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assisted ventilation


  • assisted mechanical ventilation (positive pressure ventilation) consists of a variety of techniques and modes which aim to improve inspiratory air flow.
  • it may replace spontaneous breathing or assist spontaneous breathing
  • it may be noninvasive positive pressure ventilation (NPPV or NIPPV) such as CPAP or BiPAP, or invasive (ie. administered via an endotracheal tube or similar device).
  • mechanical ventilation reverses the normal pattern of negative inspiratory airway pressures (generated by the expanding intrathoracic volume during spontaneous respiration) and replaces it with a positive inspiratory airway pressure.
  • this intrathoracic positive pressure and mechanical ventilation may have various consequences including:
    • contriction of pulmonary vasculature and thus reduction in ventricular preload and thus reduction in cardiac output
    • maldistribution of gas in the pulmonary tract resulting in V/Q mismatch
    • decreased functional residual capacity (FRC)
    • decreased compliance and surfactant
    • decreased efficiency of pulmonary gas exchange
    • asynchronous breathing - spontaneous breaths not coinciding with ventilator breaths
    • atelectasis due to lack of usual humidification of inspired air, and thus increased sputum thickness
    • baratrauma from excessive intrapulmonary pressures or volumes resulting in potential for pneumothorax and pneumomediastinum.
    • in addition, high levels of FiO2 may result in oxygen toxicity
  • two main type of humidification systems:
    • heat moisture exchanger (eg. ETT filter)
    • heated water bath (eg. Fisher and Paykel)

invasive mechanical ventilation


  • untreated pneumothorax
  • tension pneumothorax

relative contraindications

  • unilateral lung disease
  • obstructive lung disease
  • elevated peak and mean airway pressures
  • bronchopleural fistulae
  • hypovolaemia
  • elevated intracranial pressure
  • pulmonary embolism
  • recent lung surgery

ventilation phase variables

trigger of onset of a breath

  • this may be time based mandatory breaths (ie. clinician sets a specified respiratory rate as in Intermittent Mandatory Ventilation (IMV)) or triggered by patient breaths (such as with Assist-Control Ventilation (ACV) or Pressure Support Ventilation (PSV)), or a combination of both (synchronised intermittent mandatory ventilation or SIMV).
  • there are two ways to sense a patient breath to trigger a ventilation:
    • pressure-triggering:
      • a ventilator-delivered breath is initiated if the demand valve senses a negative airway pressure deflection (generated by the patient trying to initiate a breath) greater than the trigger sensitivity. A trigger sensitivity of -1 to -3 cmH2O is typically set.
    • flow-by triggering:
      • a continuous flow of gas through the ventilator circuit is monitored. A ventilator-delivered breath is initiated when the return flow is less than the delivered flow, a consequence of the patient's effort to initiate a breath.
      • this method decreases work of breathing in SIMV mode (and also in CPAP)

ventilation limit target

  • the ventilator is set a target at which it delivers ventilation for that breath
  • the target may either be:
    • flow rate limit:
      • ie. clinician sets the peak inspiratory flow rate
    • pressure limit:
      • ie. clinician sets the maximum pressure allowable during ventilation

ventilation termination target (cycle variable)

  • the ventilator is set a target at which it ceases ventilation for that breath
  • the target may either be:
    • flow rate limit:
      • a decrease in the inspiratory flow to a predetermined percentage of its peak value
    • flow volume limit:
      • ie. delivery of the set tidal volume
    • time limit:
      • ie. completion of the predetermined duration of inspiration
      • an example of a time-cycled ventilator is the Oxylog

ventilation modes


  • clinican sets ventilator flow rate and tidal volume and allows pressure to be determined by airways resistance, lung and chest wall compliance.
  • High airway pressures may be a consequence of large tidal volumes, a high peak flow, poor compliance (eg, acute respiratory distress syndrome, minimal sedation), or increased airway resistance.
  • The inspiratory time and inspiratory to expiratory (I:E) ratio are determined by the peak inspiratory flow rate. Increasing the peak inspiratory flow rate will decrease inspiratory time, increase expiratory time, and decrease the I:E ratio.
  • used in volume-limited Assist-Control Ventilation (ACV) and volume-limited SIMV
  • possible advantages of SIMV compared to AC include better patient-ventilator synchrony, better preservation of respiratory muscle function, lower mean airway pressures, and greater control over the level of support
  • AC may be better suited for critically ill patients who require a constant tidal volume or full or near-maximal ventilatory support
  • it may be regarded as volume control (VC) if ventilator-initiated breaths or volume-assist (VA) if patient-initiated breaths
  • volume limited ensures a minute volume will be delivered but otherwise no significant benefits over pressure-limited mode which is associated with lower peak airway pressures, less regional alveolar overdistension, improved patient-ventilator synchrony and earlier weaning.


