Measurement of thoracic gas volume in patients born prematurely: Should occlusion be made at end‐inspiration or end‐expiration?

It has been suggested that in infants born at term thoracic gas volume (TGV) may be more accurately estimated in a plethysmograph if end‐inspiratory (TGV I ,) rather than end‐expiratory (TGV E ,) occlusions are used. The aim of this study was to assess whether the timing of occlusion affected TGV I results in patients born very prematurely. Fifteen children with a median gestational age of 28 weeks (range 23–34) and postnatal age of 10 months (range 624) were studied. Measurements of TGV and airway resistance (R aw ) were made in a whole body plethysmograph after sedation with chloral hydrate. End‐expiratory and end‐inspiratory occlusions were performed randomly in each subject. Overall, TGV I , was significantly lower than TGV E , (median, TGV I , 233 mL; range, 130–498. Median TGV I , 250 mL; range, 132–604; P < 0.05; 95% confidence intervals for the difference, 4–50 mL). In 13 infants, TGV I , was lower than TGV E , the remaining two patients did not differ significantly from the rest of the group and neither had neonatal chronic lung disease. In only five infants did TGV I , lie below the 95% confidence intervals of TGV E , however, two‐way analysis of variance with replicated measurements showed a significant difference between TGV I , and TGV E , ( P < 0.05). The median R aw was 55 cmH 2 O/L/s (range, 36–71). A significant positive correlation was found between R aw and TGV E ,‐TGV I , (r 2 = 0.5, P < 0.01). We conclude that in children born very prematurely and with high R aw , occlusion at end‐expiration rather than end‐inspiration yields higher TGV results at follow‐up. Pediatr Pulmonol. 1994;18:295–298 © Wiley‐Liss, Inc.