An electrocardiogram, often abbreviated as EKG or ECG, is a test that measures the electrical activity of the heart. This is an important diagnostic tool that can be used to detect a number of heart conditions, such as arrhythmias, heart attacks, and other cardiac abnormalities.
In this article, we will provide an overview of EKG interpretation, including some of the most common indications, how it works, and what arrhythmias may be found on an EKG tracing.
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What is an EKG?
An electrocardiogram is a test that measures the electrical activity of the heart. It’s a noninvasive test, which means that it does not involve any needles or injections.
The electrical activity of the heart is produced by the coordinated contraction of the heart’s four chambers. The two upper chambers, known as the atria, contract first. This is followed by the two lower chambers, known as the ventricles.
The timing and sequence of these contractions are important in ensuring that the heart pumps blood effectively.
How is an EKG Measured?
The electrical activity produced by the heart’s contractions can be detected by electrodes that are placed on the patient’s skin.
These electrodes are connected to an EKG machine, which records the electrical activity and produces a graph.
Electrophysiology of the Heart
The electrophysiology of the heart refers to how signals and electrical impulses are conducted within the heart.
There are a number of different parts of the heart that are involved in this process, including the sinoatrial (SA) node, atrioventricular (AV) node, and Purkinje fibers.
The sinoatrial node, also known as the pacemaker, is responsible for setting the heart’s rhythm. The wave of depolarization that originates from the SA node is responsible for causing the atria to contract.
This is known as the P wave on an EKG reading.
The impulse is received by the AV node, which causes a short delay. This delay is interpreted as the PR interval on an EKG reading.
Then the stimulus moves through the bundle of His, through the left and right bundle branches, and into the Purkinje fibers. This produces ventricular depolarization, and contraction occurs, which can be seen as the QRS complex.
Then, the heart enters into a short period of repolarization, which is a period that no electrical activity can be detected. This is known as the ST segment on an EKG reading.
Finally, the heart enters into a period of recovery where the SA node is recharged, and another cycle can begin.
Components of an EKG
There are two primary components of an electrocardiogram:
Electrodes are small, metal discs that are placed on the patient’s skin.
They are responsible for detecting the electrical activity of the heart and transmitting it to the EKG machine.
Leads represent the electrical activity of the heart from a specific angle.
There are six different leads in an EKG reading, which are referred to by the letters I, II, III, aVR, aVL, and aVF.
EKG Chest Electrodes
- 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
Each electrode has an adhesive backing that is placed on the patient’s skin and then connected to the EKG machine.
This allows the machine to detect the electrical activity of the heart and produce a graph that can be interpreted by doctors and medical professionals.
EKG interpretation is the process of analyzing an EKG reading in order to understand the electrical activity of the heart. This involves the following steps:
- Measure the Atrial and Ventricular Rates
- Measure the PR Interval
- Assess the QRS Complex
- Assess the T Wave
- Assess the ST Segment
- Assess the R-R Interval
- Assess the Mean QRS Axis
Step 1: Measure the Atrial and Ventricular Rates
The atrial rate is the number of P waves produced by the atria in one minute. The ventricular rate is the number of QRS complexes produced by the ventricles in one minute.
Step 2: Measure the PR Interval
The PR interval is the time it takes for the electrical impulse to travel from the SA node to the AV node. This is measured from the start of the P wave to the start of the QRS complex.
Step 3: Assess the QRS Complex
The QRS complex is the wave of depolarization that passes through the ventricles. It is typically shorter than 0.12 seconds, which is three small boxes on an EKG.
However, if it is wider than three boxes, it may indicate that the electrical impulse is not passing through the ventricles properly.
Step 4: Assess the T Wave
The T wave is the wave of repolarization that passes through the ventricles. It should be pointed and asymmetrical.
If it is flat or inverted, it’s an indication that the heart is not repolarizing properly, which can occur with ischemia or hyperkalemia.
