Intubation Procedure Overview with Practice Questions

Intubation: Overview and Practice Questions

by | Updated: Dec 14, 2023

Intubation is the process of inserting a tube into the trachea (windpipe) in order to establish an airway for mechanical ventilation. This creates a passageway between the patient’s lungs and the ventilator so that air can move in and out of the lungs.

In this article, we will provide an overview of intubation, including its purpose, how it is performed, and some potential complications. We included helpful practice questions on this topic as well.

Free Cheat Sheet and Quiz Bundle Respiratory Therapy Zone

Get access to 25+ premium quizzes, mini-courses, and downloadable cheat sheets for FREE.

Free Cheat Sheet and Quiz Bundle Respiratory Therapy Zone

What is Intubation?

Intubation is the process of inserting a tube into the trachea to establish an airway for breathing support from a mechanical ventilator.

The tube serves an important role by creating a link between the patient and machine so that gas exchange can occur.

During the intubation procedure, patients are administered anesthesia and muscle relaxants to minimize discomfort during the insertion of the tube.


There are several reasons why intubation may be necessary. Some common indications include:

  • Apnea
  • Respiratory failure
  • Severe hypoxemia
  • Decreased consciousness
  • Change in mental state
  • Airway injury
  • Airway obstruction
  • Risk of aspiration
  • Cardiac arrest
  • Hemodynamic instability
  • Cardiopulmonary resuscitation

A patient cannot survive without oxygenation and ventilation. Therefore, there are no absolute contraindications for intubation and mechanical ventilation.

However, the only true contraindication is when a patient specifically and legally refuses to receive life support. This is known as an advance directive.

endotracheal intubation illustration

Types of Intubation

There are two primary types of intubation:

  1. Oral intubation
  2. Nasal intubation

There are different clinical scenarios in which one type of intubation may be preferred over the other.

Oral Intubation

During oral intubation, the tube is inserted through the mouth and down into the trachea. This is the preferred type of intubation due to the following advantages:

  • Easier and less traumatic insertion
  • Easier to suction
  • Larger tube can be inserted
  • Less airway resistance
  • Decreased work of breathing
  • Less tube kinking
  • Easier to insert a bronchoscope
  • Less nasal trauma

However, oral intubation does come with its set of disadvantages. For example, the is a greater risk of mainstem intubation, which is when the tube is inserted too far and enters the right mainstem bronchus.

This would result in the ventilation of only the right lung.

Oral intubation also has a greater risk of self-extubation in which the patient inadvertently removes the tube. There is also a greater risk of vomiting, aspiration, oral trauma, and tube occlusion (e.g., biting).

Nasal Intubation

Nasal intubation is when the tube is inserted through the nose and down into the trachea. This type is generally performed when oral intubation is not possible, such as when the patient has an obstruction, difficult airway, or oral condition.

The advantages of nasal intubation include:

  • More comfortable for long-term use
  • Less gagging
  • Ability to communicate
  • Better oral hygiene
  • Less tube biting
  • Less oral complications
  • Decreased risk of self-extubation
  • Reduced risk of mainstem intubation
  • Better nutritional support
  • Does not require sedatives or muscle relaxants

However, there are some disadvantages associated with nasal intubation as well. For example, the nasal intubation procedure is more difficult to perform and a smaller tube must be used.

It can also result in nasal complications, such as sinusitis, epistaxis, and otitis.

In addition, suctioning is more difficult and there is an increased work of breathing and airway resistance with nasal intubation.

Intubation Equipment

intubation equipment

In order to successfully perform endotracheal intubation, the proper equipment must be available. This includes:

  • PPE (gloves, gowns, masks, face shields, etc.)
  • Oxygen flowmeter and delivery source
  • Suction machine and vacuum source
  • Sterile suction catheters
  • Yankauer tip suction catheter
  • Bag valve mask
  • Colorimetric CO2 detector
  • Laryngoscope blade
  • Endotracheal tubes (multiple sizes)
  • Stylet
  • Stethoscope
  • Tape or tube holder
  • 10 mL syringe
  • Magill forceps
  • Water-soluble lubricating gel

Each piece of intubation equipment serves a specific purpose and plays an important role in the success of the procedure.

What is an Endotracheal Tube?

endotracheal tube intubation illustration

An endotracheal tube is a plastic tube that is inserted through the mouth or nose and down into the trachea.

