In HFO, the strategy aimed at adequate alveolar recruitment by using a sufficiently high mean airway pressure (high volume strategy) results in improved lung function and presumably a better outcome. Measurement of LV, however, has not yet been available in HFO. We present the first data on 1) the LV of preterm infants undergoing HFO, 2) the relationship between LV and MAP and 3) the time needed for stabilization of LV after modification of MAP.

Methods: In 11 premature infants ventilated with HFO because of surfactant deficiency (gest. age 23-31 wks (range), 25 6/7 wks (median), birthweight 640-1140g, 790g, age at measurement 2-10 days, 4 days, weight 700-1250g, 850g), LV was measured repeatedly by means of the sulfur hexafluoride (SF6) washout method. After initial measurements with the clinically selected MAP, the latter was systematically varied and measurements repeated.

Results: With clinically selected MAP, LV was between 23.3 and 41.9 ml/kg (median 33.5 ml/kg). Under systematic variation of MAP, LV ranged between 14.1 and 44.2 ml/kg (median 30.3 ml/kg). LV was strongly dependent on MAP in each patient, linear regression analyses resulted in slope factors between 1.0 and 6.9 ml/cmH2O/kg (median 2.7 ml/cmH2O/kg, 95% confidence interval 1.9 - 4.7) with correlation coefficients between 0.77 and 0.99 (median 0.95). Stabilization of LV after modification of MAP took 2 - 27 minutes (median 8). The duration could not be predicted by any other value.

Conclusions: 1) Selection of MAP on a clinical basis resulted in a wide range of LVs. 2) LV was strongly dependent on MAP, but prediction of LV solely from MAP was impossible because of large patient to patient variations of linear regressions. 3) Stabilization of LV after modification of MAP took up to 27 minutes and has to be awaited before the effect is evaluated.

We speculate that LV measurement may be superior to clinical judgement for finding the individually optimal MAP in HFO ventilated preterm infants, but which lung volume is optimal remains to be defined.