Determinants of oxygenation during high frequency oscillation.

Two studies are reported in which the aim was to assess whether oxygenation on transfer to or during high frequency oscillation (HFO) was influenced by the mean airway pressure (MAP) level. Sixteen infants, median gestational age 28 weeks, were recruited into the first study and 14 with a median gestational age of 29 weeks into the second. In the initial study, blood gases were measured immediately before and 30 min after transfer to HFO and comparison made of those infants in whom oxygenation did or did not improve. In the second study the infants were studied at two MAP levels, 2 and 5 cmH2O, above that used during conventional ventilation (baseline MAP) and at two frequencies (10 and 15 Hz), arterial blood gases were measured after 20 min on each setting. In the initial study, on transfer to HFO, oxygenation improved by a median of 21 mmHg in eight infants, but was either unchanged or deteriorated (n = 7) in the other eight infants, the median impairment in oxygenation was by 17 mmHg. The infants in whom oxygenation improved had required a significantly higher MAP during conventional ventilation than the rest of the study group. In the second study, increasing the MAP from 2 to 5 cmH2O above baseline resulted in a significant increase in oxygenation, which was significantly greater at 10 rather than 15Hz. Infants whose MAP remained below 13 cmH2O had impaired oxygenation during HFO compared to that experienced during conventional ventilation. The results of these two studies demonstrate that the MAP level during HFO is an important determinant of oxygenation.