The effect of positive endexpiratory pressure, peak inspiratory pressure, and inspiratory time on functional residual capacity in mechanically ventilated preterm infants

In mechanical ventilation of preterm infants, positive endexpiratory pressure (PEEP) is widely used to prevent alveolar collapse, maintain functional residual capacity (FRC) and improve oxygenation. Prolongation of inspiratory time (t(i)) and increase of peak inspiratory pressure (PIP) are also used for this purpose. We investigated the effect of variations of PEEP, PIP and t(i) on FRC in ten infants with hyaline membrane disease and onset of bronchopulmonary dysplasia (BPD, it = 7), pulmonary hypertension (n = 1), pulmonary hypoplasia (n = 1) or severe BPD (n = 1) (gestational age 24-39 weeks, median 26 weeks; birth weight 590-2960 g, 785 g; chronological age 7-84 days, 19 days; weight 689-4650 g, 1185 g). FRC, measured using the sulphur hexafluoride washout technique, was between 6.2 and 48.3 ml/kg (median 21.5 ml/kg). PEEP was changed stepwise 2-5 times in each patient (median 3);Ind mean airway pressure (MAP) was modified independently of PEEP by changing PIP 0-2 times (median 1) and t(i) 0-2 times (median 2). Changes of FRC correlated well with modifications of PEEP in each patient (r = 0.90, range 0.71-0.99). The slope factors of linear correlations had a median value of 2.94 ml/cm H2O per kg, which was significantly different from zero (P < 0.01) and significantly higher than the slope factors of linear correlations between FRC, and MAP after modifications of PIP or t(i) (P < 0.01). The latter two were statistically not different from zero. The quotients Delta FRC/Delta MAP were significantly higher after adjustments of PEEP than after adjustments of PIP or t(i) (P < 0.01). The time lag between the change of PEEP and the stabilization of FRC on a new level ranged from 2 to 14 min (median 5). Conclusion FRC is mainly determined by PEEP but not by PIP or t(i). Stabilization of FRC after a change of PEEP can last up to 14 min. Its duration is unpredictable and has to be waited for when testing pulmonary function in ventilated preterm infants.