Step 5: Assess the ST Segment
The ST segment is the period of time between ventricular depolarization and repolarization. It should be level with the baseline.
If it is depressed or elevated, it may indicate that the patient has a serious problem with perfusion, which helps with the delivery of oxygen to tissues.
Step 6: Assess the R-R Interval
The R-R interval is the time it takes for the electrical impulse to travel from the start of one QRS complex to the start of the next. This is measured from the beginning of one R wave to the beginning of the next.
If the R-R interval exceeds 0.12 seconds, it is known as a prolonged interval. This can be an indication of a heart block, which is when the electrical impulses are not passing through the heart properly.
Step 7: Assess the Mean QRS Axis
The mean QRS axis is the average of the QRS complex. It should be between 0 and +90 degrees.
If the mean QRS axis is greater than +90 degrees, it is an indication of right-axis deviation. This is often seen in patients with COPD and cor pumonale.
If the mean QRS axis is between +90 and -90 degrees, it is an indication of left-axis deviation. This is often seen in patients with left ventricular hypertrophy.
Normal sinus rhythm is the rhythm of the heart when all of the electrical impulses are passing through the heart in a regular pattern.
This involves an upright P wave, consistent PR intervals that last from 0.12 to 0.20 seconds, and identical QRS complexes that are no longer than 0.12 seconds.
A normal sinus rhythm also involves flat ST segments, regular R-R intervals, and a heart rate that is between 60-100 beats per minute.
However, if the electrical impulses are irregular, this means that an arrhythmia is present. The most common types include:
- Sinus tachycardia
- Sinus bradycardia
- Sinus arrhythmia
- First-degree heart block
- Second-degree heart block
- Third-degree heart block
- Atrial flutter
- Atrial fibrillation
- Premature ventricular contractions
- Ventricular tachycardia
- Ventricular fibrillation
- Pulseless electrical activity
Each type of arrhythmia found on an EKG tracing has its own specific characteristics. Therefore, medical professionals must be able to identify each type in order to properly diagnose and treat their patients.
Sinus tachycardia is the most common type of arrhythmia. It is defined as a heart rate that is greater than 100 beats per minute. Some common causes include:
The primary treatment method for sinus tachycardia involves addressing the underlying cause. However, if the patient is unstable, medications such as beta blockers or calcium channel blockers may be used in order to slow down the heart rate.
Sinus bradycardia is the opposite of sinus tachycardia. It is defined as a heart rate that is less than 60 beats per minute. Some common causes include:
- Vagus nerve stimulation
- Medications (beta blockers, calcium channel blockers, etc.)
- SA node abnormalities
An abnormally low heart rate is also common in athletes with high levels of fitness and conditioning.
The primary treatment method for sinus bradycardia involves the administration of drugs such as atropine, which helps to stimulate the heart rate.
Sinus arrhythmia occurs when there is irregular spacing between QRS complexes, which is characterized by R-R intervals that are longer than 0.12 seconds.
This arrhythmia can occur as a side effect of medications such as digoxin, and it typically does not require treatment.
First-Degree Heart Block
First-degree heart block occurs when the electrical impulses are delayed as they travel from the SA node to the AV node. This is characterized by a PR interval that is greater than 0.20 seconds.
This arrhythmia is often caused by a myocardial infarction that causes damage to the AV node. It can also occur as a side effect of certain drugs, such as digoxin or beta blockers.
A first-degree heart block typically does not require treatment unless it is symptomatic. In this case, a pacemaker may be necessary.
Second-Degree Heart Block
Second-degree heart block occurs when some, but not all, of the electrical impulses are delayed as they travel from the SA node to the AV node. This is characterized by a pattern of regular P waves followed by irregular QRS complexes.
There are two types of second-degree heart block:
- Type I (Wenckebach)
- Type II
Type I second-degree heart block is caused by a delay in the conduction of electrical impulses through the AV node. This results in progressively longer PR intervals until a QRS complex is not conducted.