This process is known as endotracheal intubation.

The ET tube is then secured in place and provides a link between the patient and the ventilator. This helps maintain an open airway so that the patient can receive positive-pressure breaths.

Intubation Procedure

The endotracheal intubation procedure involves the following steps:

  1. Verify the doctor’s order. If it’s an emergency situation, they will likely be present.
  2. Assemble and prepare all the equipment that is needed. This involves testing each piece to ensure that it’s functioning properly.
  3. Administer sedatives or paralytics if indicated.
  4. Hyperoxygenate the patient with 100% oxygen for 2-3 minutes using a manual resuscitator before attempting to insert the endotracheal tube.
  5. Position the patient in the sniffing position, which is achieved by flexing the neck and extending the head.
  6. Insert the laryngoscope blade into the mouth at a slight angle. Lift and displace the tongue out of the way to visualize the vocal cords.
  7. Advance the endotracheal tube through the vocal cords and into the trachea within 15 seconds.
  8. Remove the laryngoscope while holding the tube in place and remove the stylet from the tube. The tube will remain unstable until secured; therefore, it must be held firmly until then.
  9. Inflate the cuff with 5-10 mL of air.
  10. Attach the bag valve mask and begin delivering manual breaths to the patient.
  11. Confirm correct tube placement by watching for chest expansion, performing auscultation, and using a colorimetric CO2 detector. Bilateral breath sounds should be heard in the lungs and there should be absent breath sounds over the central region of the abdomen.
  12. Secure the tube in place with tape or a commercially made tube holder.
  13. Connect the patient to the ventilator and monitor the patient’s vital signs. The chest should continue to rise and fall as mechanical breaths are being delivered.
  14. Obtain a chest radiograph to confirm that the endotracheal tube is in the correct position.
  15. Continue to monitor the patient and make adjustments to the ventilator settings as needed.

The intubation procedure involves a collaborative effort between doctors, nurses, and respiratory therapists. The success depends on many factors, including proper equipment, technique, and experience of the practitioner.

Laryngoscope blade types intubation

Medications for Intubation

There are a variety of medications that can be used to help with intubation, including sedatives and neuromuscular blocking agents. Some examples include:

  • Etomidate
  • Ketamine
  • Propofol
  • Succinylcholine
  • Rocuronium

Sedatives are typically used to help decrease the patient’s level of consciousness and anxiety. Neuromuscular blocking agents cause skeletal muscle paralysis and help relax the muscles of breathing.


What is Endotracheal Intubation?

Endotracheal intubation is a medical procedure that involves the insertion of a tube into the trachea. This helps establish a connection between the patient and the ventilator so that positive pressure breaths can be delivered.

What is Rapid Sequence Intubation?

Rapid sequence intubation (RSI) is a type of intubation that is typically used in emergency situations in which the patient is at risk of aspiration.

It is most often indicated in patients who cannot protect their airway due to an altered mental status.

Why Would a Person Need to Be Intubated?

There are many reasons why a person might need to be intubated. Some of the most common examples include:

  • Inability to maintain an airway
  • Respiratory distress
  • Unconsciousness
  • Severe asthma or COPD exacerbation
  • Cardiac arrest

Mechanical ventilation is most commonly indicated in patients who have trouble maintaining adequate ventilation or oxygenation. An endotracheal tube helps establish an airway so that this type of therapy can occur.

Who Should Not be Intubated?

There are no absolute contraindications for intubation and mechanical ventilation; however, if a patient has a DNI order (i.e., do not intubate), an endotracheal tube should not be inserted.

How Long Does Intubation Take?

The intubation procedure itself is typically quick and can be completed in as little as 30 seconds. However, the patient will need to be monitored closely afterward and may need to remain on mechanical ventilation for an extended period of time.

An intubation attempt should only last up to 15 seconds. After this amount of time has passed, the patient should receive oxygenated breaths with a manual resuscitator before another attempt is performed.

Can a Person Talk or Eat When Intubated?

No, a person cannot talk or eat when intubated. An endotracheal tube goes through the vocal cords and into the trachea. This prevents the person from being able to speak.

Additionally, all food and drink must be stopped before intubation. Intubated patients will receive all of their nutrition and hydration through an IV.

