Ventilator Management

Ventilator Management is a topic covered in the 5-Minute Emergency Consult.

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Basics

Description

  • Mechanical ventilation is machine generated flow of gas into and out of the lungs that acts as a substitute for normal respiratory function
Basic Concepts: Physiology and Pulmonary Mechanics
  • 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

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