Pulmonary Function Testing (PFT) refers to a set of noninvasive tests that are used to examine a patient’s respiratory system and identify the severity of pulmonary impairment. The tests measure how well the lungs are able to take in and release air, and how efficiently they are able to exchange oxygen and carbon dioxide.

In this guide, we will discuss the different types of PFTs, what they assess, and how they are performed.

We will also explore some key considerations associated with PFTs, including patient preparation, proper technique, and interpretation of the pulmonary test results. So, if you’re ready, let’s get into it.

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What is Pulmonary Function Testing?

Pulmonary Function Testing (PFT) is a set of noninvasive tests that are used to assess various aspects of a patient’s lung function.

This includes the measurement of lung volume, capacity, flow rate, and gas exchange in order to help healthcare providers come up with an accurate diagnosis and determine the best course of medical intervention.

Indications

There are many different reasons to perform pulmonary function tests, including the following:

  • To assess changes in a patient’s pulmonary function
  • To evaluate the effectiveness of treatment
  • To diagnose pulmonary diseases
  • To assess the progression of pulmonary diseases
  • To differentiate between obstruction and restrictive diseases
  • To differentiate types of obstructions
  • To assess the risk for surgical procedures
  • To assess postoperative complications
  • To measure the effects of occupational exposure
  • To evaluate level of fitness
  • To evaluate impairment or disability

Keep in mind that each pulmonary function test has different indications. These are just a few generals examples that can be applied to various types of tests.

Contraindications

PFTs are generally safe for most patients. However, they may be contraindicated in certain situations, such as the following:

  • Recent myocardial infarction
  • Unstable angina
  • Uncontrolled hypertension
  • Hemoptysis
  • Pneumothorax
  • Acute pulmonary embolism
  • Acute asthmatic attack
  • Acute COPD exacerbation
  • Recent cataract surgery

Again, each type of PFT may involve different risks and contraindications. These are just a few general examples of when pulmonary function tests should not be performed.

Types of Pulmonary Function Tests

There are several different types of PFTs, each of which is used to assess a specific aspect of lung function. Some examples include:

  • Spirometry
  • Lung volume measurements
  • Forced vital capacity (FVC)
  • Maximal voluntary ventilation (MVV)
  • Maximal inspiratory/expiratory pressures (MIP/MEP)
  • Body plethysmography
  • Diffusing capacity
  • Cardiopulmonary exercise tests
  • Bronchoprovocation tests

Each test is performed using specialized equipment, and the results are interpreted by doctors and respiratory therapists to gain insights about the health of a patient’s lungs.

Spirometry

Spirometry is a pulmonary function test that measures a patient’s breathing patterns and lung functionality. It uses a spirometer, which is a diagnostic device that measures the volume and speed of airflow during a breathing cycle.

Spirometry is used to diagnose lung conditions such as chronic obstructive pulmonary disease (COPD), restrictive lung diseases, and other disorders that affect lung function. In addition, this test is also used to assess how well a medication is working in patients with asthma.

Spirometry patient with spirometer device

Lung Volume Measurements

Pulmonary function testing often involves the measurement of a patient’s lung volumes. This includes:

  • Tidal volume
  • Inspiratory reserve volume
  • Expiratory reserve volume
  • Residual volume

Various techniques can be used to measure lung volumes, including nitrogen washout, helium dilution, and body plethysmography.

Forced Vital Capacity

Forced vital capacity (FVC) is a common pulmonary function test that measures the total volume of air that can be rapidly and forcefully exhaled after a maximum inspiration.

It is helpful in determining if that patient has an obstructive or restrictive lung condition. The maneuver is highly effort-dependent and requires continuous coaching from the respiratory therapist in order to achieve reliable results.

Body Plethysmography

Body plethysmography is a pulmonary function test that measures the amount of air that is inhaled and the amount of air that remains in the lungs after exhalation.

It is helpful in assessing how a disease is affecting a patient’s lung function. It can also help confirm the presence of a restrictive lung disease that may significantly reduce the total lung volume.

In addition, body plethysmography is one of the only tests capable of measuring residual volume.

Body plethysmography

Maximal Voluntary Ventilation

Maximal voluntary ventilation (MVV) is a pulmonary function test that measures the amount of air a person can inhale and exhale in a short period of time. It requires the patient to breathe as deep and fast as possible for 12 seconds.

