High Frequency Oscillation

Equipment

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High Frequency Oscillation

Overview

High-frequency oscillatory ventilation (HFOV) employs swift ventilation rates featuring diminutive tidal volumes, often surpassing anatomical dead space, coupled with active inspiratory and expiratory phases. This technique maintains a constant distending airway pressure on the alveoli, striving to optimize functional residual capacity and enhance ventilation/perfusion matching. Small tidal volumes are superimposed at a high rate during this process. 

The objective of utilizing HFOV is to mitigate ventilator-associated lung injury, especially when conventional ventilation modes necessitate elevated airway pressures and volumes to sustain adequate gas exchange. When initiated promptly, high-frequency oscillatory ventilation has the potential to enhance oxygenation and diminish the risk of lung injury in neonates and infants. 

1. Patient Eligibility 

High-frequency oscillatory ventilation (HFOV) presents itself as a viable alternative to conventional ventilation across various disease contexts. It can be strategically employed as a "rescue therapy" when achieving sufficient oxygenation and/or ventilation proves challenging under conventional mechanical ventilation (CMV). Alternatively, HFOV may be utilized proactively to mitigate lung injury by circumventing the necessity for elevated inspiratory pressures or FiO2 levels in CMV. 

2. Specific disease scenarios where HFOV might be considered appropriate

include respiratory challenges associated with: 

  • Neonatal Respiratory Distress Syndrome (RDS) 

  • Chronic lung disease of prematurity 

  • Meconium Aspiration Syndrome (MAS) 

  • Congenital Diaphragmatic Hernia (CDH) 

  • Neonatal Air-leak Syndrome with pulmonary interstitial emphysema 

  • Pneumonia 

  • Pulmonary Hemorrhage 

  • Hypoplastic lungs 

  • Premaurity

 

3. Initial Settings 

Initial settings will be prescribed by medical provide/ hospital policy, however the following is a guide: 

  • Mean Airway Pressure (MAP): Generally the starting MAP is set 2-3cm above the current CMV mean airway pressure.   

 

Typical operating range for MAP will be between 10 to 16 cmH2O. If Higher MAPs are required, for example in in severe lung disease with very poorly compliant lungs, then they should be used with caution and careful monitoring to avoid over distension and air leak. In situations of severe gas trapping, or air leak, a lower MAP may be selected. 

 

  • Amplitude (ΔP): Set the amplitude (ΔP) to see a chest “wiggle” from the level of the nipple to the umbilicus.  The starting amplitude required to achieve adequate “wiggle”, may in due course be reduced after initial lung recruitment, to avoid hypocarbia. 

 

Typical operating ranges for ΔP (amplitude) will between 20 to 30. Higher ΔP (amplitude)   should only be used with caution only in severe lung disease. 

 

  • Frequency/Hertz: Set appropriate frequency / hertz (determined by lung pathology and clinical condition): Usually frequency is set at 8-14 Hz. Lower frequencies are typically used for term babies and higher frequencies for more premature babies. Always consult with your provider before choosing ventilator settings.  

 

  • Inspiratory Time: Typically, inspiratory time is set at 33%. 

 

  • Fio2: Fio2 should always be as low as possible to keep the patient in the appropriate SpO2 range for their gestational age. If the patient has a cardiac disease, consult your provider as they may be more comfortable with a lower SPO2 range. 

 

4. Considerations 

  • Consider a blood gas within 20-30 minutes and adjust settings as appropriate. 

  • Consider obtaining a chest x-ray after commencing HFOV to determine lung expansion,

    ideally within one hour. X-ray should show lung expansion to around the 8-9 rib space. 

 

5. Ventilation  

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 : 

  1. Increase ΔP (amplitude) in response to a raised pCO2 

  1. Reduce ΔP (amplitude) in response to a low pCO2 

 

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. Always obtain an order before making any setting on the ventilator. 

 

6. Considerations  

  • Consider a blood gas within 20-30 minutes and adjust settings as appropriate. 

  • Consider obtaining a chest x-ray after commencing HFOV to determine lung expansion, ideally within one hour. X-ray should show lung expansion to around the 8-9 rib space. 

