vent
Table of Contents
assisted ventilation
see also:
introduction
- 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
contraindications
- 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
volume-limited
- 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.
pressure-limited
- 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:
- 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
ARDS patients
- generally require initial settings of smaller tidal volumes (eg. 6ml/kg) and higher respiratory rate
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 06:41 by 127.0.0.1