Abstract
Extract: Functional residual capacity (FRC) and thoracic gas volume (TGV) were measured in 20 infants, of whom 11 were healthy preterm infants, 5 were recovering from the respiratory distress syndrome (RDS), and 4 had other pulmonary problems. In addition, some of the theoretical aspects of rebreathing techniques, including the lung to bag N2 difference at equilibrium, were studied by constructing a simple digital computer model of the system.
In both the normal preterm infants and the post-RDS group, the TGV was significantly greater than the FRC (0.02 > P > 0.01), indicating the presence of trapped gas (Fig. 2). The mean time taken to reach equilibrium during rebreathing was 44 sec in the normal infants and 52 sec in the post-RDS group (Table 2). In one infant with generalized cystic lung disease, equilibrium was not achieved even after 3 min of rebreathing (Fig. 1).
The computer-generated analysis of rebreathing for a normal 3-kg infant is shown in Figure 3, with an in vivo curve for comparison. The rapid equilibration of N2 was completed within 55 sec but, as in the in vivo experiments, there was a change in slope of the line after 1.7 min. It can be seen from Table 3 that, if a small initial bag volume (100 ml) is used, 30 sec of rebreathing is adequate for equilibration only when FRC and dead space are normal, and errors would occur if either were enlarged.
The computer study showed that the gradient for N2 between bag and lung at equilibrium is of the order of 0.2–0.37% which would cause a negligible error when calculating FRC and assuming that lung N2 is equal to the measured bag N2.
The combined in vivo and computer studies served to validate the rebreathing technique as a method for measuring FRC, and have enabled it to be modified for use even in small or very sick infants with poor ventilation. The most useful procedure to employ would seem to be to use an initial bag volume of 150–200 ml and to continue the rebreathing for 1.5–2 min, with samples being taken for analysis at approximately 20-sec intervals.
Speculation: Computer simulation of rebreathing by infants could be used to explore gas mixing under various conditions. The pattern of mixing might reflect maturation of lung function in the preterm or sick neonate.
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Ronchetti, R., Stocks, J., Keith, I. et al. An Analysis of a Rebreathing Method for Measuring Lung Volume in the Premature Infant. Pediatr Res 9, 797–802 (1975). https://doi.org/10.1203/00006450-197510000-00011
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DOI: https://doi.org/10.1203/00006450-197510000-00011
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