This test is used to assess the overall function of the respiratory system. Decreased MVV results is an indication that an obstructive disease is present.

Maximal Inspiratory/Expiratory Pressures

The maximal inspiratory and expiratory pressure (MIP/MEP) measurements are obtained to assess a patient’s respiratory muscle strength. They measure the force at which a patient can inhale and exhale, respectively.

MIP is the lowest pressure that can be generated during a maximal inspiratory effort against an occluded airway. MEP is the highest pressure that can be generated during a maximal expiratory effort against an occluded airway.

Diffusion Capacity

The diffusing capacity for carbon monoxide (DLCO) is a pulmonary function test that measures the amount of gas exchange taking place in the alveoli of the lungs.

During the test, the patient inhales a low concentration of carbon monoxide (CO) and a tracer gas and performs a breath-hold maneuver for 10 seconds. This allows doctors and respiratory therapists to assess the patient’s diffusion capacity.

The DLCO test is often indicated to monitor pneumoconiosis, evaluate obstructive diseases, and monitor changes in lung function.

Cardiopulmonary Exercise Tests

Cardiopulmonary exercise tests are often used to assess a patient’s oxygen uptake, carbon dioxide production, and heart rate during physical activity. These measurements can be helpful in determining the severity of a respiratory disease.

An example is the 6-minute walk test (6MWT), which measures the total distance a patient can walk in 6 minutes. The distance traveled has a correlation with the clinical outcome of multiple pulmonary diseases.

Cardiopulmonary Exercise Test

Bronchoprovocation Test

A bronchoprovocation test can be used to assess a patient’s airway hyperresponsiveness, which is the narrowing of the airways in response to certain triggers, such as allergens and irritants.

One example is the methacholine challenge test in which a patient inhales increasing concentrations of methacholine in order to mimic the effects of asthma. If there are significant changes in the patient’s FEV1, it helps with the diagnosis of asthma.

Types of Lung Diseases

There are two primary types of lung diseases:

  1. Obstructive
  2. Restrictive

An important aspect of pulmonary function testing is being able to recognize the type of lung disease. This will allow the practitioner to provide or recommend the appropriate forms of treatment.

Obstructive

Obstructive lung diseases include conditions that make it difficult to exhale air out of the lungs. They cause the patient to experience shortness of breath due to narrowing of the airways within the lungs, which results in decreased flow rates.

A simple way to memorize the obstructive lung diseases is use the CBABE mnemonic, which stand for cystic fibrosis, bronchiectasis, asthma, bronchitis (chronic), and emphysema.

Restrictive

Restrictive lung diseases include conditions that make it difficult for patients to fully fill their lungs with air on inhalation. Essentially, their lungs are restricted from fully expanding, which results in decreased lung volumes.

As a general rule of thumb, any pulmonary condition that isn’t classified as one of the obstructive disease can be considered a restrictive lung disease.

Pulmonary Function Testing Normal Values

A patient with normal, healthy lungs will achieve pulmonary function test results with the following ranges:

  • FEV1: > 80% of predicted
  • FEV1/FVC%: ≥ 70%
  • Forced Vital Capacity (FVC): > 80% of predicted
  • Slow Vital Capacity (SVC): > 80% of predicted
  • Airway Resistance (RAW): 0.6 – 2.4 cmH2O/L/sec
  • Diffusing Capacity for Carbon Monoxide (DLCO): 20 – 25 mL/CO/min/mmHg
  • Peak Expiratory Flow Rate (PEFR): 10 L/sec
  • Exhaled Carbon Monoxide: < 7 for nonsmokers

If a patient’s PFT results are not within the normal value ranges, it helps determine the type and severity of their respiratory condition.

Pulmonary Function Testing Equipment

Pulmonary function testing requires the use of several different types of devices, machines, and equipment, including:

  • Spirometer
  • Peak flowmeter
  • Body plethysmograph
  • Pulmonary gas analyzer
  • Oxygen analyzer
  • Gas-conditioning device
  • Blood gas analyzer
  • Silverman pneumotachometer

As technology continues to develop each year, new PFT equipment is constantly being implemented into clinical use. These are just a few of the most common examples.