This type is typically transient and does not require treatment.
Type II second-degree heart block occurs when there is a blockage in the bundle of His that prevents electrical impulses from being conducted to the ventricles. This results in some P waves that are not followed by a QRS complex.
Both types of second-degree heart block can be caused by a myocardial infarction, and they may require treatment with a pacemaker.
While this type is less common, it is often an indication of a more serious problem, such as ischemia or a myocardial infarction.
Third-Degree Heart Block
Third-degree heart block, also known as complete heart block, occurs when there is a complete blockage of electrical impulses as they travel from the SA node to the AV node.
This is characterized by a complete absence of QRS complexes after each P wave.
Third-degree heart block is a serious condition that requires immediate medical attention. It can be caused by a myocardial infarction or drug toxicity and can result in damage that renders the heart unable to pump blood effectively.
Treatment for a third-degree heart block involves addressing the underlying problem and may require medications that work to speed up ventricular contractions. It may also require a temporary external pacemaker until a permanent one can be placed.
Atrial flutter is an arrhythmia that occurs when electrical impulses travel in a rapid, organized fashion around the atria. This results in a sawtooth pattern on an EKG tracing.
Atrial flutter is often caused by the following:
- Coronary heart disease
- Rheumatic heart disease
- Pulmonary embolism
- Renal failure
This arrhythmia can be treated with medications such as digoxin, calcium channel blockers, or beta blockers. Cardioversion may be required in some cases, which is a procedure that uses an electrical shock to reset the heart’s electrical impulses.
Atrial fibrillation is an arrhythmia that occurs when electrical impulses travel in a rapid, chaotic fashion around the atria. This results in an irregular heartbeat with no true P waves.
Atrial fibrillation is a common arrhythmia, and it often affects people over the age of 60. Its causes are similar to those of atrial flutter and include coronary heart disease, hypertension, and stress.
Treatment for atrial fibrillation typically involves medications that work to slow the heart rate. These include beta blockers, calcium channel blockers, and other anti-arrhythmia medications.
This arrhythmia may also benefit from anticoagulant drugs, which work to prevent blood clots from forming. If the patient does not respond well to these treatments, cardioversion may be necessary.
Premature Ventricular Contractions
Premature ventricular contractions (PVCs) are extra beats that occur in the ventricles. These premature beats can cause the heart to beat too fast, which can result in symptoms such as shortness of breath and chest pain.
PVCs are a relatively common arrhythmia, and they often occur in people with no underlying heart problems. However, PVCs can also be a symptom of more serious conditions such as coronary heart disease or congestive heart failure.
The treatment method for PVCs is typically based on the causes and frequency of the contractions. In some cases, no treatment is necessary.
If the PVCs are frequent or symptomatic, however, treatment must occur quickly in order to avoid the progression of more serious arrhythmias. In this case, anti-arrhythmia medications are often indicated.
Ventricular tachycardia is an arrhythmia that occurs when electrical impulses travel in a rapid, chaotic fashion around the ventricles. This results in a rapid heart rate that can be life-threatening.
Ventricular tachycardia is often caused by the following:
- Coronary heart disease
- Myocardial infarction
- Congestive heart failure
- Hypertensive heart disease
- Untreated obstructive sleep apnea
Ventricular tachycardia can be a very serious arrhythmia, and it requires immediate medical attention. However, if the patient is asymptomatic with recurrent, non-sustained ventricular tachycardia, beta blockers can be administered.
Otherwise, if the patient is symptomatic, cardioversion is typically necessary in order to reset the heart’s electrical impulses.
Ventricular fibrillation is an arrhythmia that results in a zigzag pattern on an EKG tracing and irregular fluctuations. It is characterized by erratic, uncoordinated contractions of the ventricles.
Ventricular fibrillation is a serious arrhythmia that can often lead to death. Its causes are similar to those of ventricular tachycardia and include coronary heart disease, myocardial infarction, and cardiomyopathy.
