Principles of Mechanical Ventilation: An Overview (2024)

Mechanical ventilation is a critical intervention in the management of patients with respiratory failure or compromised breathing. It involves the use of machines to assist or replace spontaneous breathing, ensuring the vital exchange of oxygen and carbon dioxide.

The principles of mechanical ventilation are foundational in optimizing patient outcomes.

Understanding these principles enables healthcare providers to effectively tailor ventilation strategies, addressing each patient’s unique physiological needs.

What are the Principles of Mechanical Ventilation?

The principles of mechanical ventilation focus on supporting or replacing natural breathing by ensuring adequate ventilation and oxygenation, managing lung compliance, minimizing airway resistance, optimizing deadspace ventilation, and preventing or treating respiratory failure. This involves precise adjustments to ventilator settings tailored to individual patient needs and conditions.

Principles of Mechanical Ventilation

• Ventilation
• Oxygenation
• Lung compliance
• Airway resistance
• Deadspace ventilation
• Respiratory failure

Ventilation

Ventilation refers to the process of moving air in and out of the lungs. It is a critical aspect of respiratory physiology, enabling the exchange of oxygen and carbon dioxide between the body’s cells and the external environment.

The process involves two primary phases: inhalation, where air is drawn into the lungs, filling the alveoli, and exhalation, where air is expelled from the lungs, removing carbon dioxide from the body.

Ventilation is driven by differences in pressure between the atmosphere and the lung’s interior, controlled by the respiratory muscles, including the diaphragm and intercostal muscles.

Efficient ventilation is essential for maintaining the body’s acid-base balance and meeting its metabolic demands for oxygen, especially during periods of increased activity.

Oxygenation

Oxygenation is the process through which oxygen molecules in the air are transferred into the bloodstream and ultimately to the body’s tissues.

This process occurs primarily in the lungs’ alveoli, where oxygen from inhaled air diffuses across the alveolar membrane into the blood in the pulmonary capillaries. The oxygen is then bound to hemoglobin molecules in red blood cells, facilitating its transport throughout the body.

Oxygenation is crucial for the survival of the body’s cells, as oxygen is needed for cellular respiration, the process by which cells generate energy.

Adequate oxygenation depends on several factors, including the proper functioning of the respiratory system, the efficiency of the gas exchange process in the lungs, and the ability of the cardiovascular system to distribute oxygenated blood to all parts of the body.

Lung Compliance

Lung compliance refers to the ease with which the lungs can expand and contract during breathing. It measures the lung’s flexibility and elasticity, indicating how much effort is required to fill the lungs with air.

Compliance is determined by the structural characteristics of the lungs, including the connective tissue and the surface tension of the alveoli, as well as the chest wall and pleura.

High lung compliance means the lungs can easily expand when pressure is applied, whereas low lung compliance indicates stiff or restricted lungs, requiring more effort to inhale.

Diseases such as pulmonary fibrosis reduce lung compliance due to the thickening and scarring of lung tissue, while conditions like emphysema increase compliance due to the loss of elastic recoil, making it difficult to exhale.

Optimal lung compliance ensures efficient ventilation and minimizes the work of breathing.

Airway Resistance

Airway resistance is the opposition to airflow that occurs as air moves through the respiratory tract, from the nose and mouth down to the smallest bronchioles. It is influenced by the diameter of the airways, with narrower passages presenting more resistance to airflow.

Several factors can affect airway resistance, including inflammation, mucus production, bronchoconstriction, and physical obstructions.

Diseases such as asthma and chronic obstructive pulmonary disease (COPD) are characterized by increased airway resistance, leading to difficulty breathing.

The body can regulate airway resistance through the autonomic nervous system by dilating or constricting the airways in response to various stimuli. Managing airway resistance is crucial for ensuring adequate ventilation and the efficient exchange of gases in the lungs.

Deadspace Ventilation

Deadspace ventilation refers to the portion of inspired air that does not participate in gas exchange because it remains in the airways or reaches alveoli that are not perfused with blood.

This includes both the anatomical deadspace, which is the volume of the conducting airways, and the physiological deadspace, which also includes areas of the lung where alveoli are not adequately perfused with blood.