How is the Endotracheal Tube Removed During Extubation?

Extubation is the process of removing the endotracheal tube from the trachea. This is typically done when the patient’s condition has improved and they are no longer in need of ventilatory support.

What are the Risks of Intubation?

Intubation is generally considered a safe procedure; however, there are some risks that should be noted. These include:

  • Aspiration
  • Vocal cord paralysis
  • Hemorrhage
  • Injury to teeth or mouth
  • Injury to the throat
  • Bleeding
  • Anesthesia risks

Each patient should be closely monitored for any complications after intubation. This is a job duty of both respiratory therapists and nurses.

Why Perform a Tracheostomy after Intubation?

A tracheostomy is a surgical procedure that involves making an incision in the neck and creating an opening into the trachea. This is typically done when a patient needs to be on the mechanical ventilator for an extended period of time.

Therefore, a physician may decide that a patient needs a tracheostomy after they’ve already been intubated. In this case, the endotracheal tube is replaced with a tracheostomy tube after the incision has been made.

What Helps with a Sore Throat After Intubation?

A common side effect of intubation is a sore throat. This is due to the endotracheal tube irritating the throat while inserted.

There are a few things that can be done to help alleviate a sore throat, such as:

  • Drink plenty of fluids
  • Drink ice water
  • Try numbing lozenges (with benzocaine)
  • Suck on hard candy
  • Eat popsicles
  • Avoid citrus
  • Try over-the-counter remedies

Each person may find that different things work better for them. Therefore, one can experiment to find what works best for their individual case.

How Long Does Hoarseness Last After Intubation?

Hoarseness is another common side effect of intubation. It typically lasts for 1-2 weeks after the tube has been removed.

During this time, it is important to rest the voice as much as possible. This means avoiding yelling and other activities that put a strain on the vocal cords.

Additionally, one can try some of the sore throat home remedies that were mentioned earlier.

The Steps of Intubation Should Ideally Be Completed Within Which Duration?

The steps of intubation should ideally be completed within 30 seconds. The procedure is typically quick, but the patient will need to be monitored closely afterward.

An attempt to insert an endotracheal tube through the vocal cords should last no longer than 15 seconds. After this amount of time has passed, the patient should receive oxygenated breaths with a bag valve mask before another attempt is made.

What is the Best Patient Position for Intubation?

The best patient position for intubation is known as the sniffing position. This is when the patient’s neck is flexed and the head is hyperextended.

This position helps open up the airway and makes it easier to visualize the vocal cords. By doing so, it simplifies the process of inserting the tube into the trachea.

What Oxygen Level Requires Intubation?

Intubation and mechanical ventilation may be considered when the patient’s oxygen saturation level drops below 85%.

However, this decision should be made on a case-by-case basis. There are other factors that need to be taken into consideration, such as the patient’s underlying cause of hypoxemia.

Intubation Practice Questions

1. What is the definition of endotracheal intubation?
The insertion of an endotracheal tube through the mouth or nose into the trachea in order to protect the airway and establish a connection for mechanical ventilation

2. What should you do immediately after intubation?
You should verify that the tube is in the trachea by checking for symmetry of chest expansion, auscultating for bilateral breath sounds, and ordering a chest x-ray to confirm proper placement.

3. How much pressure should be in the endotracheal tube cuff?
15-20 mmHg

4. What can occur if the cuff pressure is too high?
This could cause bleeding, ischemia, or pressure necrosis.

5. What can occur if the cuff pressure is too low?
Aspiration pneumonia

6. How long can a patient be intubated?
It is preferred that the patient is intubated for no longer than 14-21 days. A tracheostomy is indicated for long-term mechanical ventilation.

7. What are the disadvantages of endotracheal intubation?
Discomfort, decreased cough reflex, thickened secretions, depressed swallowing reflex, increased risk of aspiration, and ulceration of the larynx or trachea

8. What can happen due to the inadvertent removal of an endotracheal tube?
Laryngeal swelling, hypoxemia, bradycardia, hypotension, and death

9. What manual maneuver is used during intubation?
Head tilt, chin lift, and jaw thrust

10. What are the indications for the head tilt, chin lift maneuver?
It can be used as long as the patient does not have a cervical injury. The patient must also have the ability to protect their own airway in order to prevent aspiration.

