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NICU guideline identifier

HFOV - on the VN500

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Terminology Abbrev/Unit Description/Comment
Mean Airway Pressure MAPhf
(cmH2O)
 
Frequency
Hertz
Fhf
(Hz)
HFV rate (Hz = cycles per second, i.e. 10 Hz = 10 cycles/sec = 600 cycles/min)
Inspiratory to expiratory ratio I:Ehf Start on 1:2, which is comparable to 33% on Sensormedics
1:1 is more powerful as the stroke volume per cycle increases but it can lead to more air-trapping compared to 1:2
Pressure Amplitude
(ΔP or power)
Ampl hf
(displayed in settings)
or ΔP
(displayed on screen)
(cmH2O) 
Difference between the maximum and minimum pressure of the oscillation
NB: In ventilator settings it's called Ampl hf, whereas on screen it displays ΔP which is the amplitude the baby achieves.
Max pressure amplitude
(in VG mode)
Ampl hf max
(cmH2O)
Max pressure variation allowed around the MAP to ensure VThf delivery
High frequency tidal volume VThf
(ml)
Stroke volume per cycle as measured by heated wire in flow sensor / or at the airway opening (leak compensated)
Dissociation coefficient for CO2 with HFO DCO2
(ml2/sec)
Guide for CO2 clearance. Calculated by ventilator DCO2 = VThf 2x Fhf
Flow  L/min Variable, controlled by the ventilator

The decision to start HFV on the VN500 and the settings that the baby is started on should be discussed with the consultant on call.

Oxygenation is dependent on MAP and FiO2. MAP provides a constant distending pressure comparable to CPAP. This inflates the lung to a constant and optimal lung volume maximising the area for gas exchange.

Ventilation can be separated from oxygenation on HFO as ventilation and oxygenation are much less dependent on each other than on conventional ventilation. Ventilation or CO2 elimination is dependent on stroke volume per cycle.

Stroke volume per cycle or the high frequency tidal volume is much smaller (~1-3 ml/Kg) than on conventional ventilation during which only bulk ventilation occurs.

Stroke volume will increase if:

  • Amplitude increases (less powerful than on Sensormedics and not a linear response)
  • Inspiratory time increases (i.e. change I:E ratio from 1:2 to 1:1)
  • Frequency decreases

DCO2 value is calculated by the VN500 as:

             DCO2 (ml2/sec) = VThf2 x Fhf

DCO2 is a relatively new value and can be a marker of alveolar ventilation during HFV. PaCO2 increases as DCO2 decreases, indicating alveolar hypoventilation. DCO2 is dependent on the size of the baby (a rough target is 40-80 ml2/sec for a 1,000 g baby) and should therefore be relatively static. Documenting the DCO2 hourly and maintaining this at a stable level can be helpful during ventilation.

HFV without VG-mode activated. If a decrease or increase in DCO2 occurs, further evaluation may be indicated (clinical exam, blood gas, X-ray, reconsider ventilator settings), esp if the frequency is maintained. A decrease in DCO2 may indicate decreased compliance for example due to a blocked tube (suction or tube change is needed), or a pneumothorax leading to a decrease in VT hf. An increase in DCO2 may indicate improvement in compliance.

HFV with VG-mode activated. DCO2 will be more static than without VG-mode as the ventilator maintains the set VT hf (stroke volume) by altering the amplitude given to the baby. The machine will alarm if the VT hf is not reached, this may be due to inadequate Ampl hf max or insufficient ventilator power (consider 1:1 ratio or lower oscillation frequency).

Frequency

Different frequencies can be chosen dependent on underlying lung pathology. Frequency should be chosen in discussion with a consultant.

For mature infants a frequency of 10Hz is appropriate. Frequency range of 7-11 is suggested for Term babies, Meconium aspiration and severe PIE. Frequency range of 12-15 for early PIE and significant preterm HMD.(Graph below sourced from Jane Pillow Dräger)2

HFOV frequency

Higher frequencies are found to be more lung protective.

It may be desirable to perform a recruitment manoeuvre. This should only be done with a consultant present.

On the VN 500, the oscillatory frequency may need to be set 1-2 Hz lower than on the Sensormedics to achieve a comparable stroke volume at the airway opening. For the Sensormedics the stroke volume is proportional to amplitude regardless of oscillatory frequency. For the VN500 the amplitude is significantly dampened at frequencies ≥10 Hz because of the oscillator mechanism in the ventilator and the compliant and flexible breathing circuit.

