Ministry of Health

NZ Government

    
 ©Copyright
 Published: 20/02/2009

  Click here to open the HFOV guideline

Introduction

As well as being a conventional ventilator, the Babylog 8000plus is able to provide high frequency oscillatory ventilation (HFOV). However, it needs to be set up and used differently to other oscillators (for example, the Sensormedics) for it to be effective.

Although the Babylog is apparently able to ventilate infants up to 4kg in weight, you should consider the Sensormedics oscillator in babies over 2kg who have significant respiratory disease.

The Babylog oscillates by rapid switching of the expiratory valve. The use of a jet Venturi system provides an active expiration phase.

Requirements

• You must use non-compliant tubing to use the HFOV function.
• Remember that the pressures generated at the ventilator are attentuated by the ventilator tubing, the endotracheal tube, and the baby’s airways so that the high pressures are not transmitted to the alveoli.

Setting up the Oscillator

• In general, set this 2-3 mbar higher than the current MAP on conventional ventilation.
• Remember: High MAPs can impair venous return to the heart and impair cardiac output. Consider this as a potential problem in babies who require high MAPs for oxgenation but deteriorate.
• Obtain a chest radiograph after the baby has been on the oscillator to check for lung expansion – aim for 8-9 posterior ribs.

• The default setting in NWH NICU is 10Hz. With the Sensormedics, our recommendation is to leave the frequency at 10Hz except under special circumstances.
• However, because of the limitations with Amplitude with the Babylog (particularly at low MAPs), frequency may need to change if the baby is not ventilating well at the maximum effective amplitude (see next paragraph).
  • Decreasing the frequency will increase the ventilation
  • Increasing the frequency will decrease the ventilation.

• Although you can choose an amplitude between 0% and 100%, the amplitude delivered depends on the MAP – the lower the MAP, the lower the amplitude before the maximum is reached.
• The amplitude is calculated the pressure fluctuation as a percentage of the difference between MAP and 60 mbar.
• For example, if the MAP is 15, then 100% amplitude would be 45 mbar.  Therefore, the pressure would be from -7.5 to 37.5 mbar
• However, the airway pressure is limited to –4 mbar. Therefore, this limits the maximum effective MAP – you can set it at 100% but in the example above it will only deliver -4 to 34 mbar (total 38 mbar) which equals 84%!  Therefore, the lower the MAP the lower the amplitude that will be effective.
• The formula for this is:  Maximum effective amplitude (%) = (2 x (MAP + 4))/(60-MAP) x 100

  

• This is calculated using the formula: DCO₂ = VT² x frequency
• This value, by itself, is not very useful. However, it is very useful to set this up as a continuous value on the computer monitor so you can look for trends
• For example, with the changes in settings, has DCO₂ changed? Is DCO₂ changing without any change in the settings (i.e. is the baby’s respiratory status changing?)
• Be aware that DCO₂ is affected by leaks around the ET tube.

• Set up the computer screen so that the following continuous parameters are displayed:
  • FiO₂
  • MAP
  • Amplitude
  • DCO₂

Adjusting Ventilation on HFOV

Weaning or increasing ventilation should be done according to the clinical condition of the baby and the blood gas results.

It is important to understand however the limitations of the machine when deciding how to increase or decrease ventilation. For example, if the baby has a MAP of 14 and an amplitude of 78%, increasing the amplitude will have no effect on ventilation unless the MAP is also increased.

Similarly, if a baby is on a MAP of 15 with an amplitude of 84%, decreasing the MAP to 13 will limit the maximum deliverable amplitude to 72%.

However, altering the frequency may have quite significant effects on ventilation.  For babies that are difficult to ventilate who are already at the maximum effective amplitude, consider reducing the frequency to increase ventilation.