This arrhythmia is a medical emergency and must be treated with rapid defibrillation, which is a procedure that uses a transthoracic electrical shock to reset the heart’s electrical impulses.
It also typically requires cardiopulmonary resuscitation (CPR), which is a life-saving technique that should be performed by trained medical personnel.
When ventricular fibrillation is present, medical professionals may also administer oxygen therapy and anti-arrhythmic medications while attempting to reverse the underlying causes.
Patients who survive ventricular fibrillation often receive an implantable cardioverter-defibrillator (ICD). It’s a device that is implanted in the chest and monitors the heart for arrhythmias and can reset an abnormal heartbeat back to normal.
Pulseless Electrical Activity
Pulseless electrical activity (PEA) is an arrhythmia that results in a flat line on an EKG tracing. It occurs when the electrical impulses in the heart are not strong enough to result in a heartbeat.
PEA can be caused by a variety of conditions, including:
- Electrolyte imbalance
- Myocardial infarction
- Drug overdose
- Acid-base disturbance
This arrhythmia is a medical emergency, and patients often require CPR and defibrillation. In addition, medications such as epinephrine and vasopressin may be indicated to improve blood pressure and heart function.
EKG Practice Questions:
1. A P-R interval of 0.32 seconds is an indication of what?
It’s an indication of a disturbance in the heart’s conduction pattern.
2. What type of monitor is worn for 24 hours to evaluate the performance of the heart during routine activities?
3. A patient is showing symptoms of a heart attack, and the physician suspects that the left ventricle is involved. Which EKG leads should you assess?
Leads I and aVL, and Leads V5 and V6
4. What do depressed T waves on an EKG tracing represent?
5. What causes elevated ST segments on an EKG tracing?
Myocardial ischemia and certain cardiac medications
6. Every small square on the horizontal axis of the EKG grid paper represents how much time?
7. How many electrophysiologic phases are there in an action potential?
8. Where does the electrical impulse of the heart originate in a normal conduction system?
The SA node
9. In which phase of repolarization is there a slow influx of calcium?
10. Stimulation of the parasympathetic system causes what?
It causes a decreased heart rate and decreased conduction.
11. What does the P wave on an EKG tracing represent?
12. What does the T wave on an EKG tracing represent?
13. What is excitability?
It is the ability of a cardiac cell to reach its threshold potential and respond to a stimulus.
14. What is the normal P-R interval?
It is usually between 0.12 and 0.20 seconds.
15. What part of the autonomic nervous system can cause an increase in heart rate?
Sympathetic nervous system
16. How many times can EKG electrodes be used?
Most types are disposable and should only be used once.
17. Two or more waveforms represent what?
18. What does the term repolarization mean?
It means that cardiac cells are returning to their resting membrane potential.
19. What lead is often viewed when continuous cardiac monitoring is used?
20. Which electrolytes are primarily involved in the cardiac cycle?
Sodium, calcium, and potassium
21. Can a 12-lead EKG be used to determine cardiac output?
22. What is counted vertically on the EKG grid paper?
23. What is the duration of each large square on EKG paper?
24. Which phase can a strong stimulus cause depolarization of cardiac cells?
The relative refractory period
25. Which leads are considered bipolar leads?
Lead I, Lead II, and Lead III
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26. Which leads provide information about the lower surface of the heart?
Lead II, Lead III, and aVF
27. What happens 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+.
28. When do U waves become present during an EKG complex?
The U waves become visible only in the presence of electrolyte imbalances or heart disease.
29. Which electrodes use the center of the heart as the negative reference point?
V1, V3, and V6
30. What is the normal duration for a QRS complex?
31. What leads would be required during continuous EKG monitoring?
Lead I, V2, Lead II, and Lead III
32. What are some good things about an EKG test?
The test is inexpensive, noninvasive, and easy to obtain.
