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This document is a guide to the theory and practical techniques for the use of High Frequency Oscillatory ventilation (HFOV) in the neonate. This mode of ventilation may be useful in settings where conventional modes are failing to achieve adequate ventilation or may result in significant pulmonary injury, or where HFOV is considered to be better suited to underlying lung pathophysiology. The decision to use HFOV is individualized and must be made by experienced senior clinicians.
High frequency oscillatory ventilation (HFOV) utilises rapid ventilation rates with small tidal volumes (often less than anatomical dead space) and active inspiratory AND expiratory phases. A constant distending airway pressure is applied to the alveoli which aims to maximise functional residual capacity and ventilation/perfusion matching, over which small tidal volumes are superimposed at a high rate.
The aim of using HFOV is to reduce ventilator associated lung injury when high airway pressures and volumes in conventional ventilation modes are required to maintain adequate gas exchange. When initiated early, high frequency oscillatory ventilation may improve oxygenation and reduce risk of lung injury in neonates and infants.
HFOV may be used as an alternative to conventional ventilation in a number of disease settings. HFOV may be reserved as a “rescue therapy” when adequate oxygenation and/or ventilation cannot be achieved on CMV. Alternatively, HFOV may be employed to minimise lung injury by avoiding use of high inspiratory pressures or FiO2 on CMV.
Particular disease settings where HFOV may be appropriate include lung disease in:
Please discuss the decision to commence HFOV with the attending consultant.
Initial settings will be prescribed by medical staff, however the following is a guide:
Expect to stay with the baby and make adjustments to the MAP and ΔP (amplitude) in the first minutes of the initial hour of starting HFOV, based on oxygenation, CO2 clearance, hemodynamic (BP) and chest x-ray.
On commencement of HFOV it is useful to ensure that optimal lung volume has been achieved (as discussed earlier). MAP is used to achieve and maintain the maximal recruitment of alveoli. Both under-inflation and over-inflation of the lungs will give sub optimal oxygenation.
Ventilation (CO2 clearance) in HFOV is controlled by the DP (amplitude), for a given level of lung inflation. It is also influenced by the frequency of oscillation (Hz). Decreasing the frequency can cause markedly increased CO2 elimination and should not be done without discussion with the attending consultant.
In response to pCO2 measurements adjust the DP (amplitude) in increments of 2-4.:
Typical operating ranges for ΔP (amplitude) will be between 20 to 30cm H2O. Higher ΔP (amplitude) should only be used with caution only in severe lung disease.
Always observe the chest wall to make sure that it is still vibrating. Recheck a blood gas 20 minutes after making a change.
Ventilation parameter during HFOV
End tidal CO2 monitoring does not work during HFOV and other trends in ventilatory parameters should be monitored and charted.
VThf is strongly influenced by the internal diameter of the tracheal tube. When HFOV is used without volume guarantee, halving the tracheal tube internal diameter (e.g., due to secretions) will increase the resistance by 16 fold, and markedly reduce tidal volume. Small increases in resistance are also evident with longer tracheal tubes. VThf is also influenced by the ΔP (amplitude), frequency and I:E ratio set for the patient.
(NB – When HFOV is used with VG, ventilators, which will make modifications to the DP to try to maintain tidal volume; however evidence is still evolving and being studied in neonates at this time. See section 3.8 for more details
There is no gold standard clinical test of lung volume. CXR can assist in assessing lung inflation. Obtain a CXR within the first 30 minutes of commencing HFOV and consider repeating within the first 12 hours. Thereafter, consider repeating CXRs with acute changes in the baby’s condition. Frequent CXR may be needed whilst establishing recruitment but may be able to be done less frequently once stable or cautiously weaning
The CXR should confirm the diaphragm to be lying between the 8th and 9th posterior ribs for optimal lung inflation.
Overinflation is indicated by:
Underinflation is indicated by:
In specific situations such as non-uniform lung disease with one side of the lungs collapsed, you might have to tolerate slight over distension in the good lung to encourage recruitment in the other side.
