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 (TGVl) rather than end-expiratory (TGVE) occlusions are used. The aim of this study was to assess whether the timing of occlusion affected TGV 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 6-24) 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, TGVl was significantly lower than TGVE (median, TGVl 233 mL; range, 130-498. Median TGVE 250 mL; range, 132-604; P < 0.05; 95% confidence intervals for the difference, 4-50 mL). In 13 infants, TGVl was lower than TGVE; 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 TGVl lie below the 95% confidence intervals of TGVE, however, two-way analysis of variance with replicated measurements showed a significant difference between TGVE and TGVl (P < 0.05). The median R(aw) was 55 cmH2O/L/s (range, 36-71). A significant positive correlation was found between R(aw) and TGVE-TGVl (r2 = 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.