|
| |
- volume or pressure can be set as well as a minimum frequency
- ventilator delivers preset volume or pressure when triggered by negative
inspiratory effort above triggering threshold
- if patient does not take sufficient spontaneous breaths ventilator will
deliver breaths to ensure minimum frequency is achieved
- volume controlled assist/control ventilation may be associated with
significant work of breathing, especially if trigger threshold is high.
- tachypnoea due to agitation, lung disease or central hyperventilation may
cause significant alkalosis
- both assist and control breaths are same ie same preset pressure or volume, in
contrast to SIMV
- NB in this mode it is important to set the control rate to close to the
patient's actual rate as inspiratory time will be governed by the control rate
and the inspiratory time. Thus if the patient's actual rate is much higher than
the control rate expiratory time is severely curtailed.
- another potential problem is inadequate inspiratory flow rate (avoid rates
<40l/min) in volume-targeted assist control. This occurs if the preset
ventilatory rate is set too low. This may result in a long inspiratory time with
a consequent fall in flow. With pressure targeted assist control the ventilator
provides sufficient flow to allow the set pressure level to be achieved rapidly
Pressure support ventilation
- = inspiratory assist
- constant preset airway pressure supplied at the start of each breath
- pressure support ceases after a given fraction of inspired time or when
inspiratory flow rate falls below a predetermined fraction of initial flow rate
(usually 25% in older ventilators, 5% in some modern ventilators)
- allows patient to control inspiratory and expiratory times, inspiratory flow
and tidal volumes. This control can improve patient comfort
- can be combined with SIMV or CPAP or used on its own in patients whose gas
exchange is improving and in whom weaning is being attempted
- pressure support of 3.5-14.5 cmH2O required to overcome the
additional work of breathing due to breathing through ETT and demand valve.
Patients who require pressure support of < 6 cmH2O can probably be
extubated
- may unload respiratory muscles and reduce diaphragmatic fatigue
- disadvantages: VT and minute ventilation variable, excessively
large VT may be delivered and high initial flow and termination
algorithm may be unsuitable for severe airflow obstruction (newer ventilators
allow adjustment of initial flow rate or pressure slope)
- when limit variable is volume instead of pressure = volume support (VS)
|