  • clinician sets a peak inspiratory pressure and ventilation terminates when either time limit reached (as in pressure control (PC) or pressure assist (PA) modes), or terminated when pressure falls to a set percentage of peak inspiratory pressure (Pressure Support Ventilation (PSV)).
  • it can be used with ACV, IMV or SIMV or with CMV when it is called Pressure Controlled Ventilation (PCV).
  • all PCV is time-cycled.
  • ventilator pressure = resistive pressure + elastic pressure + PEEP
    • resistive pressure = airway flow x airway resistance
    • elastic pressure (alveolar pressure) = lung volume / lung compliance
    • airway flow = volume of air delivered / time
    • ventilator “plateau” pressure at end-inspiration hold (0.3-0.5sec) maneuver (when flow = zero and PEEP = zero) = elastic pressure
    • resistive pressure = peak inspiratory pressure - plateau pressure
    • elastic (alveolar) pressure = plateau pressure
    • an acute rise ventilator pressure may be caused by:
      • if associated with a high end-inspiratory pressure could be caused by:
        • asynchronous breathing
        • a fall in compliance (eg. endobronchial intubation, pneumothorax, abdominal distension)
        • increased lung volume and raised intrinsic PEEP (eg. air trapping due to inadequate expiratory time)
      • if associated with a normal or unchanged end-inspiratory pressure:
        • increased airway resistance (eg. partially blocked ETT, bronchospasm)
  • “Auto-PEEP” (intrinsic positive end-expiratory pressure) interferes with pressure triggering.
    • Auto-PEEP refers to end-expiratory pressure that is created when inspiration begins before expiration is complete.
    • this can be measured by end-expiratory hold maneuver:
      • immediately before a breath, close expiratory port for 2 sec ⇒ zero flow
      • pressure at this time is the alveolar pressure at end expiration = intrinsic PEEP
      • may be falsely high if patient makes efforts to breathe
    • can be identified by failure of expiratory flow to fall to zero before next breath
    • intrinsic PEEP is caused by:
      • airflow obstruction
      • high minute volume
      • inadequate expiratory time ⇒ shorten inspiratory time &/or reduce resp. rate
        • the normal inspiration/expiration (I/E) ratio is 1:2 to 1:3. This is reduced to 1:4 or 1:5 in the presence of obstructive airway disease in order to avoid air-trapping (breath stacking) and intrinsic PEEP

extrinsic PEEP

  • applied PEEP (extrinsic positive end-expiratory pressure) is generally added to mitigate end-expiratory alveolar collapse.
  • some potential indications for use of PEEP:
    • low levels of 3-5cm H20 for most patients to provide “physiologic PEEP” and reduce ventilator-induced lung injury
    • higher levels of PEEP to improve oxygenation and allow less toxic levels of FiO2:
      • hypoxaemic respiratory failure
        • acute pulmonary oedema
        • shunt > 30%
        • reduced lung compliance
        • recurrent atelectasis and low functional residual capacity (FRC)
  • higher PEEP levels need to be used with care to avoid adverse consequences such as:
    • over distension of alveoli
    • reduced venous return and thus reduced cardiac output
    • may worsen hypoxaemia in patients with focal lung disease
    • can interfere with bronchial circulation
    • reduced urine output and sodium excretion

ventilator settings

  • ventilator settings are dependent upon degree of spontaneous breathing, condition being managed, age, sex, comorbidities, etc
  • minute volume = RR x tidal volume
  • I:E ratio is mainly determined by inspiratory flow rate
  • inspiratory flow rate may be “set” on ventilators via either:
    • set inspratory time (Tinsp) and tidal volume
  • high flow rates (eg. > 60L/min) result in shorter inspiratory time, higher peak inspiratory pressure (PIP), more turbulent air flow, but may be needed to achieve high minute volume demands.
  • low flow rate results in longer inspiratory time, lower peak inspiratory pressure (PIP), more laminar air flow and perhaps better distribution of gas
  • if inspiratory flow is inadequate, patient will have increased work of breathing and “air hunger”

average initial settings for many patient groups

  • SIMV (+/- PSV) is the most common setting for most intubated patients, however IPPV (ie. CMV) may be used in paralysed patients
  • if using pressure controlled, usually set pressure to 20mmHg initially then titrate as per minute volume
  • if using volume controlled, initial tidal volume 8ml/kg ideal body weight
  • Pmax alarm:
    • usually 40 cm H2O
    • if high alarm:
      • check tube blockage/kink, patient agitation, or pneumothorax
      • if COPD/bronchospasm, try disconnect tube and allow exhalation of stacked breathes
      • if plateau pressure on inspiratory hold maneuver > 30 then reduce tidal volume
  • respiratory rate:
  • PEEP:
    • 5cm H20 initially
    • increase incrementally (to max. 14cm H2O) if need > 40% FiO2 to maintain SaO2 of > 88%
  • I:E ratio:
    • generally aim for I:E ratio 1.1.5 to 1.3
      • adjust either:
        • Tinsp set to 1.3 secs
        • peak flow rate (usually 40-60L/min)
        • inspiratory slope
    • but I:E ratio 1:4 or more in those with COPD/asthma
  • FiO2:
    • the lowest FiO2 which maintains an adequate SaO2

ARDS patients

asthma/COPD patients

  • generally require initial settings of lower respiratory rate and longer expiratory times
  • permissive hypercapnoea as long as pH > 7.1
vent.txt · Last modified: 2015/12/21 17:41 (external edit)