11. What are the contraindications for the head tilt, chin lift maneuver?
Patients who are conscious and those with cervical or spinal cord injuries

12. When is the modified jaw thrust maneuver indicated?
It’s indicated in an unresponsive patient with a possible c-spine injury.

13. When is the modified jaw thrust maneuver contraindicated?
When the patient is conscious

14. What are the essential components of a bag-valve-mask?
Self-refilling bag, mask, oxygen reservoir, and a supplemental oxygen source

15. What are the disadvantages of bag-valve-mask ventilation?
Gastric distention and it is difficult to maintain a leak-proof seal

16. What can be done if the chest is not rising while using a bag-valve-mask for ventilation?
You can reposition the airway, check for an obstruction, lift the jaw, suction the airway, and intubate if necessary.

17. What are some reasons for performing intubation?
Respiratory depression due to drugs, support for gas exchange, to increase lung volumes, to maintain an obstructed airway, presence of a foreign body, bleeding, edema, trauma, altered level of consciousness, potential aspiration, and before an elective operation

18. What are the pieces of equipment needed for intubation?
Oral airway, bag-valve-mask, flow meter, suction equipment, laryngoscope handle and blades, ET tubes, 10 cc syringe, stylet, tube tape, stethoscope, and colorimetric detector

19. What laryngoscope blade size should be used in infants?
Size 0-1

20. What laryngoscope blade size should be used in adults?
Size 3-4

21. The laryngoscope blade should reach between what two things?
The lips and larynx

22. Which type of laryngoscope blade directly lifts the epiglottis upward?
Miller (straight)

23. Which type of laryngoscope blade indirectly lifts the epiglottis?
Macintosh (curved)

24. What size endotracheal tube is used for males?
Size 7.5–9.0

25. What size endotracheal tube is used for females?
Size 7.0-8.5

Mechanical Ventilation Basics PDF Book Cover

Grab your FREE digital copy of this eBook now, no strings attached.

Mechanical Ventilation Basics PDF Book Cover

26. What do the colors means on a CO2 colorimetric detector?
If it turns yellow, it means that the is exhaling CO2 and the tube is likely positioned in the trachea. Remember that yellow is mellow and gold is good. Purple, on the other hand, is a problem.

27. What are some common errors in intubation technique?
Positioning errors, poor head placement, wrong bed height, not hunched over enough, damaging the patient’s teeth, and inserting the tube too deep

28. What is the estimated depth for endotracheal tube placement?
22 cm for males and 21 cm for females

29. What are the contraindications for nasal intubation?
Nasal fracture, basilar skull fracture, nose bleeds, and sinusitis

30. What are the complications of nasal intubation?
Nosebleeds, submucosal dissection, inflammation, edema, and gag reflex stimulation

31. When are the patient’s vocal cords open the most?
They are the most open during inspiration, so that is when you should advance the endotracheal tube.

32. How can you provide supplemental oxygen to a patient while performing nasotracheal intubation?
A nasal cannula can be placed in the opposite nare, a face mask can be placed over their mouth, and oxygen tubing can be placed near the mouth.

33. What are some things to try if the passage of the nasotracheal tube into the trachea is difficult?
Turn the patient’s head to the side, change the degree of flexion or extension, apply cricoid pressure, push the larynx toward the opposite side of the nares, have the patient stick out their tongue, use a laryngoscope, use Magill forceps, or use a fiberoptic bronchoscope.

34. What can happen if the patient doesn’t receive enough sedation during intubation?
A lack of sedation could cause poor patient cooperation resulting in trauma and aspiration. This would make it more difficult to perform intubation successfully.

35. What can happen if the patient receives too much sedation?
Hypoventilation, apnea, cardiac arrest, and a loss of airway-protective reflexes

36. What two types of drugs are used for sedation?
Narcotic analgesics are used to relieve pain and sedative hypnotics are used to decrease anxiety and induce sleep.

37. What are three examples of narcotic analgesics?
Morphine, fentanyl, and demerol

38. What is the reversal agent for narcotic drugs?

39. What is the most common sedative hypnotic?
Versed (Midazolam)

40. What is the onset and duration of Versed?
Onset 1-3 minutes, duration 20-45 minutes

41. What produces a temporary loss of sensation or feeling in a confined area of the body?
Local anesthesia

42. What are the benefits of rapid induction?
It’s easier to perform, relieves side effects associated with awake intubations, minimal change in vital signs, and avoids trauma.

