Flow limitation in normal infants: a new method for forced expiratory maneuvers from raised lung volumes.

Forced expiratory maneuvers generated by rapid thoracic compression have been used to assess airway function in infants. It remains unclear whether flow limitation can be achieved in healthy infants because low pressure transmission across the chest wall and inspiratory effort may limit the maximum transpulmonary pressure developed during the maneuver. We have found that several rapid inflations to a lung volume set at an airway pressure of 30 cmH2O (V80) briefly inhibit respiratory effort and allow forced expiration to proceed from V80 to residual volume. We used a water-filled esophageal catheter to measure isovolume pressure-flow curves in seven healthy infants (3-88 mo). Forced vital capacity (FVC) was defined as the volume between V80 and residual volume. Pressure transmission between the compression jacket and the esophagus decreased with decreasing lung volume and averaged 60 and 37% at 50 and 75% of expired FVC, respectively. Subjects demonstrated plateaus in their isovolume pressure-flow curves at 50% of expired FVC and lower lung volumes. We conclude that this new methodology enables forced expiratory maneuvers to achieve flow limitation in healthy infants over at least the lower portion of their lung volume.