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 p ressure (PEEP) is widely used to prevent alveolar collapse, maintain f unctional residual capacity (FRC) and improve oxygenation. Prolongatio n of inspiratory time (t(i)) and increase of peak inspiratory pressure (PIP) are also used for this purpose. We investigated the effect of v ariations of PEEP, PIP and t(i) on FRC in ten infants with hyaline mem brane disease and onset of bronchopulmonary dysplasia (BPD, it = 7), p ulmonary hypertension (n = 1), pulmonary hypoplasia (n = 1) or severe BPD (n = 1) (gestational age 24-39 weeks, median 26 weeks; birth weigh t 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 wa s changed stepwise 2-5 times in each patient (median 3);Ind mean airwa y 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 corre lated 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 val ue of 2.94 ml/cm H2O per kg, which was significantly different from ze ro (P < 0.01) and significantly higher than the slope factors of linea r 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 ad justments of PEEP than after adjustments of PIP or t(i) (P < 0.01). Th e 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 main ly determined by PEEP but not by PIP or t(i). Stabilization of FRC aft er a change of PEEP can last up to 14 min. Its duration is unpredictab le and has to be waited for when testing pulmonary function in ventila ted preterm infants.