The goal 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.
This is controlled mainly by adjusting amplitude to achieve optimal pCO2 (40-50 mmHg). Although the amplitude of each breath appears large by comparison to conventional ventilation pressures, the attenuation of oscillation through the endotracheal tube means that the transmitted amplitude at the level of the alveolus is very small.
Higher amplitude (% on the Draeger Medical Babylog) will increase tidal volume and hence CO2 removal. With increasing ventilator frequency, lung impedance and airway resistance increases so the tidal volume delivered to the alveoli decreases. This leads to the paradox that increasing ventilator frequency may reduce CO2 elimination.
This is determined by MAP level and lung volume. If there is no improvement in oxygenation within a few hours then HFOV alone may not work and HFOV + Nitric Oxide or HFOV +vasodilators should be considered.
X-ray reveals diaphragm flattened, lung fields expanded to greater than 8th rib posteriorly, thin cardiac silhoutte.
X-ray reveals lungs fields "whiteout" and expanded to less than 6th rib posteriorly.
PaO2 too high
Reduce oxygen concentration in increments of 30%. If PaO2 still high then reduce MAP.
PaO2 too low
Perform a chest x-ray to check the appearance;
If overdistension - reduce MAP
If underdistension - increase MAP
Measure BP as hypotension due to hypovolaemia may occur during HFOV
Carbon dioxide elimination
PaCO2 too high
Check the chest wall is "bouncing". Check that the largest possible sized endotracheal tube has been used. Increase oscillatory power. If oscillatory power is at its maximum reduce the frequency (Hertz).
PaCO2 too low
Reduce the oscillatory power.
Meconium aspiration syndrome
As there is gas trapping with chemical inflammation and atelectasis it is considered prudent to wait 48 hours until the chest x-ray shows a more homogenous appearance. The settings will then be the same as for RDS.
Persistent Pulmonary Hypertension
Where there are no additional respiratory problems, it is easy to cause overdistension. A low volume strategy should be used.
These infants tend to be hypotensive. Their BP must be checked and treated before commencing HFOV.
Air Leak Syndrome
The aim is to reduce gas flow through the leak, therefore the HFOV settings are different to the norm. MAP equal to or less than during CMV FiO2 increased to 100% to maintain a PaO2 at 50-55 mmHg. When the leak/interstitial emphysema has been absent for 48 hours then RDS type settings can be used.
i] Elective high frequency oscillatory ventilation versus conventional ventilation for acute pulmonary dysfunction in preterm infants . Henderson-Smart DJ, Bhuta T, Cools F, Offringa M. Cochrane Review - abstract