear similar and there is a sawtooth pattern. There are numerous P waves to each QRS complex. The R-R interval may be normal or vary. The causes of Atrial Flutter may include heart disease, stress, renal failure, and hypoxemia.

106. What is a First-Degree Heart Block?
The PR interval is longer than .20 seconds. There is one P wave before each QRS complex. Typically the QRS complex has a normal configuration. The R-R intervals are regular. It is common following a heart attack that damages the AV node or may be a complication of certain medications such as digoxin or Beta blockers.

107. What is Normal Sinus Rhythm?
An upright P wave that is identical from one complex to the next and has a consistent PR interval. It also has identical QRS complexes no longer than 0.12 seconds. It has a flat ST segment and the R-R interval is regular and does not vary more than 0.12 seconds. The heart rate is between 60-100 bpm.

108. What are Premature Ventricular Contractions?
They cause a unique and bizarre QRS complex which is wider than normal. The is no P wave preceding and may occur as a single event. A PVC may occur at every other beat, every third beat, or as a run, and it often occurs as a result of stress.

109. What is a PR interval?
It is the distance (time) between the start of atrial depolarization and the start of ventricular depolarization.

110. What is the P wave?
The wave of depolarization in the atria.

111. What is the QRS?
The wave of depolarization over the ventricles.

112. What is a Sinus Arrhythmia?
It is recognized by irregular spacing between the QRS complex. The R-R interval varies more than 0.12 seconds. It may occur with the effects of breathing on the heart or as a side effect of medications such as digoxin. Most of the time, it doesn’t need treatment.

113. What is Sinus Bradycardia?
The heart rate is less than 60 bpm, but the rest of the tracing looks normal. This represents a significant clinical problem if it causes the patient’s blood pressure to drop significantly or causes symptoms such as fatigue, lightheadedness, or syncope. It is most often caused by hypothermia or abnormalities in the SA node.

114. What is Sinus Tachycardia?
The heart rate exceeds 100 bpm but the rest of the ECG tracing looks normal.
It is most often caused by anxiety, pain, fever, hypovolemia, or hypoxemia. It may also be caused by certain medications such as bronchodilators.

115. What are the steps to reading an EKG?
(1) Identify the atrial rate, (2) Measure the PR interval, (3) Evaluate the QRS complex, (4) Evaluate the T wave, (5) Evaluate the ST segment, (6) Identify the R-R interval, and (7) Identify the mean QRS axis.

116. What is the ST segment?
It is the time from the end of ventricular depolarization to the start of repolarization.

117. What is a Third Degree Heart Block?
It is the most serious kind. It is indicated when the conduction system between the atria and ventricles is completely blocked, and impulses generated in the SA node are not conducted to the ventricles. There is no relationship between the P waves and the QRS complex. The P-P intervals and the R-R intervals regularly march out, but they have no correlation with each other. The QRS complexes are normal if ventricles are paced by the AV node. The QRS complexes are not normal in the configuration if the ventricles are paced by an ectopic site and are abnormally wide. This heart block is often caused by a myocardial infarction or drug toxicity.

118. What is the T wave?
It is the wave of repolarization over the ventricles.

119. What is the Type one second-degree heart block?
Also known as the Wenckebach or Mobitz type I. It occurs when an abnormality in the AV junction delays or blocks conduction of some of the impulses through the AV node. There is a prolonged PR interval until one impulse does not pass on to the ventricles at all. There is a P wave that is not followed by a QRS complex.

120. An elevated or depressed ST segment is common in what?
A myocardial infarction and is a life-threatening arrhythmia.

121. The interpretation of the ECG is completed by whom?
The doctor.

122. Sinus bradycardia is a clinical problem if what occurs?
If it causes the patient’s blood pressure to decrease significantly or if the patient is symptomatic.

123. Where is atrial repolarization seen on an ECG?
It is obscured by the QRS complex.

124. Why do Respiratory Therapists need to understand EKG’s?
To able to recognize serious arrhythmias, and to be able to respond quickly and appropriately.

Final Thoughts

So there you have it! That wraps up our study guide on EKG’s and interpreting the electrocardiogram. I truly hope that this information was helpful for you and I hope you can use it to ace your exams in RT school. Not only that — I hope the information sticks with you throughout your career as a Respiratory Therapist.

Thank you so much for reading and as always, breathe easy my friend.