Pulmonary Function Testing Practice Questions:

1. What are the contraindications of pulmonary function testing?
Hemoptysis, Pneumothorax, Myocardial Infarction, Pulmonary embolism, Acute chest or abdominal pain, Nausea, Vomiting, Recent cataract removal surgery, Patients with dementia or confusion may not achieve optimal or repeatable results.

2. What are the indications of pulmonary function testing?
To identify and quantify changes in pulmonary function, To evaluate the need and quantify therapeutic effectiveness, To perform epidemiologic surveillance for pulmonary disease, To assess patient for risk of postoperative pulmonary complications, and To determine pulmonary disability.

3. What are the three categories of pulmonary function tests?
Dynamic flow rates of gases though the airways, Lung volumes and capacities, and The ability of the lungs to diffuse gases.

4. What are the three components to pulmonary function testing?
(1) Performing spirometry for measuring airway mechanics, (2) Measuring lung volumes and capacities, and (3) Measuring the diffusing capacity of the lung (DL).

5. What are the two general types of measuring instruments?
Instruments that measure gas volume, and instruments that measure gas flow.

6. What are the two major categories of pulmonary disease?
Obstructive pulmonary disease and Restrictive pulmonary disease.

7. What can disqualify a Forced Vital Capacity trial?
Cough, an inspiration, Valsalva maneuver, a leak, or an obstructed mouthpiece.

8. What does a decrease in flow rate signify?
An increase in airways resistance and the presence of airway obstruction when patient effort creating the difference between mouth pressure and lung pressure is constant.

9. What does an FEV₁/FVC of less than 70% indicate?
An obstructive impairment.

10. What is Airway resistance (Raw)?
The difference in pressure between the ends of the airways divided by the flow rate of gas moving through the airway.

11. What is the accuracy of flow?
95%

12. What is the accuracy of volume?
± 3% or within 50 mL or reference value

13. What is the minimum objective standard for FEV₁?
025 L

14. What is the most commonly measured lung volume?
Vital Capacity,

15. What is the most commonly performed test of pulmonary mechanics?
Forced Vital Capacity.

16. What is the normal tidal volume amount for an adult?
500 to 700 mL.

17. What is the normal value for ERV?
1200 ml

18. What is the normal value for IRV?
3100 ml

19. What is the normal value for peak expiratory flow?
9.5 L

20. What is the normal value for RV?
1200 ml

21. What is the normal value for TLC?
6000 ml

22. What is the normal value for VC?
4800 ml

23. What is the normal value of FRC?
2400 ml

24. What is the normal value of Inspiratory Capacity?
360 ml

25. What is the predicted normal FEV₁ for a 20 year old man?
4.7 L

26. What is the predicted normal FVC for a 20 year old man?
5.60 L

27. What is the primary problem in obstructive pulmonary disease?
An increase in airway resistance.

28. What is the primary problem in restrictive lung disease?
Reduced lung compliance, thoracic compliance, or both.

29. What is the primary problem with Obstructive Pulmonary disease?
An increase in airway resistance, An increase in total lung capacity, An increase in functional residual capacity, A decrease in flow.

30. What is the primary problem with Restrictive Pulmonary disease?
Reduced compliance, thoracic compliance or both. An inability for the lungs to expand, a reduction in volumes.

31. What is the primary purpose of pulmonary function testing?
To identify pulmonary impairment and to quantify the severity of pulmonary impairment if present.

32. What makes up the Functional Residual Capacity?
Expiratory Reserve volume (ERV) and the Residual Volume (RV).

33. What pulmonary test poses the greatest risk for fainting?
MVV

34. What test is used to measure FRC?
A diffusion test.

35. What type of gas is used in a diffusion test?
Carbon monoxide.

36. When a patient performs a FVC how much volume in % must be exhaled in 1 second?
70%

37. When are flow rates measured?
Because the radius of the airways normally lessens slightly during expiration, flow rates are usually measured during expiration.

38. When is a dilution test used?
It is used in patients with an obstructive lung disease.

39. When performing pulmonary function testing, what establishes test validity?
It strictly follows testing procedures, it ensures patient effort and performance, and it ensures equipment accuracy and calibration.

40. When will performing a bronchial provocation be indicated?
When the patient’s history suggests episodic symptoms of hyperactive airways and airway obstruction, such as seasonal or exercise induced wheezing.

