Ventilator Management



  • Mechanical ventilation is machine-generated flow of gas into and out of the lungs that acts as a substitute for normal respiratory function
  • Successful ventilator management requires understanding of the cause of respiratory failure and pathophysiology that influences pulmonary mechanics
Physiology and Pulmonary Mechanics
  • Our natural respiratory pattern involves negative pressure ventilation:
    • At the lung's functional residual capacity (FRC, neutral state) surface tension of alveoli is balanced by elastic recoil of chest wall; alveoli pressure equals atmospheric pressure
    • During inspiration, alveolar pressure becomes negative and air travels into the lungs
    • Exhalation is normally passive, but can be made active with the use of accessory muscles
    • Work of breathing (WOB) equals the force required to overcome resistance to airflow (resistive WOB) and the lung's neutral FRC (elastic WOB) multiplied by respiratory rate
  • Mechanical ventilation is positive pressure ventilation as forced gas delivery generates positive pressure during inspiration
  • Ventilation refers to functional removal of CO2 from the blood; this influences PaCO2, pH, and ETCO2:
    • MV = RR × TV; minute ventilation = respiratory rate (breaths/min) × tidal volume (standard breath)
    • Dead space (DS) = volume of breath that does not participate in gas exchange; DS increases with damaged airways or reduced blood supply to alveoli (e.g., pulmonary embolism)
    • ETCO2 is usually around 2–5 mm Hg <PaCO2; value affected by DS, ventilation/perfusion (V/Q) mismatch, changes in metabolic CO2 production and venous return
  • Oxygenation (PaO2) is influenced by inspired oxygen (FiO2) and positive end-expiratory pressure (PEEP):
    • PEEP helps maintain alveolar recruitment and minimize V/Q mismatch
    • PEEP can improve oxygenation but high levels risk alveolar injury
  • Compliance refers to lung distensibility (change in volume with given change in pressure):
    • Compliance decreases with damaged parenchyma (e.g., pneumonia, ARDS) or problems with the chest wall/pleura (e.g., obesity, abdominal distension)
  • Compliance determines plateau pressure, which is the steady state pressure distributed to the smallairways and alveoli during positive pressureventilation; measure using inspiratory holdmaneuver; goal ≤30 mm Hg
  • Resistance refers to change in pressure with given flow; this is primarily determined by airway radius:
    • Resistance increases due to problems with the airways (e.g., bronchospasm), with the endotracheal tube (e.g., secretions), or with ventilator tubing
    • Resistance determines peak pressure (pressure in large airways); goal ≤40 mm Hg
Basic Modes of Ventilation
  • Noninvasive positive pressure ventilation (NIPPV, see “Ventilation Management, Noninvasive”):
    • Continuous positive airway pressure (CPAP): Set pressure support throughout respiratory cycle
    • Bilevel positive airway pressure (BiPAP): Inspiratory pressure higher than expiratory pressure to support WOB and augment ventilation
  • Continuous mandatory ventilation (CMV):
    • Breaths are delivered only at a set rate – time is the trigger for every breath:
      • Volume-controlled (or volume cycled) CMV: Delivers set volume with each breath and guarantees certain MV
      • Pressure-controlled (or pressure cycled) CMV: Delivers constant flow of gas until set inspiratory pressure to limit peak pressure
      • Rests respiratory muscles but often requires heavy sedation, less commonly used
  • Volume-control/assist-control (VC/AC):
    • Patient or time triggers a breath, but the same machine-controlled breath (set volume or flow) is delivered
    • Reduces WOB and guarantees set TV but may lead to hyperventilation or excessive inspiratory pressures
  • Synchronized intermittent mandatory ventilation (SIMV):
    • Patient or time triggers a breath at a set RR
    • Patient breaths are pressure controlled, machine breaths are flow or pressure controlled
    • Reduces interference with cardiovascular function but increases WOB and patient must adjust to two types of breaths
  • Pressure support ventilation (PSV):
    • Ventilator augments patient's spontaneous breaths with set amount of pressure
    • Improves patient comfort and reduces WOB but risks hypoventilation or apnea

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