assisted ventilation

see also:

• lung protective ventilation to reduce lung injury in ARDS
• pathophysiology of mechanical ventilation
• managing ventilator problems
• Assist-Control Ventilation (ACV)
• Intermittent Mandatory Ventilation (IMV)
• Pressure Support Ventilation (PSV)
• Airway Pressure Release Ventilation
• the LTV 1200 ventilator

• 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)

• untreated pneumothorax
• tension pneumothorax

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

• 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)

• 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

• 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

• 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.

• 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 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”

• 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:
  • 16/minute for those > 20kg
    • increase according to desired minute volume if using Pressure Support Ventilation (PSV)
  • BUT perhaps aim for 6/min if COPD/asthma
• 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

• generally require initial settings of smaller tidal volumes (eg. 6ml/kg) and higher respiratory rate
• see lung protective ventilation to reduce lung injury in ARDS

• generally require initial settings of lower respiratory rate and longer expiratory times
• permissive hypercapnoea as long as pH > 7.1