End-expiratory lung volume increased 17.5 +/- 5.4 ml (mean +/- SD) in full term infants and 7.7 +/- 2.1 ml in premature infants when 5--7 cm H2O continuous negative pressure (CNEG) was applied around the thorax. In the full term infants, respiratory rate decreased from 52--43 min-1 (P less than 0.001), mean inspiratory duration (ti) was unchanged, and mean duration of expiration (te) increased from 0.62 +/- 0.14 (SE)-0.84 +/- 0.22 sec (P less than 0.001) after application of CNEG. Te of occluded efforts on CNEG was also prolonged (P less than 0.005), although less than te of spontaneous breaths on CNEG (P less than 0.005). Te increased in four of eight premature infants when CNEG was applied. We conclude that phasic vagal feedback regulates Ti and te is controlled by tonic vagal activity.
Speculation: Premature infants may have an optimum functional residual capacity (FRC) which can be achieved with continuous distending pressure, resulting in regularization of respiration.