Description

• 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

• 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

• 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