33. Why would an EKG test be ordered on a patient?
Chest pain, shortness of breath, dyspnea, weakness, lethargy, and dizziness
34. What does an EKG measure?
It measures cardiac activity, repolarization, and depolarization.
35. What does an EKG not measure?
It does not measure the force of contraction of the heart.
36. What is the normal pacemaker of the heart?
The SA node
37. What is a segment?
A portion of the baseline
38. What is an interval?
It contains at least one wave.
39. How do you perform the rate calculation on an EKG?
This calculation can be performed by counting the number of large boxes between two R waves and then dividing that number by 300.
40. How many seconds do the small squares represent on an EKG?
41. How many seconds do the large squares represent on an EKG?
42. What is the rate of the SA node?
43. What is the rate of the AV node?
44. What is the rate of the Purkinje fibers?
45. What two nerve fibers in the right atrium can alter the heart rate when stimulated?
Sympathetic and parasympathetic
46. What does the P wave represent?
It represents atrial depolarization (contraction).
47. What does the QRS complex represent?
It represents ventricular depolarization and atrial repolarization.
48. What does the T wave represent?
It represents ventricular repolarization.
49. What does the Q wave represent?
It represents the conduction of an impulse down the interventricular septum.
50. What does the U wave represent?
It represents repolarization of the bundle of His and Purkinje fibers.
51. What is the PR interval?
It is the beginning of the P wave to the beginning of the QRS.
52. What does it mean if the PR interval is increased?
This would indicate a 1st-degree heart block.
53. Which rhythm would you likely see with acute hypoxemia?
54. What happens with the P waves and QRS during a third-degree heart block?
There is no relationship between the P waves and the QRS complexes during a third-degree heart block.
55. What is implied by an abnormally prolonged PR interval?
An atrioventricular block
56. Why is the electrical impulse temporarily delayed at the atrioventricular (AV) node?
To allow filling of the ventricles
57. What medication is used to treat sinus bradycardia?
58. What term is used to describe the ability of certain cardiac cells to depolarize without stimulation?
59. Which EKG abnormality is life-threatening?
An elevated ST segment
60. An EKG rhythm strip shows a sawtooth-like pattern occurring at a rate of 270/min. What is the most likely interpretation?
61. What is suggested by inverted T waves on an EKG?
62. What structure normally paces a healthy heart?
The sinoatrial (SA) node
63. Is atrial flutter considered to be a life-threatening arrhythmia?
64. Is a narrow QRS a common characteristic of a premature ventricular complex (PVC)?
65. Is pericarditis a common cause of ventricular tachycardia?
66. In what part of the cardiac conduction system does the electrical impulse travel most rapidly?
The Purkinje fibers
67. What medication is most useful for the treatment of premature ventricular contractions?
68. An EKG that has one P wave for every QRS complex with a PR interval that is 0.30 seconds would represent what?
First-degree heart block
69. What wave represents depolarization of the ventricles?
The QRS wave
70. What is the width of a normal QRS complex?
Not wider than 3 mm
71. Which waves represents repolarization of the ventricles?
72. What arrhythmia would an electronic pacemaker be indicated for treatment?
Third-degree heart block
73. What is the normal maximum length of a P wave?
74. What is an electrocardiogram primarily used for?
To evaluate a patient with symptoms suggestive of acute myocardial disease
75. What is possibly a serious complication associated with atrial fibrillation?
76. What is the normal period of time for the PR interval?
Not longer than 0.20 seconds
77. What structure serves as the backup pacemaker for the heart?
The atrioventricular (AV) node
78. How is voltage measured on EKG paper?
The vertical axis
79. Is an occasional premature ventricular contraction (PVC) a major concern?
No, it is not a major concern.
80. What condition is often associated with right-axis deviation?
81. What type of drugs can lead to a first-degree heart block?
82. What is atrial fibrillation?
It is an arrhythmia characterized by atrial muscle quivers in an erratic pattern that does not result in a coordinated contraction. There are no true P waves, and the ventricular rate may be abnormal, resulting in an abnormal R-R interval.
