Ministry of Health

NZ Government

    
 ©Copyright
 Published: 20/02/2009

Volume Guarantee (VG) is a new mode, available on the Babylog 8000plus ventilator.  Volume ventilation has not been available for neonatologists until recently.  Recent advances have meant that the sophisticated electronics responsible for bringing information about respiratory mechanics and respiratory function monitoring can be integrated into the delivery system of the ventilator.

This means that instead of using pressure as a means of determining ventilation, expired volume (specifically tidal volume, VT) can be specified.  The ventilator will try to deliver the prescribed volume with the lowest possible pressure.  This is done on a breath-to-breath basis, looking at how much volume was delivered with the previous breath and adjusting the Peak Inspiratory Pressure up or down to increase or decrease the VT accordingly.

• The preferred ventilation mode in which to use VG is Pressure Support Ventilation (PSV) but you can use it in SIMV or SIPPV.
  • In PSV, the inspiratory time (T_i) is limited by the mechanics of the baby's lungs.
  • This is in contrast to SIMV and SIPPV where the T_i is determined by the person altering the settings.
  • PSV is more "natural" in that when the baby's lungs are full, the inspiration will be terminated so the baby can breath out.  If the Ti as set in SIMV and SIPPV may be longer than the baby needs to fill his lungs, then the inspired pressure will be maintained at the PIP, regardless of how full the lungs are.

Principles behind VG

The goal of VG is to reduce volutrauma and barotrauma to the baby's lungs.  It is a mode which has not been well studied in infants.  However, the underyling premise is that delivering the lowest possible pressure required to deliver the volume will result in less lung injury.  Studies have shown less variation in delivered VT but more variation in PIP in VG modes.  Mean airway pressure is generally reduced.

Choose your patient carefully!

Patient selection is important!  The key thing to consider is

• Is there a leak?
  • As the set VT that the machine tries to deliver is the expired VT, a significant leak will mean that that volume cannot be achieved.
  • If the leak is greater than 40%, do not use VG.

Another thing to consider is what you expect to happen to the baby in their disease process.  For example, if the lungs are stiff and non-compliant because the baby has RDS, then PSV+VG is a good mode to use after you give surfactant and you expect the baby's compliance to improve.  The ventilator will essentially wean itself.

In our experience, it can be less successful for babies with Chronic Lung Disease.

Setting up the ventilator

The main issue is to choose an appropriate VT at the start and to give enough pressure to get it in.

1. Look at what VT the baby is currently getting on his current ventilation settings.
2. The usual VT to aim for is 4-8ml/kg per breath. Starting with 5ml/kg is usually a safe point but may be a bit high for some babies.
3. Look at the pressure settings.  The pressure settings are - in essence - alarm settings.  If the ventilator cannot deliver the VT at the pressures prescribed, it will alarm "Low Tidal Volume".  You may need to increase the PIP to stop it from alarming.  It is safe to increase the PIP by at least 2 mbar in VG mode as safety features in VG will prevent too large a breath from being given.
4. The rate setting is a "back-up" rate if the baby is not breathing.  However, it should be set 10-15 breaths below the baby's apparent rate so that the baby has an opportunity to trigger.

Weaning from VG

Weaning (or, for that matter, increasing ventilation) should be on the basis of volume.  If the baby is over-ventilated, reduce the prescribed volume by an appropriate amount.

• There is no point reducing the rate if the baby is breathing above the ventilator on PSV or SIPPV.  With PSV and SIPPV, each breath is assisted.  On SIMV, reducing the rate will reduce the number of assisted breaths but this may increase the work of breathing as it will increase the time spent receiving ET CPAP.
• There is no point reducing the pressures without reducing the set VT - all you will do is make the ventilator alarm because it is less likely to be able to deliver the volume prescribed with the pressures set.

Conversely, if you need to increase the amount of ventilation, you may need to increase the PIP so that the ventilator can deliver the set VT.