43. What are the side effects associated with awake intubations?
Hypertension, tachycardia, and increased bronchospasm due to airway stimulation

44. What are three adverse effects of rapid induction?
Myocardial depression, hypotension, and increased airway obstruction due to the loss of laryngeal muscle tone

45. What should be monitored during rapid induction?
Heart rate and rhythm, respiratory rate and depth, SpO2, skin color, blood pressure, and level of consciousness

46. How soon should the patient awaken after rapid induction?
10-15 minutes

47. How do you prepare the patient for rapid induction?
Pre-oxygenate with an FiO2 of 100%

48. What are the criteria for extubation?
Adequate respiratory mechanics, stable VS, normal electrolytes, Hgb, Hct, adequate UO, no acute processes, adequate LOC, CXR stable, recovery from airway reflexes, and recovery from anesthesia

49. Why is stridor most likely heard when a patient self-extubates?
Because the cuff is being pulled out while inflated

50. What is the treatment for moderate to severe stridor?

Respiratory Therapy PEEP T-shirt

Get access to 25+ premium quizzes, mini-courses, and downloadable cheat sheets for FREE.

Free Cheat Sheet and Quiz Bundle Respiratory Therapy Zone

51. What are the two ways that intubation can be performed?
Orotracheal and nasotracheal intubation

52. What are the complications of intubation?
Trauma, bleeding, and breaking teeth

53. What are the contraindications of intubation?
The only true contraindication is when the patient’s living will indicates their desire to not be intubated.

54. What are the components of an endotracheal tube?
An endotracheal tube is composed of a 15 mm adapter, pilot balloon, cuff, beveled tip, curved body with cm markings, radiopaque line, Murphy eye, and vocal cord line.

55. What is the 15 mm adapter?
It’s an adapter that connects to common equipment such as the bag-valve-mask and the ventilator circuit.

56. What is the pilot balloon?
It plays an important role in inserting and deflating air from the cuff and also helps with cuff pressure monitoring.

57. What is the cuff?
It surrounds the endotracheal tube, seals off the lower airway for ventilation, and helps prevent aspiration.

58. What is the purpose of a beveled tip?
It helps reduce trauma during insertion.

59. What is the purpose of the curved body with centimeter markings?
It indicates the distance from the end of the tube and is used as a landmark for securing it in place.

60. Why does an endotracheal tube have a radiopaque line?
It has a line that is visible on a chest x-ray to help visualize the position of the tube.

61. What is the Murphy eye of an endotracheal tube?
It is an extra hole in the tube that serves as a safeguard in case the tube becomes obstructed.

62. What is the vocal cord line?
It is used to indicate when to stop ET tube insertion.

63. What size ET tube should be used for males?
Size 8.0

64. What size ET tube should be used for females?
Size 7.5

65. What size ET tube should be used for neonates?
The size is based on their weight.

66. What is the typical tube position for males?
It should be placed 21-23 cm at the lip.

67. What is the typical tube position for females?
It should be placed 19-21 cm at the lip.

68. What are the two types of laryngoscope blades?
Mac and Miller

69. What is the Miller blade?
It is a straight laryngoscope blade that directly lifts the epiglottis.

70. What size Miller blades should you use for adults?
Size 2-3

71. Where do you aim the Miller blade during intubation?
It should be inserted directly over the epiglottis to move it out of the way.

72. What is the Macintosh blade?
It is a curved laryngoscope blade that indirectly lifts the epiglottis by lifting the vallecula.

73. What size Mac blades should you use for adults?
Size 3-4

74. Where do you aim with the Mac blade during intubation?
It should be inserted at the vallecula, which indirectly lifts the epiglottis out of the way so that the vocal cords can be visualized.

75. What are the types of laryngoscopes?
Regular (small light bulb) and fiberoptic (fiberoptic channel)

Grab your FREE digital copy of this eBook now, no strings attached.

