Ventilation Terminology



  1. Continuous positive airway pressure (CPAP) – the delivery of continuous level of positive airway pressure.
    • Functionally similar to positive end expiratory pressure (PEEP), the ventilator does not cycle during CPAP
    • No additional pressure above the level of CPAP is provided
    • Patients must initiate all their own breaths.
    • Most often used for obstructive sleep apnea.


  1. Bilevel positive airway pressure (BPAP/BiPAP)- bilevel positive airway pressure is a mode used during noninvasive positive pressure ventilation (NPPV).
  • BPAP delivers a preset inspiratory positive airway pressure (IPAP) and expiratory positive airway pressure (EPAP)
  • Tidal volume correlates with the difference between IPAP and EPAP.
  • BPAP offers several advantages compared to CPAP.
    • Notably it is an active ventilation rather than solely pneumatic splinting of the upper airway.
    • A lower mean airway pressure (which means it is often tolerated better)
    • Allows for rest of ventilator muscles
    • Quicker resolution of respiratory acidosis
    • However, it does have an issue with asynchrony


  1. Assist Control (A/C) – physician determines minimal minute ventilation by setting respiratory rate and tidal volume.
  • Patient can trigger additional breaths, which are delivered with the same tidal volume as the previous breaths. 


  1. Synchronized Increased Minute Ventilation (SIMV) –SIMV delivers a minimum number of fully assisted breaths per minute that are synchronized with the patient's respiratory effort
  • These breaths are patient- or time-triggered, flow-limited, and volume-cycled.
    • Any breaths taken between volume-cycled breaths are not assisted
    • The volumes of these breaths are determined by the patient's strength, effort, and lung mechanics
    • A key concept is that ventilator-assisted breaths are different than spontaneous breaths
    • Another important concept is that AC and SIMV are identical modes in patients who are not spontaneously breathing due to heavy sedation or paralysis.
  • SMV is seen as better because it increases patient work to preserve respiratory muscle function
  • SIMV allows for better titration of ventilator support


  1. Volume control (VC) breaths are ventilator-initiated breaths with a set inspiratory flow rate.
  • Inspiration is terminated once the set tidal volume has been delivered.


  1. Volume assist (VA) breaths are patient-initiated breaths with a set inspiratory flow rate.
  • Inspiration is terminated once the set tidal volume has been delivered.


  1. Pressure control (PC) breaths are ventilator-initiated breaths with a pressure limit.
  • Inspiration is terminated once the set inspiratory time has elapsed.
  • The tidal volume is variable and related to compliance, airway resistance, and tubing resistance.
  • A consequence of the variable tidal volume is that a specific minute ventilation cannot be guaranteed.


  1. Pressure support (PS) breaths are patient-initiated breaths with a pressure limit.
  • The ventilator provides the driving pressure for each breath, which determines the maximal airflow rate.
  • Inspiration is terminated once the inspiratory flow has decreased to a predetermined percentage of its maximal value.


  1. Extrinsic positive end-expiratory pressure (Applied PEEP) is generally added to mitigate end-expiratory alveolar collapse.
  • A typical initial applied PEEP is 5 cm H2O, and this is adjusted accordingly depending on if atelectasis persists.


  1. Flow rate — The peak flow rate is the maximum flow delivered by the ventilator during inspiration.
  • The need for a high peak flow rate is particularly common among patients who have obstructive airways disease with acute respiratory acidosis.
  • In such patients, a higher peak flow rate shortens inspiratory time and increases expiratory time (ie, decreases the inspiratory to expiratory [I:E] ratio).
  • These alterations:
    • Increase carbon dioxide elimination
    • Improve respiratory acidosis
    • Decrease the likelihood of dynamic hyperinflation (auto-PEEP)


  1. End Tidal CO2 (Capnography)- provides instantaneous information about:
  • Ventilation (how effectively CO2 is being eliminated by the pulmonary system)
  • Perfusion (how effectively CO2 is being transported through the vascular system)
  • Metabolism (how effectively CO2 is being produced by cellular metabolism)
    • Used frequently in kids with vents as an easy way of assessing ventilation status.
    • Can be used to trend acid/base states, especially in conjunction with VBGs.
    • Capnography can be useful in evaluating respiratory status and is much quicker than pulse oximetry.
    • Make sure to look at wave forms to make sure that ETCO2 readings are accurate (look for a smooth hump) or else capnography readings may be abnormally high or low due to physiologic dead space.



The CO2 waveform: A, Expiratory pause begins; A–B, Clearance of anatomic dead space; B–C, Dead space air mixed with alveolar air; C–D, Alveolar plateau; D, End-tidal partial pressure of CO2 registered by capnograph (arrow) and beginning of inspiratory phase; D–E, Clearance of dead space air; E–A, Inspiratory gas devoid of CO2.


  1. Chest Physiotherapy (CPT)- involves hyperoxygenation by bagging (or vent) with 100% oxygen, deep endotracheal instillation of 0.25-0.5ml/kg sterile saline, bagging with momentary inspiratory hold, followed by release of the hold and simultaneous forced exhalation and vibration to stimulate cough, and endotracheal suctioning.
  • Commonly done in children with persistent atelectasis or cystic fibrosis


  1.  Cough Assist – patients with respiratory muscle weakness can’t generate the force necessary to produce an effective cough.
  • Mechanical insufflation-exsufflation can be delivered via  mechanical device to patients who are spontaneously breathing or mechanically ventilated (works best in patients with trachs).
  • Positive pressure is applied during inhalation, and rapid exsufflation quickly follows generating a negative pressure differential which leads to a simulated cough. 


 ventilation_0.png   Jim Beck, MD, Division of Pulmonary and Critical Care Medicine, University of Michigan 



  1. Mechanical Ventilation: Basic Review, University of Colorado School of Medicine, Department of Internal Medicine
  2. Ventilator Management, Medscape, 2015
  3. Management and Prognosis of patients requiring prolonged mechanical ventilation, Up To Date, 2015
  4. Overview of Mechanical Ventilation, Up To Date, 2015
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