Please check achieved amplitude (ΔP) against set Ampl hf (or if in VG mode, check that set VT hf is achieved).

Achieved amplitudes at the airway opening (∆P Ao) were measured in a benchtop model for both the Sensormedics and VN500 and it was demonstrated that achieved amplitude decreased significantly with increasing frequencies for the VN500 but not for the Sensormedics (SM).

HFVVN1
Adapted from John J. et al.3

Oscillatory power can be increased to treat hypercapnia by decreasing the I:E ratio to 1:1 or reducing the frequency.

Inspiration:Expiration ratio

Is normally started at 1:2, which means that inspiration takes 33% and expiration 66% of cycle time. This is equal to the Sensormedics where an inspiratory time of 33% is used. At 10 Hz this will give an inspiratory time of 0.03 seconds.

I:E ratio can be changed to 1:1, where inspiration and expiration each take 50% of cycle. This is a decision made by the consultant. This I:E ratio is more powerful as it will increase stroke volume per cycle. It can however lead to air trapping as shorter time is available for expiration.

We don't use I:E ratio 1:3.

Starting VG in HFV mode

Volume guarantee in HFV mode is different from VG in conventional mode. The high frequency tidal volume (VThf) or stroke volume is much smaller (~ 1-3 ml/Kg) and reflects dead space ventilation. The VG mode can be used to maintain stable VThf or stroke volume delivery, by matching required amplitude to changes in compliance. Amplitude will be increased up to the set maximum Amplitude (Ampl hf max). Record the amplitude achieved (the VN500 displays this as ∆P on the screen).

The decision to start VG should be discussed with the consultant on call.

If starting HFV with VG-mode from the onset

Start VThf at ~2 ml/Kg and set the Ampl hf max at 5 cmH2O or ~10-15% above the current Ampl hf needed to deliver ~2 ml/Kg. Check CO2 levels with a blood gas about 1 hour after commencing this mode of ventilation (and check that the achieved DC02

If starting VG-mode when baby has already been on HFV on VN500

Assuming the baby was normocapnic, start with current averageVT as the set VThf and set the "Ampl hf max" at 5 cmH2O or ~10-15% above the current Ampl hf setting. Check CO2 levels with a blood gas about 1 hour after commencing this mode of ventilation.

Making Adjustments to ventilation once established on HFV on the VN500

Hypoxia Hyperoxia Hypercarbia Hypocarbia
Increase FiO2 Decrease FiO2 Increase Ampl hf
(~ 10%) a
Decrease Ampl hf
(~10%)
Increase MAPhf b
(1-2 cms H20) 
Decrease MAPhf
(1-2 cms H20)
If in VG mode:c
Increase VThf
(steps of 0.1-0.2 ml)
 If in VG mode:
Decrease VThf
(0.1-0.2 ml)
    Decrease Frequency d
(steps of 1Hz)
Increase Frequency d
(steps of 1Hz)
    Decrease I:E to 1:1 e  Increase I:E to 1:2 e

a Check if ΔP (achieved amplitude) is same as set Ampl hf.
b Check for atelectasis and hyperinflation. Consider recruitment maneuvers (to be done by consultant).
c Check if VThf (stroke volume) is achieved
d Changes in frequency should only be made in discussion with consultant
e Changes in I:E ratio should only be made in discussion with consultant

When changes are made, especially for ventilation, check resulting DCO2 or VThf values. Changes in DCO2 will become visible within a minute. Decide when next blood gas needs to be taken.

Chest radiograph

  • Initial chest radiograph at 1-2 hrs to determine the baseline lung volume on HFV (aim for 8 posterior ribs, be mindful of flattening of the diaphragm or bulging of lung tissue between the ribs)
  • Consider a repeat chest radiograph 4-6 hours after starting HFV to determine inflation
  • Repeat chest radiography with acute changes in baby's condition