The presence of deadspace is a normal part of respiratory physiology; however, an increase in deadspace can be indicative of certain pathological conditions, such as pulmonary embolism or vascular disease, which impair the distribution of blood to the alveoli.

Effective ventilation requires not only moving air in and out of the lungs but also ensuring that a sufficient portion of this air reaches areas of the lung capable of gas exchange.

Respiratory Failure

Respiratory failure occurs when the respiratory system fails in one or both of its gas exchange functions: oxygenation or elimination of carbon dioxide from the blood.

This condition can result from any disruption in the ventilation-perfusion relationship, lung compliance, airway resistance, or control of breathing.

Respiratory failure is classified into two types based on blood gas levels: Type I (hypoxemic) respiratory failure, characterized by a failure to adequately oxygenate the blood, and Type II (hypercapnic) respiratory failure, where there is inadequate carbon dioxide elimination.

Causes of respiratory failure vary widely and include diseases affecting the airways (like COPD or asthma), the parenchyma (such as pneumonia or ARDS), the neuromuscular system (e.g., Guillain-Barré syndrome or ALS), or the chest wall (like kyphoscoliosis).

Treatment depends on the underlying cause and may involve supplemental oxygen, mechanical ventilation, or addressing the specific cause of respiratory failure.

Principles of Mechanical Ventilation Practice Questions

1. What is ventilation?
The movement of air into and out of the alveoli.

2. What are the mechanics of ventilation?
Elasticity, compliance, resistance, pressure, and gravity.

3. What is Boyle’s Law?
It states that the pressure of a given mass of an ideal gas is inversely proportional to its volume when the temperature is kept constant. In other words, if the volume of a gas increases, its pressure decreases, and vice versa, assuming the temperature remains unchanged.

4. What are the indications for mechanical ventilation?
Acute or chronic respiratory failure, oxygenation failure, pulmonary or cardiovascular conditions, sepsis, head injury management, nervous system disorders, and muscular weakness.

5. What is considered respiratory failure?
A pH less than 7.25 and a PaCO2 greater than 50.

6. What is the most common use of mechanical ventilation?
Post-operative patients recovering from anesthesia.

7. What conditions may indicate the need for mechanical ventilation in adults?
Apnea, impending respiratory arrest, acute exacerbation of chronic obstructive pulmonary disease (COPD), acute severe asthma, neuromuscular diseases, acute hypoxemic respiratory failure, heart failure and cardiogenic shock, acute brain injury, and flail chest.

8. What common conditions lead to the need for mechanical ventilation?
Patients typically require mechanical ventilation due to ventilatory failure, respiratory failure, or both.

9. What is external respiration?
External respiration is the gas exchange between the alveoli and the blood in pulmonary capillaries, involving oxygen uptake and carbon dioxide elimination.

10. What is internal respiration?
Internal respiration refers to the oxygen and carbon dioxide exchange at the cellular level between the bloodstream and body cells.

11. What does transpulmonary pressure refer to?
Transpulmonary pressure is the difference in pressure between the alveolar space and the pleural space, crucial for maintaining alveolar inflation.

12. What is transrespiratory pressure?
Transrespiratory pressure is the pressure difference between the airway opening and the body surface, necessary for inflating the lungs and airways during positive pressure ventilation.

13. What is transairway pressure?
Transairway pressure is the difference in pressure between the airway opening and the alveoli, representing the pressure gradient needed to overcome airway resistance during airflow.

14. What is transthoracic pressure?
Transthoracic pressure is the pressure difference between the alveolar space (or lung) and the body surface, essential for simultaneous expansion or contraction of the lungs and chest wall.

15. What is elastance?
Elastance measures a structure’s ability to return to its original shape after being stretched or compressed by an external force.

16. What is airway compliance?
Airway compliance indicates how easily the airways can expand under pressure.

17. What is airway resistance?
Airway resistance quantifies the frictional forces encountered during airflow through the respiratory tract.

18. How does lung compliance affect peak inspiratory pressure measured during mechanical ventilation?
Lung compliance does not typically change significantly in mechanically ventilated patients. However, if a patient actively breathes in or out during the measurement of plateau pressure, the resulting value may not accurately reflect true lung compliance.

19. What is the most common reason for mechanical ventilation after anesthesia?
Impaired respiratory drive secondary to the residual effects of anesthesia.