76. What are the methods for confirming the placement of an endotracheal tube?
Colorimetry device, capnography, esophageal detection device, chest x-ray, direct laryngoscopy, videoscope/glidescope, bronchoscope, light wand (lighted stylet), presence of vapor trail, chest rise, and bilateral breath sounds

77. What is the next step after the ET tube is in place?
After the tube is inserted and the laryngoscope is removed, the next step is to remove the stylet, inflate the cuff, and confirm the tube position.

78. How long should an intubation attempt last?
Intubation attempts should not exceed 15 seconds. You must watch the patient’s oxygen saturation and manually resuscitate as needed.

79. What color is good for the colorimetry device?
You want the color to change from purple to yellow.

80. What is the best way to confirm endotracheal tube placement?
Chest x-ray

81. Where should the endotracheal tube be placed?
It should be inserted 2-6 cm above the carina.

82. What are the causes of a difficult airway?
Obesity and mandible malformation

83. What is a common specialized insertion technique used during intubation?
Applying cricoid pressure to the throat

84. What is the normal endotracheal tube cuff pressure?
25-35 cmH2O

85. What does the Murphy’s eye protect against?
It is an extra hole on the side of the tube that is there to help the patient breathe if there is an obstruction.

86. What is the purpose of the stylet?
It helps keep the tube rigid and holds it in place so that it can be inserted into the trachea.

87. What is the purpose of using lidocaine during intubation?
Lidocaine is an agent that helps numb the vocal cords to decrease the chances of laryngospasm.

88. What equipment is needed for extubation?
Yankauer, pulse oximeter, towels, and an oxygen delivery source

89. What is a common complication of extubation?

90. What is the difference in the intubation procedure in different types of patients?
The process of intubation is typically the same in patients of all ages. The only difference is in the size of the equipment used during the process of insertion.

91. Who can perform intubation?
Intubation can be performed by doctors, respiratory therapists, anesthesiologists, nurse anesthetists, and trained EMTs.

92. What is the most common reason for re-intubation?
The most common reason for re-intubation is improper placement of the endotracheal tube.

93. What is an artificial airway?
A tube that is inserted into the patient’s trachea in order to maintain breathing and ventilation

94. What are the types of artificial airways?
Endotracheal tubes, tracheostomy tubes, and pharyngeal airways

95. What is airway management?
Airway management is the process of maintaining ventilation so that gas exchange can occur.

96. What should you do if secretions accumulate during intubation?
Perform airway suctioning

97. What is glidescope intubation?
Glidescope intubation is a type of intubation where a fiberoptic camera is used to visualize the vocal cords. It involves the use of a videolaryngoscope.

98. What is the primary goal of endotracheal intubation?
To secure the patient’s airway

98. What is the purpose of pre-oxygenation prior to intubation?
It helps prevent hypoxemia during insertion

99. What can be inserted after multiple failed intubation attempts?
Laryngeal mask airway (LMA)

100. What is the last resort after a failed intubation?

Final Thoughts

Intubation is an important medical procedure that involves the insertion of a tube into the trachea. This helps establish a link between the patient and the ventilator so that ventilatory support can be delivered.

Respiratory therapists play an important role in the intubation procedure. They are responsible for preparing the patient and equipment, as well as monitoring the patient once mechanical ventilation is initiated.

We have similar guides on ventilator management and weaning that I think you’ll find helpful. Thanks for reading!

Written by:

John Landry, BS, RRT

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.


  • Faarc, Kacmarek Robert PhD Rrt, et al. Egan’s Fundamentals of Respiratory Care. 12th ed., Mosby, 2020.
  • Chang, David. Clinical Application of Mechanical Ventilation. 4th ed., Cengage Learning, 2013.
  • Rrt, Cairo J. PhD. Pilbeam’s Mechanical Ventilation: Physiological and Clinical Applications. 7th ed., Mosby, 2019.
  • Faarc, Heuer Al PhD Mba Rrt Rpft. Wilkins’ Clinical Assessment in Respiratory Care. 8th ed., Mosby, 2017.
  • “Tracheal Intubation in Critically Ill Patients: A Comprehensive Systematic Review of Randomized Trials.” US National Library of Medicine National Institutes of Health, 20 Jan. 2018,
  • “Intubations and Airway Management: An Overview of Hassles through Third Millennium.” PubMed Central (PMC), Apr. 2015,
  • “Clinical Consensus of Emergency Airway Management.” PubMed Central (PMC), 1 Nov. 2017,

Recommended Reading