41. Why is Vital Capacity reduced in restrictive lung disease?
Because the patient’s inhaled volume is reduced.

42. Airway collapse most likely occurs when?
It most likely occurs during forced exhalation.

43. What are some examples of combined Obstructive/Restrictive diseases?
Sarcoidosis, pregnancy, and asthma.

44. What is the % FEV1?
It is the amount of gas expired in 1 second; decreased indicates large airway obstruction.

45. What is the % FEV3?
It is the amount of gas expired in 3 seconds; decreased indicates small airway obstruction.

46. What is a description of Obstructive Diseases?
They have decreased flows and increased airway resistance.

47. What is the PFT hallmark for obstructive diseases?
They have decreased flows.

48. What is the PFT hallmark for restrictive diseases?
They have decreased volumes.

49. What is the main purpose of pulmonary function tests?
They are used mainly to differentiate between obstructive or restrictive diseases.

50. What is a description of Restrictive Diseases?
They have decreased volumes and decreased lung compliance.

51. What are the tests used to determine FRC and RV?
Nitrogen wash; Helium Dilution; Body Box. If the patient can’t do spirometry, they can’t perform these tests.

52. What are the three methods to determine FRC?
Nitrogen Washout, Helium Dilution, and Body Box.

53. What are the values that will remain normal for restrictive diseases?
Vt, FEV1, FEF200, and PEFR.

54. Because of the potential for acute hyperventilation and fainting or coughing, the patient should be in what position?
The seated position.

55. What is the Best curve?
The trial that meets the acceptability criteria and gives the largest sum of FVC plus FEV₁.

56. What is a Capacity?
The range or limits of how much it can measure.

57. What is Compliance?
The volume of gas inspired per the amount of inspiratory effort; effort is measured as the amount of pressure created in the lung of in the pleural space when the inspiratory muscles contract

58. What correlation exists between the measurements of pulmonary mechanics and age?
A negative correlation.

59. What correlation exists between the measurements of pulmonary mechanics and height?
A positive correlation.

60. What is Diffusing capacity of the lung (DL)?
The number of milliliters of gas that transfer from the lungs to the pulmonary blood per minute for each 1 mm Hg partial pressure difference between the alveoli and pulmonary capillary blood.

61. What is Diffusing capacity of the lung for carbon monoxide (DLCO)?
The difference between the volume of carbon monoxide inhaled and the volume of carbon monoxide exhaled, considering the partial pressure of carbon monoxide in the lung at the time of measurement.

62. An exhalation of how long is acceptable for children younger than 10 years?
3 seconds.

63. What is Expiratory reserve volume (ERV)?
The total amount of gas that can be exhaled from the lung after a quiet exhalation.

64. What is Extrapolated Volume?
The volume exhaled before the zero time point

65. The FEF 25% to 75% is a measure of what?
The flow during the middle portion of FVC, or the time necessary to exhale the middle 50%.

66. The Fleisch pneumotachometer measures what?
It measures the changes in pressure as gas flows through a minimal, constant resistance according to the formula: V=∆P/R.

67. Flow measuring devices are commonly called what?
Pneumotachometers.

68. What is a Flow Volume Loop?
It is the forced expiratory VC sometimes is followed by a forced inspiratory VC to produce a complete image of forced breathing.

69. For baseline testing, patients should temporarily abstain from what?
Bronchodilator medications, Short-acting bronchodilators, Long-acting beta-agonist bronchodilators, Oral therapy with aminophylline should be stopped for 12 hours.

70. What is a Forced Vital Capacity?
The maximum volume of gas that the subject can exhale as forcefully and as quickly as possible.

71. For tests of pulmonary function, what four important general principles should be considered?
Test specificity, Sensitivity, Validity, Reliability.

72. For volume measurements, standard reference values are provided by a?
Graduated 3.0 L calibration syringe.

73. What is the Functional residual capacity (FRC)?
It is the total amount of gas left in the lungs after a resting expiration.

74. FVC is an effort dependent maneuver that requires what?
It requires careful patient instruction, understanding, coordination, and cooperation.

75. FVC maneuver must be completely exhaled or an exhalation time of at least?
6 seconds for adults and children over the age of 10.

76. How can airway radius be reduced?
Excessive contraction of the bronchial and bronchiolar muscles (Bronchospasm), Excessive secretions in the airways, Swelling of the airway mucosa, Airway tumors, Collapse of the bronchioles.