7. Chest X-Ray  

No definitive clinical gold standard exists for assessing lung volume. Chest X-rays (CXR) can aid in evaluating lung inflation. Obtain a CXR within the initial 30 minutes of initiating High-Frequency Oscillatory Ventilation (HFOV) and consider repeating it within the first 12 hours. Subsequently, periodic CXRs may be warranted in response to acute changes in the infant's condition. Frequent CXRs may be necessary during the recruitment phase but could be performed less frequently once stability is achieved or during cautious weaning. 

  

The CXR should confirm optimal lung inflation by ensuring the diaphragm lies between the 8th and 9th posterior ribs. 

  

8. Indicators of overinflation include: 

- Diaphragm extending beyond the 10th rib 

- Intercostal bulging of the lungs 

- Flattened diaphragm 

- Thin cardiac silhouette 

  

9. Indicators of underinflation include: 

- Lung fields exhibiting areas of collapse or consolidation 

- Lung fields expanded to less than the 6th rib posteriorly 

- Elevated diaphragm 

- Atelectasis

  

In specific situations, such as non-uniform lung disease with one side experiencing collapse, tolerating slight overdistension in the healthy lung may be necessary to promote recruitment in the affected side. 

10. Trouble Shooting During HFOV: 

Low PO2: Consider: 

  • ET tube patency and position – Check for chest movement/wiggle & breath sounds, consider suction, could use Pedi-cap while giving manual breaths with neo-puff to check if ETT is in the airway. 

  • Condensation/ rain-out in the ventilator circuit- slight elevation of the oscillation to assist with rain-out and enable good drainage 

  • Air leak/ Pneumothorax- Transilluminate, Urgent CXR, drain if needed 

  • Sub-optimal lung recruitment- Increment MAP or recruitment manoeuver, consider CXR 

  • Over inflated lung – Check BP, reduce MAP; does oxygenation improve, Consider CXR 

 

High PCO2: Consider: 

  • ET tube patency and position – Check for chest movement/wiggle & breath sounds, consider suction, could use Pedi-cap while giving manual breaths with neo-puff to check if ETT is in the airway. 

  • Under-inflated lungs, insufficient alveolar ventilation – Increase ΔP (amplitude) and look for improvement in chest wall movement, could decrease frequency(Hz) after discussion with consultant. 

  • The disadvantage of high MAP is “Over inflation of lungs” which leads to decreased cardiac output and reduced BP, high pulmonary vascular resistance, decreased venous return and pulmonary leaks. It can lead to poor oxygenation and tissue perfusion, eventually leading to high CO2 levels in blood.  It should be timely identified by means of clinical assessment, BP and CXR and corrected by reducing the MAP. 

 

11. Weaning 

 

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.   

  1. It is important to maintain the lung volume during weaning. Reduce FiO2 as tolerated, and once less than around 30% begin weaning the MAP in increments of 1-2 cm H2O (except when over-inflation is evident). 

  1. In air leak syndromes ,reducing MAP may take priority over weaning the FiO2. 

  1. Reduce ΔP (amplitude) in increments of 2-4, according to the paCO2. Remember to observe the chest wall to confirm vibration. 

  1. If an acceptable paCO2 cannot be maintained by adjusting the DP (amplitude) alone then adjustments to the frequency (Hz) may be required. Changes in frequency should be discussed with the provider. 

  1. Deterioration in oxygenation may be due to either a MAP which is too low (atelectasis), or too high (over distension during the baby’s recovery phase). A CXR may help to distinguish these. 

  1. Patients may be extubated from HFOV e.g. to Vapotherm or CPAP. Alternatively patients may be transitioned to conventional ventilation prior to extubation. When changing to conventional ventilation set an appropriate PEEP and then choose a PIP to give a MAP 1 - 2 cm H2O below the HFOV setting. 

  1. Consider weaning sedation and starting caffeine citrate, if indicated, in advance of extubation. 

 

12. Potential Complications 

 

  • Decreased cardiac output (therefore decreased mean blood pressure) associated with high mean airway and intra-thoracic pressure.  Increased intra-thoracic pressure compresses the major vessels returning blood to the heart, it also acts to increase pulmonary vascular resistance thereby reducing pulmonary venous return to the left side of the heart and cardiac output to the systemic circulation.   

  • Decreased cardiac output is a particular risk when initiating HFOV in an infant where central venous pressure may already be low e.g sepsis, NEC, low intravascular volume.  Such patients may require volume expansion prior to commencing HFOV 

  • Over-distension leading to pulmonary air-leaks 

  • IVH due to changes in cerebral circulation related to treatment with the oscillator.