Paediatrics: High frequency oscillatory ventilation
A continuous positive distending pressure (mean airway pressure) is applied and, around this pressure amplitude (or ∆p) is oscillated by a diaphragm or an interrupter device in the ventilator circuit.
High-frequency oscillatory ventilation
A continuous positive distending pressure (mean airway pressure) is applied and, around this pressure amplitude (or ∆p) is oscillated by a diaphragm or an interrupter device in the ventilator circuit. High-frequency oscillatory ventilation (HFOV) has 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.
- Pulmonar y hypoplasia.
- Congenital diaphragmatic hernia (see Fig. 6.7).
- Mean airway pressure (Paw).
- 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, however, further Paw ‘rise’ will ‘fall’ PaO2 because of overdistension.
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 the complicated airflow physics of HFOV that augment CO2 diffusion. Once the frequency is set, CO2 removal is increased by ‘rise amplitude and vice versa.
If ventilating for the first time, appropriate initial settings at term are:
- Amplitude 20cmH2O.
- Frequency 10Hz.
- FiO20.5, i.e. 50% inspired O2 concentration.
If transferring from IPPV:
- Set initial HFOV Paw2cmH2O higher then Paw used in IPPV.
- Start on the same FiO2, and set the frequency at 10Hz.
- 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 PaO2is 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 CO2elimination occurs at 10Hz, and, hence, the frequency does not usually need to be changed.
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 minimum 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.