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Chapter: Paediatrics: Neonatology

Paediatrics: High frequency oscillatory ventilation

A continuous positive distending pressure (mean airway pressure) is ap-plied and, around this pressure amplitude (or ∆p) is oscillated by a dia-phragm or an interrupter device in the ventilator circuit.

High frequency oscillatory ventilation

 

A continuous positive distending pressure (mean airway pressure) is ap-plied and, around this pressure amplitude (or ∆p) is oscillated by a dia-phragm or an interrupter device in the ventilator circuit. High frequency oscillatory ventilation (HFOV) has an efficacy equivalent to IPPV in the primary treatment of RDS. It may be indicated for:

·  Rescue treatment when IPPV has failed.

 

   Pulmonary air leaks.

 

   Meconium aspiration syndrome.

 

   PPHN.

 

   Pulmonar y hypoplasia.

 

   Congenital diaphragmatic hernia (see Fig. 6.7).

 

Ventilation parameters

   Mean airway pressure (Paw).

   FiO2.

   Airway pressure difference generated around Paw (amplitude or ∆p).

   Oscillation frequency per second.

   Circuit gas flow.

 

Oxygenation (PaO2) is dependent on both Paw and FiO2. As Paw ‘rise’, PaO2 will improve as functional residual capacity (FRC) ‘rise’. At some point, how-ever, further Paw ‘rise’ will ‘fall’ PaO2 because of over distension.

 

CO2 removal (PaCO2) is dependent on alveolar ventilation and, so, on both the frequency and amplitude. Unlike IPPV, ventilator constraints make tidal volume inversely proportional to the frequency. It is normal for generated tidal volumes to be less than that physiologically required, yet adequate ventilation occurs—this apparent paradox is explained by com-plicated air flow physics of HFOV that augment CO2 diffusion. Once the frequency is set, CO2 removal is increased by ‘rise’amplitude and vice versa.

 

Commencing ventilation

 

If ventilating for the first time, appropriate initial settings at term are:

   Paw 8cmH2O.

   Amplitude 20cmH2O.

   Frequency 10Hz.

 

   FiO2 0.5, i.e. 50% inspired O2 concentration.

If transferring from IPPV:

   Set initial HFOV Paw 2cmH2O higher then Paw used in IPPV.

   Start on the same FiO2, and set frequency at 10Hz.

Monitoring ventilation

 

Once ventilated, observe the infant’s chest expansion and oscillation, and alter settings as required.

Perform a CXR after 1hr to assess chest expansion: 8 posterior ribs visible above the diaphragm is appropriate until the baby is stable.

 

Monitoring ventilation is otherwise as for IPPV. Be aware that rapid elimination of CO2 can occur leading to over-ventilation. Anticipate and monitor blood gas/transcutaneous readings closely.

 

If PaO2 is too low: ‘rise’either the FiO2 or mean airway pressure (MAP) by 1–2cmH2O every 30–60min (avoid chest overexpansion), and vice versa.

If CO2  is too high: ‘rise’amplitude by 2cmH2O increments and vice versa.

Optimal CO2 elimination occurs at 10Hz, and, hence, the frequency does not usually need to be changed.

 

Weaning ventilation

 

As clinical status improves: ‘fall’ FiO2 to 0.5 and then ‘fall’ Paw by 2cmH2O steps until 6–7cmH2O is tolerated. Also progressively ‘fall’ amplitude to the mini-mum required to maintain normal CO2.

 

Some babies will tolerate weaning to what is essentially CPAP, whilst others, below a certain Paw, do better if changed to slow rate IPPV.

 

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Paediatrics: Neonatology


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