Weaning

  • Reduce FiO2 to <40% before weaning MAPhf (except when over-inflation is evident).
  • Reduce MAPhf when chest radiograph shows evidence of over-inflation (>9 ribs or flattening of diaphragm).
  • Reduce MAPhf in 1-2 cmH2O increments to 8-10 cmH2O.
  • In air leak syndromes, reducing MAPhf takes priority over weaning the FiO2.
  • Wean the Ampl hf in ~10% or 2-4 cmH2O increments if not in VG-mode. Be mindful that relationship between amplitude and stroke volume are not linear on VN500, whereas they are on the Sensormedics. Therefore, the result of a change in amplitude may not be as predictable on the VN500.
  • Do not wean the set VThf if the baby is on VG-mode and is normocapnic. The achieved amplitude will decrease automatically if the baby improves (with improving compliance of the lung)
  • Consider to increase the frequency, especially when lower frequency was used, as pressure transmission and, therefore, potential VILI development will reduce at higher frequencies. Change of frequency should always be done following discussion with on call consultant.
  • Discontinue weaning when MAPhf 8-10 cmH2O and Ampl hf 20-25 cmH2O. Consider extubation to CPAP or change to conventional ventilation, if the baby is stable (oxygenating well with satisfactory blood gases). Discuss with consultant.

Suctioning

  • All babies on HFV should have inline suction in place, with the smallest catheter possible
  • Ventilation settings can continue as set during a suction; however, if the baby is on HFV with VG mode then please check that the Ampl hf max is set at ≤ 5 cmH2O above the amplitude the baby is currently receiving
  • If the baby is unstable, FiO2 can be increased. If this is not successful the MAP can be increased by 1-2 cmH2O until the baby has recovered from the suction. If this is not successful, consider recruitment maneuvers to be done by consultant. NB: A manual breath is not advised; however, if this is given check that the sigh breath pressure is set at set MAP + 2 cmH2O (additional settings, HFO sigh, change P sigh) as this does not automatically change with changes in MAP!

Please consider a suction if the DCO2 trend has decreased by 10% or more (if not in VG mode) or if the machine alarms at VThf not reached.

Prescription and documentation

During HFV on the VN500 the following values should be prescribed and recorded hourly on the level 3 nursing chart

  Prescription Documentation (hourly)
Ventilation mode PC-HFO ± VG PC-HFO ± VG
    FiO2
MAPhf  MAP MAP
If not in VG-mode    
Ampl hf Ampl ΔP (is achieved amplitude)
    VThf
    DCO2 (will fluctuate a bit)
If in VG-mode    
Ampl hf max Ampl hf max ΔP (is achieved amplitude)
VThf Set VThf and set VThf/Kg VThf (achieved)
    DCO2 (should be relatively stable)

NB: When a baby is switched from conventional to HFV or switched from HFV to conventional the minute volume alarms need to be adjusted otherwise the machine will keep alarming. Minute volume alarm limits during HFV are set at 25% below and above current displayed value for minute volume.

NB: Be mindful to re-adjust MV limits and Psigh following a recruitment maneuver.

Transfer on VN500 in HFV mode

If baby needs to be transferred between PICU and NICU organize D-size bottles and a set of spare bottles. D-size will give approximately 30 min, whereas the standard NICU bottle will only give 8 min worth of ventilation.

References

  1. Pillow J.J. High frequency oscillation ventilation: Mechanisms of gas exchange and lung mechanics. Crit Care Med 2005; 33 (3 suppl.): S135-S141
  2. Pillow Jane. High Frequency Oscillatory Ventilation: Theory and Practical Applications. Dräger. 2016
  3. John J., Harcourt E.R., Davis P.G., Tingay D.G. Drager VN500's oscillatory performance has a frequency-dependent threshold. J Paed Child Health 2014; 50: 27-31
  4. Luna M.S., Gonzalez M.S., Cortijo F.T. High-frequency oscillatory ventilation combined with volume guarantee in a neonatal animal model of respiratory distress syndrome. Crit Care Research and Practice 2013; 593915
  5. Mazela J., Gradzka-Luczewska A., Korzan S., et al. Normalised tidal volumes during high frequency oscillatory ventilation with the VN500 ventilator. Arch Dis Child 2014; 99:A500 doi:10.1136/archdischild-20140307384.1387
  6. Vento G., Tana M., Gianduzzo A., et al. High-frequency oscillatory ventilation (HFOV) in preterm infants: Nursing management experience of a III-level neonatal intensive care unit (NICU) at the Catholic University of the Sacred Heart of Rome. JNEP 2014; 4 (1): 62-67

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Document Control

  • Date last published: 07 October 2018
  • Document type: Clinical Guideline
  • Services responsible: Neonatology
  • Owner: Newborn Services Clinical Practice Committee
  • Editor: Sarah Bellhouse
  • Review frequency: 2 years