Ventilation modes

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Volume Assist Control (AC)

  • The most common mode for ED ventilation
    • Prevents patient fatigue by offering full respiratory support
    • The safety and ease of this mode typically outweighs the theoretical benefits of other modes (primarily a perception of increased patient comfort)[1]
  • Preset rate and TV
  • Patient able to trigger additional breaths (full assisted tidal volume)
  • Spontaneous breathing (above rate) is not allowed
  • Beneficial for patients requiring a high minute-ventilation (reduces oxygen consumption and CO2 production of the respiratory muscles)
  • May worsen obstructive airway disease by air trapping or breath stacking
  • Set: RR, FiO2, PEEP, TV, I:E ratio

Synchronous Intermittent Mandatory Ventilation (SIMV)

  • Senses not to give with breathing if present, extra breaths of patient's TV
  • Preset breaths in coordination with the respiratory effort
  • Spontaneous breathing allowed between breaths
  • Synchronization attempts to limit the barotrauma by not delivering a breath when already maximally inhaled (vs. IMV)
  • For each additional breath triggered by pt, ventilator delivers variable TV depending on patient effort and condition of lung
  • Because of need for patient effort, not recommended for tired or septic pt
  • Set: RR, FiO2, PEEP, TV, I:E ratio

^For the paralyzed patient, there is no difference in minute-ventilation or airway pressures between A/C and SIMV

Pressure Support (PS)

  • Controls via pressure, good if pressures getting too high
  • Limits barotrauma and decreases the work of breathing in the spontaneously breathing patient
  • Level of pressure set (not TV) to assist spontaneous efforts
  • Most ventilators allow back-up respiratory rate (in case of apnea)
  • Mode of choice in patients whose respiratory failure is not severe and who have an adequate respiratory drive (improved patient comfort, reduced cardiovascular effects, reduced risk of barotrauma, and improved distribution of gas)
  • Set: RR, FiO2, PEEP, PS, I:E ratio

Pressure Regulated Volume Control (PRVC)

  • Provides constant pressure through inspiration with decelerating inspiratory flow pattern
  • Ventilator adjusts pressure breath to breath based on patient’s airway resistance and compliance
  • Benefits: minimum PIP, guaranteed tidal volume, patient can trigger more breaths, improved oxygenation, breath by breath changes
  • Not recommended for asthma or COPD
  • Set: FiO2, RR, TV, upper pressure limit, I:E ratio, PEEP


  • Needs spont breathing pt
  • Not for fatiguing pt
  • Set: FiO2, PEEP, back up RR

Control Mode

  • only in OR
  • machine initiates and delivers breath
  • fixed rate and TV

See Also

Mechanical Ventilation Pages

  1. Weingart SD. Managing Initial Mechanical Ventilation in the Emergency Department. Ann Emerg Med. 2016;68:614-617