Ensure that related aspects of care are optimised, including:
Lung volume loss during ETT disconnection e.g. suctioning:
If HFOV is briefly interrupted (e.g. due to ventilator disconnection, ETT suction) then there may be a loss of lung volume due to the drop in MAP. A recruitment manoeuvre may be required at this time to restore lung volume and optimise oxygenation and ventilation. Use of in-line suction, where available, can also prevent this.
↓PO2/ Poor Oxygenation
↑ PO2/Over Oxygenation
Increase ΔP (amplitude)
Decrease ΔP (amplitude)
Decrease Frequency** (1-2Hz) if amplitude maximal
Increase frequency** (1-2Hz) if amplitude minimal
*Consider recruitment manoeuvres – discuss with consultant
**Changes in frequency should only be made after discussion with consultant
Trouble Shooting During HFOV:
Low PO2: Consider:
High PCO2: Consider:
Weaning is primarily through reduction in the MAP as compliance improves, however it may also be necessary to adjust DP +/- frequency (Hz), as CO2 clearance improves, to avoid hypocarbia.
A lower MAP many minimise air leak.
Mean Airway Pressure - a continuous distending pressure measured in cm H20
Amplitude/D P - the peak to trough measure of the pressure waveform. This is the measure of pressure the ventilator uses to push air into the circuit. ΔP (amplitude) creates the wiggle seen in HFOV.
Frequency - the rate at which oscillations are delivered. Expressed in hertz where 1 hertz = 60 breaths per minute
Functional Residual Capacity - the volume of air present in the lungs at the end of expiration
Dead Space - the air in the nose, mouth, larynx, trachea, bronchi and bronchioles where air does not come into contact with the alveoli of the lungs i.e. the portion of tidal volume that does not take part in gas exchange
Theories of of ventilator induced lung injury:
The use of large tidal volumes to achieve adequate ventilation damages the pulmonary capillary endothelium, alveolar and airway epithelium. This mechanical damage causes fluid, protein, and blood to leak into the airways, alveoli and interstitial tissue of the lung and leads to progressive respiratory failure.
Refers to the damage caused to a lung unit by repetitive opening and closing of the alveoli. This opening, collapse and reopening of alveoli causes surface forces which have the potential to damage the surface epithelium of the airways.
Mechanical damage caused to the airways by the application of high positive airway pressures
The administration of excessive levels of oxygen and the action of oxygen free radicals is implicated in lung disease and in the development of retinopathy of prematurity. Un-scavenged oxygen free radicals have a direct effect on pulmonary epithelial cells leading to cell membrane injury. The lungs, in response to this damage tries to regenerate but the repair is fibrotic, which is typical of chronic lung disease in premature infants.
HFOV involves applying a pressure waveform over a continuous distending pressure. In most ventilators both the inspiratory and expiratory cycles are active i.e. gas is pushed in and pulled out. An alternative means of creating oscillation is by flow interruption.
During HFOV alveoli are kept open utilising a continuous distending pressure (the mean airway pressure: MAP) and not subjected to large pressure and volume swings causing the traumatic ‘inflate-deflate’ cycle to maintain gas exchange (Figure 1). In contrast during conventional ventilation the alveoli open and close on every breath delivered and large pressure swings (PIP/PEEP) are required to move relatively large volumes of gas to enable adequate gas exchange to occur.
Figure 1: Ventilation waveform in Conventional ventilation (PCV = Pressure Controlled Ventilation) and HFOV.
(Reproduced from Chan KP, Stewart TE, Mehta S. High-frequency oscillatory ventilation for adult patients with ARDS. Chest. 2007; 131(6):1907-1916.)
In HFOV oxygenation is decoupled from ventilation and each can be controlled independently. Oxygenation is determined by inspired FiO2 and lung recruitment, which is determined by MAP. Carbon dioxide clearance (i.e. ventilation) is controlled separately by the tidal volume which is dependent on oscillation frequency (oscillations per minute) and amplitude of the waveform ΔP (amplitude).
