Ventilator Management

Description

• Mechanical ventilation is machine generated flow of gas into and out of the lungs that acts as a substitute for normal respiratory function

• Mechanical ventilation is positive pressure ventilation indicating that forced gas delivery generates positive pressure during inspiration
• Negative pressure ventilation:
  • Natural respiratory pattern
  • At rest (functional residual capacity) surface tension of alveoli is balanced by elastic recoil of chest wall; alveoli pressure equals atmospheric pressure at this point
  • In inspiration, lungs expand causing alveolar pressure to become negative compared with atmospheric pressure and air travels down pressure gradient into lungs
  • Exhalation is normally passive, but can be made active with the use of accessory muscles in the setting of airway obstruction/increased airway resistance
• Minute ventilation (MV):
  • Total volume of breaths in 1 min
  • Breaths in 1 min is respiratory rate (RR)
  • Standard breath is called tidal volume (TV)
  • MV = TV × RR: Each component can be adjusted to control ventilation
• Oxygenation is controlled with adjusting fraction of inspired oxygen (FiO₂) and positive end-expiratory pressure (PEEP)
• Compliance:
  • Describes lung distensibility
  • Defined as change in volume with given change in pressure
  • Decreased lung compliance can be caused by problems with the lung parenchyma (i.e., pneumonia, ARDS) or problems with the chest wall/pleura (i.e., abdominal distension)
  • Lung compliance determines plateau pressure:
    • Plateau pressure is the steady state pressure; represents the attenuated pressure that is distributed to the small airways and alveoli during positive pressure ventilation
    • Goal ≤30 mm Hg
• Resistance:
  • Defined as change in pressure with given flow
  • Main determinant is airway radius
  • Increased resistance can be caused by problems with the airways (i.e., bronchospasm), problems with the endotracheal tube (i.e., secretions), or problems with ventilator tubing
  • Resistance determines peak pressure:
    • Peak pressure is the pressure seen in the larger airways before delivered volume is distributed to smaller airways and alveoli
    • Also determined by TV delivered
    • Goal ≤40 mm Hg