20. What changes in airway conditions can lead to increased airway resistance?
Airway resistance is increased when an artificial airway is inserted, and a smaller internal diameter of the tube equals greater resistance to flow. A decrease in the diameter of the airways can also cause airway resistance to increase.

21. What is the principle of operation of a negative pressure mechanical ventilator?
It attempts to mimic the function of the respiratory muscles to allow breathing through normal physiological mechanisms.

22. What is the principle of operation of a positive pressure mechanical ventilator?
The ventilator delivers air into the patient’s lungs through an endotracheal tube or positive pressure mask.

23. What is the principle of operation of a high-frequency mechanical ventilator?
It uses above-normal rates with below-normal volumes to ventilate the patient.

24. What is peak inspiratory pressure?
The highest pressure recorded at the end of inspiration.

25. What is the baseline pressure?
It is usually zero (or atmospheric), which indicates that no additional pressure is applied at the airway opening during expiration and before inspiration.

26. What is positive end-expiratory pressure (PEEP)?
It occurs when the baseline pressure is higher than zero. When PEEP is set, the ventilator prevents the patient from exhaling to zero, which helps with oxygenation.

27. What is plateau pressure?
It is measured after a breath has been delivered to the patient and before exhalation begins.

28. Why is mean airway pressure clinically important?
For any specific tidal volume, PaO2 can be affected by mean airway pressure are the parameters used to achieve this pressure. An increased mean airway pressure improves the FRC. Increasing the mean airway pressure improves oxygenation in patients with pulmonary disorders that are refractory to conventional ventilatory modes.

29. The pressure difference between the alveolus (Palv) and the body surface (Pbs) is called what?
Transthoracic pressure

30. What is the formula used to calculate lung compliance?
Compliance = change in volume/change in pressure

31. What are some other names for airway pressure?
Mouth pressure, airway opening pressure, and mask pressure.

32. Intra-alveolar pressure at the end of inspiration during a normal quiet breath is approximately what?
0 cmH2O

33. Which of the following is associated with an increase in airway resistance?
Decreasing the flow rate of gas into the airway.

34. Which of the following statements is true regarding negative pressure ventilation?
These ventilators mimic normal breathing mechanics.

35. What are the basic types of power sources used for mechanical ventilators?
Electrical or pneumatic (compressed gas).

36. What is the difference in function between positive and negative pressure ventilators?
Positive pressure ventilator: Gas flows into the lung because the ventilator establishes a pressure gradient by generating a positive pressure at the airway opening. Negative pressure ventilator: The ventilator generates a negative pressure at the body surface that is transmitted to the pleural space and then to the alveoli.

37. What is the user interface (control panel) on a mechanical ventilator?
It is located on the surface of the ventilator and is monitored and set by the respiratory therapist. It has various controls for the setting components, such as tidal volume, rate, inspiratory time, alarms, and FiO2.

38. What are the ventilator’s internal and external pneumatic circuits?
It’s a series of tubes that allow gas to flow inside the ventilator and between the ventilator and the patient.

39. What is the difference between a single-circuit and a double-circuit ventilator?
Single-circuit ventilators allow the gas to flow directly from its power source to the patient. Double-circuit ventilators require the primary power source to generate a gas flow that compresses a mechanism (bellows/bag in chamber), then the gas in the bellows flows to the patient.

40. What are the components of an external (patient) circuit?
(1) Main inspiratory line: connects the ventilator output to the patient’s airway adapter or connector; (2) Adapter: connects the main inspiratory line to the patient’s airway (Y-connector); (3) Expiratory line: delivers expired gas from the patient to the exhalation valve; and (4) Expiratory valve: allows the release of exhaled gas from the expiratory line into the room air.

41. In what conditions is mean airway pressure important?
ARDS, pneumonia, ventilatory induced lung injury, pulmonary contusion, pulmonary edema, and morbid obesity.

42. What does barotrauma cause?
Hyperinflation over a sustained period of time and large changes in volume.

43. What effect can positive pressure ventilation have on the central nervous system?
It can cause an increased intracranial pressure.

44. What effect can positive pressure ventilation have on the cardiovascular system?
It can cause a decrease in residual volume preload which leads to a decrease in cardiac output.

