Terms Used in the Physiology of the Ventilated Patient

• Airway resistance:
  • Refers to the resistive forces encountered during the:
    • Mechanical respiratory cycle
  • The normal airway resistance is:
    • ≤ 5 cmH2O
• Lung compliance:
  • Refers to the:
    • Elasticity of the lungs, or
      • The ease with which they stretch and expand:
        • To accommodate a change:
          • In volume or pressure
  • Lungs with a low compliance, or high elastic recoil:
    • Tend to have difficulty with the inhalation process, and are colloquially referred to as:
      • “Stiff” lungs:
        • An example of poor compliance would be:
          • A patient with a restrictive lung disease, such as:
            • Pulmonary fibrosis
  • In contrast:
    • Highly compliant lungs, or lungs with a low elastic recoil:
      • Tend to have more difficulty during the exhalation process:
        • As seen in obstructive lung diseases
• Atelectasis:
  • Is a complete or partial collapse:
    • Of the entire lung or area (lobe) of the lung:
      • It occurs when the tiny air sacs (alveoli) within the lung:
        • Become deflated or possibly filled with alveolar fluid
• Derecruitment:
  • Is the loss of gas exchange surface area:
    • Due to atelectasis
  • Derecruitment is one of the most common causes of:
    • Gradual hypoxemia in intubated patients and can be minimized by:
    • Increasing PEEP
• Recruitment:
  • Is the restoration of gas exchange surface area:
    • By applying pressure:
      • To reopen collapsed or atelectatic areas of the lung
• Predicted Body Weight:
  • Is the weight that should be used in determining ventilator settings:
    • Never use actual body weight
  • Lung volumes are determined largely by:
    • Gender and height, and therefore, these two factors are used to determine predicted body weight:
      • The formula for men is:
        • PBW (kg) = 50 + 2.3 (height (in) – 60) and
      • The formula for women is:
        • PBW (kg) = 45.5 + 2.3 (height (in) – 60)

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