As per the Cochrane review (2015), there is evidence that the use of elective HFOV compared with CV results in a small reduction in the risk of CLD, but the evidence is weakened by the inconsistency of this effect across trials. Probably many factors, both related to the intervention itself as well as to the individual patient, interact in complex ways. In addition, the benefit could be counteracted by an increased risk of acute air leak. Adverse effects on short-term neurological outcomes have been observed in some studies but these effects are not significant overall. Most trials reporting long-term outcome have not identified any difference.
Gas transport in HFOV is thought to be complex and accomplished by a combination of the following mechanisms (see also Figure 2):
Figure 2: Proposed mechanisms of gas transport in HFOV
(Reproduced from Slutsky, AS, Drazen, JM Ventilation with small tidal volumes.N Engl J Med2002; 347,630-631)
It is important to appreciate that in HFOV the pressures set at the ventilator and delivered at the ET tube are not the same as those reaching the alveoli. Approximately 10% of the ΔP (amplitude) delivered at the endotracheal tube is transmitted to the terminal bronchioles and alveoli, compared with 90% of PIP transmission during conventional ventilation (Figure 3)
Figure 3: Reduction in HFOV waveform along respiratory tree
(Reproduced from Rajiv P K .Essentials of Neonatal Ventilation 2010)
The determinants of oxygenation and CO2 clearance (ventilation) during HFOV are discussed in detail below and summarised in Figure 4.
Figure 4: Ventilation parameters and effect on oxygenation and ventilation in HFOV and conventional ventilation (CMV)
(Reproduced from Royal Children’s Hospital, Melbourne Newborn Services Guidelines – 2014)
Determinants of oxygenation in HFOV
Oxygenation is determined by lung volume and oxygen concentration delivered (FiO2). The mean airway pressure (MAP) setting on the HFOV is used to modify lung volume by recruiting atelectatic lung units and optimising the alveolar surface area for gas exchange. The MAP is considered to act as a continuous distending pressure (CDP) on the alveoli. The goal of recruitment manoeuvers in HFOV is to open atelectatic lung units to gas exchange and then find the lowest possible mean airway pressure to keep them open (see Optimal ventilation strategy below)
Determinants of ventilation (CO2 clearance) in HFOV
In conventional ventilation modes CO2 clearance is determined by minute ventilation which is: minute volume tidal volume x frequency. The HFOV equivalent of minute volume is the gas transport coefficient or DCO2.
During HFOV CO2 clearance is determined by the amplitude (ΔP) (amplitude) and frequency (measured in hertz):
Successful application of HFOV is dependent upon ventilation with the lung recruited, known as the open lung strategy or the high lung volume strategy.
Continuous distending pressure (MAP) during HFOV recruits the lung if sufficient pressure is applied on initiation to open most lung units and retains volume if sufficient pressure is maintained to keep most lung units open. In other words, the goal of the open lung strategy is to open collapsed alveoli to gas exchange and then find the lowest possible MAP that will keep them open.
The ‘art’ of HFOV relates to achieving and maintaining optimal lung inflation. Optimal oxygenation is achieved by gradual increments in MAP to recruit lung volume and monitoring the effects on arterial oxygenation. The aim is to achieve maximum alveolar recruitment without causing over-distension of the lungs.
Optimising lung inflation with MAP: It is useful to conceptualise HFOV as like taking the lung around one sustained pressure volume hysteresis loop (Figure 5). Using the principles of P/V relationship, lung recruitment and and optimal range of lung volume we can apply these to optimise lung volume and oxygenation (Figure 6).