45. What is the function of an externally mounted exhalation valve?
During inspiration, the expiratory valve closes so that gas can flow only into the patient’s lungs.

46. What is a commercially available ventilator that is entirely pneumatically powered?
Bird Mark 7

47. What type of ventilator delivers pressures below ambient pressure on the body surface and mimics the physiology of normal breathing?
Negative pressure ventilator

48. What is the purpose of increasing mean airway pressure?
You would increase it in order to re-inflate collapsed alveoli and thus ultimately improving oxygenation.

49. A Drager Evita Infinity V500 ventilator is set to deliver a minute ventilation of 5 L/min. The patient breathes six times in 1 minute and receives a mandatory breath of 500 mL with each breath. The ventilator detects the difference between the actual and the set minute ventilation and adds four more breaths (500 mL each) to make up the difference. Which of the following best describes this type of ventilator?
Closed loop

50. The controls set by the ventilator operator are considered part of the what?
The user interface.

51. The gas-conducting tubes that carry gas from the ventilator to the patient are referred to as what?
The circuit

52. A ventilator in which the gas that enters the patient’s lungs is also the gas that powers the unit is referred to as what?
Single-circuit ventilator

53. In a spring-loaded bellows volume-delivery device, the amount of pressure is determined by what?
The tightness of the spring.

54. What are two other names for pressure ventilation and volume ventilation?
Pressure-controlled ventilation, pressure-targeted ventilation, volume-controlled ventilation, and volume-targeted ventilation.

55. What is mean airway pressure?
It is the average airway pressure during the inspiratory cycle.

56. What are the two most commonly used patient-trigger variables?
Pressure and flow

57. What patient-trigger variable requires the least work of breathing for a patient receiving mechanical ventilation?
Flow-triggering

58. What are the effects of an inflation hold on inspiratory time?
Because of the way the pause functions, the normal cycling mechanism no longer ends the breath, resulting in an increase in the inspiratory time and a reduction in the expiratory time.

59. What is a clinical situation in which expiratory resistance is increased?
When a patient has an endotracheal tube and can’t use the pursed-lip breathing technique.

60. What are two methods of applying continuous pressure to the airways that can be used to improve oxygenation in patients with refractory hypoxemia?
CPAP and PEEP.

61. What is airway resistance?
Obstruction in the airways.

62. What is alveolar deadspace?
Lung volume that is unable to take part in gas exchange because of reduced perfusion.

63. What is alveolar volume?
The difference between tidal volume and dead space volume.

64. What is anatomical deadspace?
The volume in the conducting airways that does not take part in gas exchange.

65. What is deadspace ventilation?
Wasted ventilation (i.e., ventilation in excess of perfusion).

66. What is a diffusion defect?
Impaired gas exchange across the AC membrane.

67. What is intrapulmonary shunting?
Wasted perfusion

68. What is oxygenation failure?
The failure of the heart and lungs to provide adequate oxygen to the tissues.

69. What is peak inspiratory pressure?
The pressure used to deliver a tidal volume by overcoming non-elastic (airway) and elastic resistance (lung parenchyma).

70. What is physiologic deadspace?
The sum of anatomic and alveolar deadspace.

71. What is the plateau pressure?
The pressure needed to maintain lung inflation in the absence of airflow.

72. What is refractory hypoxemia?
Low arterial oxygen that does not respond to a moderate to high FiO2.

73. What is ventilatory failure?
Failure of the respiratory system to remove CO2.

74. What is a V/Q mismatch?
The abnormal distribution of ventilation to perfusion.

75. What is Raw?
Airway resistance; the degree of airway obstruction in the airways.

76. When the radius of an airway decreases by 50% of its original size, the driving pressure needed to maintain the same airflow must increase by a factor of what?
16-fold

77. Airway resistance in mechanical ventilator patients is primarily affected by what?
The length, size, and patency of the airway; and the ET tube and ventilator circuit.

78. Obstruction causes what?
It causes changes inside the airway, changes in the airway wall, and changes outside the airway.

80. What is the normal adult airway resistance
0.4 to 2.5 cmH2O/L/sec

81. When the airway resistance increases, what happens to the work of breathing?
It increases.

82. What commonly gets trapped in the ventilator circuit that causes an increased airway resistance?
Condensation

83. What happens with chronically high airway resistance?
Fatigue can set in and affect the respiratory muscles which leads to ventilation and oxygenation failure.

