This is the most established role for HFOV, however most of the evidence of benefit is short term and in term babies. There is no clear evidence to support a rescue role in pre-term babies. HFOV delivered by the Draeger Medical Babylog 8000 is only likely to be effective in babies whose birth weight is less than 2 kg. Indications for rescue HFOV should be considered on a case by case basis.
In animal experiments HFOV causes less lung trauma than conventional ventilation. Whether this is also true in the human pre-term infant is still uncertain. Meta-analysis of trials that have compared prophylactic HFOV with conventional ventilation does show a significant reduction in chronic lung disease. However the findings are not consistent across the trials and lingering doubts remain about the high incidence of brain injury which was found in the HiFi trial, the first and largest of these trials.
Principles of HFOV
The use of high frequency ventilation at low tidal volume allows the primary goals of ventilation, oxygenation and CO2 removal, to be achieved without the costs of pressure-induced lung injury. HFOV has two physical goals:
Sustained inflation and recruitment of lung volume by the application of distending pressure (MAP) to achieve oxygenation
Alveolar ventilation and CO2 removal by the imposition of an oscillating pressure waveform on the MAP at an adjustable frequency (Hz) and an adjustable amplitude (dP or % on the Draeger Medical Babylog 8000).
There are two strategies used in delivering oscillatory ventilation:
High volume and low oxygen
Low volume and high oxygen
High volume Strategy
This is used where there is uniform lung disease e.g. hyaline membrane disease. The alveoli need to be expanded, therefore the MAP is increased by 2-3cmH2O above what is being achieved in CMV. In cases of severe respiratory failure HFO may be employed as a rescue therapy-very high MAP e.g. 30cmH2O may be required. If oxygenation does not improve within 6 hours alternative or additional therapy should be used e.g. HFO and nitric oxide therapy or HFO + a pulmonary vasodilator.
Low volume Strategy
This is employed where there is non-homogeneous lung disease e.g. meconium aspiration or even if there is no lung disease e.g. PPHN. In these instances over distension of the alveoli must be prevented.
HFOV is a method of mechanical ventilation that employs supra-physiological breathing rates and tidal volumes frequently less than dead space. Because conventional ventilation relies on the production of large pressure changes to induce mass flow of gas in and out of the lungs, it may be associated with deleterious consequences of volume and pressure changes at alveolar level. These include air leaks, such as PIE, pneumothorax, and bronchiolo-alveolar injury leading to chronic lung disease.