To describe the maximum expiratory flow-volume relationship in newborn infants, we simulated forced expiration by transiently applying positive pressure in a chamber surrounding the infant's body. Maximum expiratory flows were reached at any given lung volume when increases in chamber pressure failed to produce increases in flow. Maximum expiratory flows were achieved in seven of nine healthy newborn infants at lung volumes equal to functional residual capacity (FRC) and in all infants at lung volumes below FRC. The volume expired below FRC (6.4 ml/kg) was roughly equivalent to previously calculated values of expiratory reserve volume in newborn infants (7 ml/kg). The maximum expiratory flow volume curves showed that the infants were able to increase expiratory flow rates well above those achieved during tidal breathing. The convex shape of the curves at low lung volumes is compatible with flow limitation occurring in peripheral airways.

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