83. What is atrial flutter?
It is an arrhythmia characterized by rapid depolarization of the atria that results from ectopic focus that depolarizes at a rate of 250-350 beats per minute. The P waves appear similar, and there is a sawtooth pattern. There are numerous P waves to each QRS complex, and the R-R interval may be normal, or it can vary.
84. What is a first-degree heart block?
It is an arrhythmia characterized by a PR interval that is longer than 0.20 seconds. There is one P wave before each QRS complex, but the QRS complex typically has a normal configuration. The R-R intervals are regular, and this arrhythmia is common following a heart attack that damages the AV node, or it can occur as a complication of certain medications, such as digoxin or beta blockers.
85. What is a premature ventricular contraction?
It is an irregular rhythm characterized by a unique and bizarre QRS complex that is wider than normal.
86. What is a PR interval?
It is the distance (time) between the start of atrial depolarization and the start of ventricular depolarization.
87. What does the P wave represent?
It represents the wave of depolarization in the atria.
88. What does the QRS represent?
It represents the wave of depolarization over the ventricles.
89. What is sinus arrhythmia?
It is an arrhythmia that is recognized by irregular spacing between the QRS complex. The R-R interval varies by more than 0.12 seconds, and it may occur due to the effects of breathing on the heart or as a side effect of medications such as digoxin.
90. What is sinus bradycardia?
It is an arrhythmia where the heart rate is less than 60 beats/min, but the rest of the EKG tracing appears normal.
91. What is sinus tachycardia?
It is an arrhythmia where the heart rate exceeds 100 beats/min, but the rest of the EKG tracing appears normal.
92. 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
93. What is the ST segment?
It is the time from the end of ventricular depolarization to the start of repolarization.
94. Which type of heart block is the most serious?
Third-degree heart block is considered to be the most serious.
95. What does the T wave represent?
It is the wave of repolarization of the ventricles.
96. When is an elevated or depressed ST segment common?
It is common during a myocardial infarction and should be interpreted as a life-threatening arrhythmia.
97. The interpretation of an EKG is completed by which medical professional?
98. Sinus bradycardia is a clinical problem if what occurs?
It is a serious problem if it causes the patient’s blood pressure to decrease significantly or if the patient is symptomatic.
99. What are the adhesive disks that stick to a patient’s chest during an EKG?
100. Why do respiratory therapists need to understand EKGs?
They must be able to recognize serious arrhythmias in order to respond quickly and appropriately.
An electrocardiogram is a noninvasive test that is used to assess the electrical activity of the heart. Respiratory therapists, nurses, and other medical professionals are often responsible for performing this test in order to diagnose arrhythmias and other cardiac conditions.
Therefore, EKG interpretation is a very important skill for medical practitioners to learn and understand. It’s also important to be familiar with the different types of arrhythmias and how they can be treated.
We have a similar guide on cardiac electrophysiology that I think you’ll find helpful. Thanks for reading, and, as always, breathe easy, my friend.
John Landry is a registered respiratory therapist from Memphis, TN, and has a bachelor's degree in kinesiology. He enjoys using evidence-based research to help others breathe easier and live a healthier life.
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- Ashley, Euan. “Conquering the ECG – Cardiology Explained – NCBI Bookshelf.” National Center for Biotechnology Information, U.S. National Library of Medicine, 2004, www.ncbi.nlm.nih.gov/books/NBK2214.
- “What Is an Electrocardiogram (ECG)?” National Center for Biotechnology Information, U.S. National Library of Medicine, 31 Jan. 2019, www.ncbi.nlm.nih.gov/books/NBK536878.
- “Electrocardiogram Interpretation and Arrhythmia Management: A Primary and Secondary Care Survey.” National Center for Biotechnology Information, U.S. National Library of Medicine, 30 Mar. 2016, www.ncbi.nlm.nih.gov/pmc/articles/PMC4838440.