Stoke-on-Trent, United Kitlgdom (D. P.S. IChanges in respiratory mechanics and timing produced by continuous negative extrathoracic pressure (CNEP) of -0.6 kPa were assessed in 18 infants recovering from neonatal respiratory distress syndrome. The mcdian gcstational age was 28 wk (range 2 4 3 6 wk). All infants were recruited before discharge from neonatal intensive care and were measured at a median postnatal age of 58 d (range 10-127 d) and a mcdian wcight of 2.67 kg (range 1.99-3.77 kg). All had been treated for respiratory distress syndrome; 11 were diagnosed as having chronic lung disease. At the time of the study, all infants were stable breathing room air. There was a significant decrease of the respiratory rate in all but one infant from 63.6 2 10.0 to 49.3 ? 9.1 breaths per min (mean * SD) during CNEP. This was predominantly due to a marked prolongation of the expiratory timc. Passive respiratory mechanics were assessed using airway occlusion techniques. Whereas respiratory system compliance (C,,) did not change in the infants with a normal baseline measurement, there was a significant improvement of C,, in the 11 infants with low C,, values in atmosphere: In the latter, all of whom were very-low-birthweight infants, C,, assessed by the multiple occlusion technique (mean ? SD) corrected for body wcight increased from 7.9 ? 1.5 to 9.4 ? 1.9 mL-kPa-'.kg-' in CNEP (p = 0.012). There was no consistent change in respiratory system resistance in this population of 18 infants. These preliminary results suggest that the beneficial effects of CNEP observed in clinical studies may be partially explained by its effects on respiratory mechanics and timing. (Pediatr Res 36: 364-372, 1994) Abbreviations CNEP, continuous negative extrathoracic pressure RDS, respiratory distress syndrome C,, respiratory system compliance (mL.kPa I ) IPPV, intermittent positive pressure ventilation C,,.,,,, dynamic lung compliance (mL-kPa-I) VLBW, very-low-birth-weight infants ( < 1500 g) CLD, chronic lung disease MOT, multiple occlusion technique SBT, single breath technique R,,, respiratory system resistance (kPa.L-'.s) Pa,, pressure at the airway opening (kPa) t,,,,, expiratory time during end-inspiratory occlusion (s) t,, expiratory time (s) V.,., tidal volume (mL) t,, inspiratory time (s) FRC, functional residual capacity (mL) EEL, end-expiratory level PNT, pneumotachograph CI, confidence interval The use of negative extrathoracic pressure as a means described in 1889. Intermittent negative extrathoracic of respiratory support has a long history dating back to pressure ventilation was subsequently used widely for the first half of the last century (1). The first apparatus treatment of respiratory failure in children with poliomydesigned for use in the resuscitation of neonates was elitis and, during the 1960s and 1970s, both intermittent negative extrathoracic pressure and CNEP ventilation