Volume Assist Control




In volume assist-control (often labelled “volume control”) patients may receive either controlled or assisted breaths. When the patient triggers the ventilator, he/she receives a breath of identical duration and magnitude as the mandatory breath. The patient receives a breath of this type irrespective of actual minute ventilation requirement. The “interactive” element of this mode is merely that the patient receives a breath when they want one, nothing else. The advantage of this mode is that patients can breathe spontaneously without working. The patient is fully rested on the ventilator, except for triggering, assuming that the peak flow is adequate. The problem with this mode is that there is no weaning component, and patients tend to hyperventilate as they emerge.

Assist control (AC) ventilation involves the use of four variables: tidal volume and respiratory rate as before, with the addition of peak inspiratory flow (as an alternative to I:E ratio) and trigger sensitivity. The peak flow (click here) is set with two things in mind: if the flow rate is too high, the volume is rapidly delivered to only the most compliant lung tissues (and not to the inelastic diseased tissues), at very high peak pressures. If the peak flow is too low, the patient will demand more gas than the ventilator is set up to supply and dysynchrony with the machine occurs. A modern adjustment to this is the use of pressure augmentation: the ventilator senses that the patient’s demands exceed the peak flow, and automatically increases the flow.

The inspiratory flow rate is measured in liters per minute, and it determines how quickly the breath is delivered. The time required to complete inspiration is determined by the tidal volume delivered and the flow rate: Ti = VT/Flow Rate.


 In volume-assist control, the patient can receive controlled or assisted breaths – all identical.

For the ventilator to interact with the patient, it must sense that the patient is making a spontaneous effort – a trigger – and address this by delivering a breath. The original triggers were negative pressure sensors (a negative pressure is generated by the patient sucking in against a valve), these had the problem of placing an unnecessary workload on breathing and have been replaced with flow triggers (“flow by”): a continuous flow of gas travels around the breathing system, inspiration deflects this flow, and triggers the ventilator.



This patient is on volume assist control ventilation. Note the decelerating flow pattern. The peak flow is 70 l/min, the tidal volume is 700ml. In the panel on the left, the patient is receiving controlled breaths at a rate of 12 per minute. On the right the patient is receiving assisted breaths – can you discern the small downward deflection of the patient triggering just prior to inspiration? Note that in all breaths the tidal volume is identical.

One has to be careful with triggering devices: if the trigger is too sensitive the patient “overtriggers” and hyperventilates, if it is not sensitive enough, the patient becomes dysynchronous.

To start a patient on assist-control one must select a PEEP (as determined by lung compliance), a minute volume (MV 100ml/kg), a tidal volume (TV 6ml/kg), and a peak flow. The respiratory rate is the MV/TV. The peak flow is usually four times the minute ventilation. The trigger is either set as “flow-by” or a negative pressure of -2cmH2O. Refinement of the settings is determined by the patient’s plateau pressure (should be less than 30cmH2O), the patients inspiratory flow demand, and blood gas targets.





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