Using Volume Assist-Control Ventilation

Volume assist-control is a very useful mode in patients who fail to ventilate or have acute lung injury. If setup correctly the patient, even with a partial respiratory drive, should incur minimal work in breathing. Care must be taken that the patient does not hyperventilate and develop significant respiratory alkalosis. There is no natural weaning mode for assist-control; most units use pressure support ventilation in patients who are not yet ready to discontinue mechanical ventilation.
Try to avoid being dragged into the volume control versus pressure control argument by naively taking sides. Most patients with acute lung injury can be well managed on volume assist-control: although manipulating mean airway pressure can be trickier than in pressure control. As an intensivist you would be expected to be comfortable with both modes.

Set PEEP as  in protocol (click here)

Set initial rate at 12-14 breaths/minute

Set tidal volume at 6ml/kg

Set the peak flow at 50l/minute Set flow triggering Use a decelerating flow pattern

Check a blood gas after 30-60 minutes If CO2 is elevated increase the respiratory rate but be careful of auto-PEEP. Consider reducing tidal volumes to 4-5ml/kg

If the patient develops severe hypoxemia increase the PEEP as necessary add an inspiratory pause to increase mean airway pressure sedate and paralyse the patient consider prone positioning consider alternative modes: PC  ARPV  HFO

Is the patient breathing spontaneously? Ensure that the peak flow is adequate If respiratory drive intact, and lung injury is not advancing, consider switching the patient over to pressure support ventilation (PSV).

If the patient is not ready to control the rate, depth and duration of breath, and is tachypneic: increase the sedation consider BiLevel (or equivalent) ventilation.

If switching to PSV: calculate the driving pressure from Plateau pressure-PEEP (volume control breaths) Reset the PS at the driving pressure

Look at pressure waveform, adjust inspiratory flow (slope) to optimal setting.

Watch lung compliance: increase PS if compliance reduces (decreased tidal volumes) decrease PS if compliance improves (tidal volumes >6ml/kg).

Address patient sedation (tachypnea in PSV is often from inadequate sedation)

Patrick Neligan 2002