77. How do you know if there is a leak in the system when using Nitrogen washout?
The test must occur in a leak proof circuit because the presence of any air increase the measured nitrogen percentage and results in grossly elevated measurements of lung volumes.

78. How long does a patient with obstructive lung disease require to equilibrate?
20 minutes because of slow gas mixing in the lung. In normal patients, equilibration occurs in about 2 to 5 minutes.

79. How many attempts must be done when similar readings to indicate that the FVC is valid?
3 attempts.

80. How often should the machine be calibrated?
At least daily, although best practice in many labs is to verify accuracy before each test subject.

81. How to calculate minute volume/ventilation?
Respiration Rate x tidal volume = ventilation.

82. How to calculate the % of error?
% error= mean measured value – reference value÷ reference value x 100.

83. What do you use to calibrate the machine?
Use a 3.0 L calibration syringe.

84. How to measure residual volume?
When the FRC is known, RV can be calculated as the difference between FRC and ERV.

85. How to measure TLC?
RV + VC

86. If a person cannot exhale at least 70% of their VC in 1 second, what is the problem?
There must be an obstruction.

87. What is Inspiratory Capacity (IC)?
Maximum amount of air that can be inhaled from the resting end expiratory level or FRC; sum of the tidal volume and inspiratory reserve volume.

88. What is Inspiratory Reserve Volume (IRV)?
Maximum volume of air that can be inhaled after normal quiet inspiration.

89. In what patients may the validity of measuring the forced vital capacity be hindered?
Acutely ill patients or who have recently smoked a cigarette.

90. Is tidal volume alone a valid indicator of the type of lung disease?
No, no it is not.

91. Is TLC increased or decreased in obstructive lung disease?
Increased

92. Is TLC increased or decreased in restrictive lung disease?
Decreased

93. What is Maximal Voluntary Ventilation?
An effort dependent test for which the patient is asked to breathe as deeply and as rapidly as possible for at least 12 seconds. MVV is a test that reflects patient cooperation and effort, the ability of the diaphragm and thoracic muscles to expand the thorax and lungs, and airway patency.

94. Measuring pulmonary mechanics is essentially assessing what?
The ability of the lungs to move large volumes of air quickly through the airways in order to identify an airway obstruction.

95. Measuring what can identify the destruction of alveolar tissue or the loss of functioning alveolar surface area?
The diffusing capacity of the lung for carbon monoxide (DLCO).

96. What is an obstructive pulmonary disease?
Any respiratory disease characterized by decreased airway size and increased airway secretions.

97. What is Peak Expiratory Flow Rate (PEFR)?
The maximum expiratory flow rate in L/sec.

98. Plethysmography techniques apply what law?
Boyle’s Law.

99. Pneumotachometers measure what?
They measure flow.

100. What are pulmonary mechanics?
The measurements of FVC, FEV₁, several FEF values, forced inspiratory flow rates, and MVV.

100. Reduced lung compliance is a result of what?
Alveolar inflammation, Pulmonary fibrosis, Neoplasms in the alveoli, A reduced thoracic compliance may be the result of thoracic wall abnormalities such as kyphoscoliosis, Neuromuscular diseases by affecting the function of inspiratory muscles.

101. The residual volume represents how much of the TLC?
20%

102. What is Residual volume?
Volume of gas remaining in the lungs after a complete exhalation.

103. What is a restrictive pulmonary disease?
A broad category of disorders with widely variable etiologies but all resulting in a reduction in lung volumes, particularly the inspiratory and vital capacities; categorized according to origin skeletal/thoracic, neuromuscular, pleural, interstitial and alveolar.

104. Reversibility is defined as what?
A 15% or greater improvement in FEV₁ and at least a 200 mL increase in FEV₁. Reversibility of the airway obstruction indicates that the therapy was effective.

105. The severity of pulmonary impairment is based on what?
A comparison of each patients measurement with the predicted normal value for the patient.

