SHOULD TGV BE MEASURED FROM END-INSPIRATORY OCCLUSIONS RATHER THAN END-EXPIRATORY OCCLUSIONS IN WHEEZY INFANTS

It has been suggested that thoracic gas volume (TGV) measured in infants in a plethysmograph most accurately represents true lung volume when calculated from end‐inspiratory airway occlusions. The rationale proposed is that pressure measured at the mouth underestimates alveolar pressure more at end‐expiration than at end‐inspiration, presumably due to small airway closure, and this results in greater overestimation of TGV. To investigate this possibility we calculated TGV in 40 wheezy infants from occlusions at both end‐inspiration (TGVei) and end‐expiration (TGVee) using a 60 L whole body plethysmograph. TGV was corrected for equipment dead space and tidal volume. When a significant change in TGV was defined as lying outside the 95% confidence interval of the TGVee measurements, 8 of the 40 infants tested had significantly higher TGV values measured from occlusions made at end‐expiration, while two infants had significantly lower TGV values measured from occlusions made at end‐expiration. This trend was not more common in infants with “concave” flow‐volume curves. Although it is technically easier to make occlusions at end‐expiration, occluding at end‐inspiration may minimize errors of TGV measures in a few individuals due to small airway closure at low lung volumes. Pediatr Pulmonal 1990; 9:214–219. Copyright © 1990 Wiley‐Liss, Inc., A Wiley Company