Figure 5: Pressure volume relationship in the lung demonstrating optimal pressure range with maximal lung recruitment (lung volume)
(Reproduced from Royal Children’s Hospital, Melbourne Newborn Services Guidelines – 2014)
Figure 6: Pressure volume relationship in the lung demonstrating different stages of lung inflations as detailed below as Point A,B,C and D
(Reproduced from Royal Prince Alfred Hospital, Sydney Newborn Services Guidelines – 2006)
Point A in figure: Under-inflation: At this point the lung is under-inflated, PVR will be high and relatively large amplitude will produce only small changes in volume. Clinically this manifests as a high oxygen requirement with limited chest vibration. CO2 clearance is also reduced.
Point B in figure: Optimal recruitment inflation: Once the lung has opened up with higher MAP, smaller amplitude will produce a larger change in volume. Clinically this manifests as improved oxygenation (falling oxygen requirements) and good chest vibration resulting in improved CO2 clearance. PVR will also be lower.
Point C in figure: Over-inflation: At this point excessive MAP has produced over-inflated lung. Oxygenation and CO2 clearance will start to deteriorate and PVR will increase contributing to cardiovascular compromise. This is the most dangerous point in HFOV and is to be avoided at all costs. It is difficult to pick clinically because the oxygen requirement may stay low, although they will eventually rise and the reduced chest vibration is easy to miss. Chest X-ray may be helpful to detect over-inflation.
Point D in figure: Optimal inflation: The goal should be to move the babies lungs from point B to point D avoiding point C (as shown on the arrow marked *** in Figure 2). Having achieved optimal lung inflation by slowly reducing MAP it should be possible to maintain the same lung inflation, oxygenation and ventilation at a lower MAP. If MAP is lowered too far oxygen requirements will start to rise again
HFOV-VG allows the clinician to set a predefined tidal volume, irrespective of other ventilator variables such as ‘frequency’ or ‘I:E ratio’. The clinician will define a maximum ΔP (amplitude), and the ventilator will adjust the delivered ΔP (amplitude) as required (up to the predefined ΔP/amplitude max) to achieve the set tidal volume.
HFOV-VG is still not widely used due to insufficient data in neonates. There are various studies on animal and experimental lung models but Belteki et al (2019) reported the first detailed analysis of ventilation parametres when using neonatal HFOV-VG over long periods. They stated that in contrast to the traditional HFOV, ventilator tightly controls the VThf while the ΔP (amplitude) varies widely when HFOV-VG mode is used. The use of targeted VThf with autochanging ΔP (amplitude) could be useful when the respiratory mechanics change rapidly such as after surfactant therapy, drainage of a pneumothorax or treatment with sedation and muscle relaxation. Without VG, these events could lead to excessive oscillations and hypocapnia, unless the ΔP (amplitude) is reduced promptly by the medical team. Despite good overall control in HFOV-VG, there is frequently short term variability of VThf, which could be secondary to patient- ventilator interactions i.e. baby’s spontaneous breathing activity or movements than on whether VG was used or not. Also, the strong inverse relationship between VThf & DCO2 with pCO2 in blood could not be established, therefore close monitoring of EtCO2 or pCO2 in blood during HFOV-VG was recommended.
They have suggested to start VThf at 2-2.5ml/kg with close monitoring of CO2 values. Once the patient is stable, it could then be weaned in small steps i.e. no more than 0.1ml/kg at a time. Gonzalez-Pacheo et al (2018) in their study on animals showed that use of very low VThf with high frequency (20 Hz) to maintain a target pCO2, significantly reduced the ventilator lung injury seen on tissue histology, when compared to the group using higher VThf and lower frequency(10hz).
This guideline has been developed with particular reference to the HFOV Guideline of the Newborn Intensive Care Unit, Royal Children’s Hospital Melbourne and clinical practice guideline for NICU KEMH, PCH and NETS Western Australia
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Last reviewed: 10 October 2020
Next review: 01 October 2023
Author(s): Tayyaba Yasmeen – Paediatric Trainee GG&C; Althaf Ansary - Neonatal Consultant – Ayrshire Maternity Unit
Co-Author(s): Other Professionals consulted: Neil Patel – Neonatal Consultant – QUEH
Approved By: West of Scotland Neonatology Managed Clinical Network