106. The speed of expansion represents what?
The flow rate.

107. What is a spirometer?
Sometimes used as a generic term for all volume measuring and flow measuring devices.

106. To ensure validity, each patient must perform a minimum of how many acceptable FVC maneuvers?
3

107. What is Total lung capacity?
The total amount of gas in the lungs after a maximum inspiration.

108. The two largest repeated measurements should agree within?
5%

109. A typical normal value for a 20 year old healthy man is?
40 ml/min/mm Hg.

110. The validity of MVV depends on what?
The duration of the maneuver, which should be at least 12 seconds; the breathing frequency, which should be at least 90/min; and the average volume, which should be at least 50% of FVC.

111. The validity of PEF rate is based on what?
A preceding inspiration to total lung capacity and a maximal effort.

112. What is Vital Capacity?
The total amount of air that can be exhaled after a maximum inspiration; the sum of the inspiratory reserve volume, the tidal volume and the expiratory reserve volume.

113. Volume measuring devices are specifically called what?
Spirometers — they include water sealed, bellows, and dry rolling seal types. These devices expand as they collect gas volumes.

114. What are the indications of pulmonary function testing?
To identify and quantify changes in pulmonary function, To evaluate need and quantify therapeutic effectiveness, To perform epidemiologic surveillance for pulmonary disease, To assess patient for risk of postoperative pulmonary complications, To determine pulmonary disability

115. What are the three categories of pulmonary function tests measuring?
Dynamic flow rates of gases through the airways, Lung volumes and capacities, The ability of the lungs to diffuse gases.

116. What are the three components to pulmonary function testing?
(1) Performing spirometry for measuring airway mechanics, (2) Measuring lung volumes and capacities, and (3) Measuring the diffusing capacity of the lung.

117. What are the two general types of measuring instruments?
(1) Instruments that measure gas volume, and (2) Instruments that measure gas flow.

118. What does an FEV₁/FVC of less than 70% indicate?
An obstructive impairment.

119. What is the most commonly measured lung volume?
Vital Capacity (VC).

120. What is the most commonly performed test of pulmonary mechanics?
Forced Vital Capacity (FVC).

121. What is the normal tidal volume amount for an adult?
500 to 700 mL.

122. What is the primary problem with a restrictive lung disease?
Reduced lung compliance, thoracic compliance, or both.

123. What is the primary problem with an obstructive pulmonary disease?
An increase in airway resistance, An increase in total lung capacity, An increase in functional residual capacity, A decrease in flow.

FAQ

What is Pulmonary Function Testing Used for?

Pulmonary function testing is used to assess a variety of respiratory conditions, including both obstructive and restrictive lung diseases. It can also be used to monitor disease progression and determine effectiveness of treatment.

Some common pulmonary function tests include spirometry, lung volumes, lung capacities, maximal voluntary ventilation, MIP/MEP, and body plethysmography.

What is the Purpose of Pulmonary Function Testing?

The primary purpose of pulmonary function testing is to provide a detailed assessment of a patient’s respiratory health, which can be used to diagnose and monitor various respiratory conditions.

By evaluating lung function and airway hyperresponsiveness, doctors are able to make more accurate diagnoses and determine the most appropriate treatment strategies for each patient. Additionally, pulmonary function testing can help track disease progression and evaluate the effectiveness of various treatments.

What is DLCO in Pulmonary Function Testing?

DLCO, or the diffusing capacity for carbon monoxide, is a common pulmonary function test that measures gas exchange in the lungs. It can help diagnose various lung conditions and monitor disease progression over time.

How Do You Read Spirometry or PFT Results?

Spirometry results are typically presented as a graph or table, and the values are compared to normal ranges for age, sex, height, and other factors.

In general, low values indicate decreased lung function, while high values may be due to air trapping in the lungs or increased effort required to breathe. The interpretation of spirometry or PFT results is typically performed by pulmonologist or respiratory therapist.

Final Thoughts

Pulmonary function testing plays an essential role in the diagnosis and management of respiratory conditions. By measuring various aspects of lung function, doctors and respiratory therapists are able to diagnose and treat patients with obstructive and restrictive lung diseases.

If you found this to be helpful, we have a similar guide on mechanical ventilation that I think you’ll find helpful. Thanks for reading and, as always, breathe easy, my friend.

Medical Disclaimer: This content is for educational and informational purposes only. It is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Please consult with a physician with any questions that you may have regarding a medical condition. Never disregard professional medical advice or delay seeking it because of something you read in this article. We strive for 100% accuracy, but errors may occur, and medications, protocols, and treatment methods may change over time.

References

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