1. Tidal pressure/volume and flow/volume respiratory loops were constructed from records obtained during 98 studies on 48 neonates of various ages and gestations. Records were obtained with a total body plethysmograph and an oesophageal balloon. Total pulmonary resistance was computed at five separate levels in the breathing cycle and results were expressed graphically as tidal resistance profiles by plotting total pulmonary resistance against percentage tidal volume above functional residual capacity.

2. Values obtained for standard pulmonary mechanical measurements and thoracic gas volume were similar to those of other workers. In addition six distinctive resistance profile patterns were found and related to different breathing patterns.

3. The technique appeared to be particularly useful in identifying air trapping and also demonstrated that total pulmonary resistance is open to misinterpretation when measured only at the mid-tidal-volume level, as is the present convention.

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