84. What is ventilatory failure?
Failure of the respiratory system to remove CO2 from the body resulting in an abnormally high PaCO2.

85. What is oxygenation failure?
Failure of the heart and lungs to provide adequate oxygen for metabolic needs. It is severe hypoxemia that does not respond to moderate to high levels of supplemental oxygen. It is caused by hypoventilation, V/Q mismatch or intrapulmonary shunting. Mechanical ventilation may be needed to minimize the work of breathing and provide oxygen support.

86. What is lung compliance?
It is the degree of lung expansion per unit pressure change.

87. Is lung expansion easy with low compliance?
No, it would be difficult.

88. What three things does high compliance do to the lungs?
1) Causes incomplete exhalations, 2) causes air trapping, and 3) reduces CO2 elimination.

89. What is refractory hypoxemia?
It is a persistent low level of oxygen in the blood that is not responsive to medium to high concentrations of inspired oxygen. The key thing to remember is that it’s usually caused by intrapulmonary shunting.

90. What are the three types of deadspace ventilation?
1) Anatomic deadspace, 2) alveolar deadspace, and 3) physiologic deadspace.

91. What are the five mechanisms that lead to ventilatory failure?
1) Hypoventilation, 2) persistent V/Q mismatch, 3) persistent pulmonary shunt, 4) persistent diffusion defect, and 5) persistent reduction of PiO2 (inspired O2 tension).

92. What is hypoventilation?
The reduction of alveolar ventilation and an increased PaCO2.

93. What are the causes of hypoventilation?
Suppression of the CNS, neuromuscular disorders, and airway obstruction.

94. What is alveolar volume?
The difference between tidal volume and deadspace volume (Vt – deadspace) that takes part in gas exchange.

95. What is minute alveolar ventilation?
It is a function of tidal volume, deadspace volume, and respiratory rate.

96. What causes a low minute alveolar ventilation?
Hypoventilation

97. What are the three groups of mechanical ventilation patients?
1) Depressed ventilatory drive (e.g., drug overdose), 2) excessive ventilatory workload (e.g., airflow obstruction), and 3) failure of the ventilatory pump (e.g., chest trauma).

98. What does a depressed ventilatory drive lead to?
It may lead to oxygenation and ventilatory failure. The patient may have normal pulmonary function, but respiratory muscles do not have adequate impulses to function properly.

99. What are the objectives of mechanical ventilation?
To support or manipulate pulmonary gas exchange, to normalize alveolar ventilation, to maintain arterial oxygenation, and to reduce or manipulate the work of breathing.

100. What is volume-controlled ventilation?
It is volume limited, volume targeted, and pressure variable.

101. What is pressure-controlled ventilation?
It is pressure limited, pressure targeted, and volume variable.

102. What is cycling?
It is the mechanism by which the phase of breathing switches from inspiration to expiration.

103. How do you select the initial tidal volume for a patient on the mechanical ventilator?
It should be set at 6-8 mL/kg of the patient’s ideal body weight.

104. What is the minimum FiO2 used in anesthesia?
Use 30% or greater

105. The PIP should be set below what in order to avoid overdistention of the alveoli?
Keep the PIP < 45 cmH20.

106. The plateau pressure should be set below what in order to avoid overdistention of the alveoli?
Keep the plateau pressure < 30 cmH20.

107. What is the normal I:E ratio?
1:2

108. What range of PEEP will improve the functional residual capacity (FRC)?
5-10 cmH20

109. During spontaneous ventilation, the PEEP effect is achieved by using what?
CPAP

110. During mechanical ventilation, what happens at the end of inspiration?
The expiratory phase begins by opening the exhalation valve.

Final Thoughts

Understanding the principles of mechanical ventilation is required for respiratory therapists and physicians who manage critically ill patients receiving breathing support from a ventilator.

Adhering to these principles makes it possible to optimize ventilator settings to meet individual patient needs, mitigate the risk of complications, and enhance patient comfort and outcomes.

Continual education and proficiency in mechanical ventilation principles remain paramount for healthcare providers to deliver safe and effective respiratory care.