MODIFICATION OF THE OPEN-CIRCUIT N-2 WASHOUT TECHNIQUE FOR MEASUREMENT OF FUNCTIONAL RESIDUAL CAPACITY IN PREMATURE-INFANTS

We compared the standard nitrogen (N-2) washout technique for measurin g functional residual capacity (FRC) with a modified technique that us es a helium/oxygen mixture (heliox) at different ratios instead of pur e oxygen. The tests were made with a standard lung function system equ ipped with an ultraviolet (UV) analyzer for measurement of N, concentr ations in the expired gas. We examined models of ''spontaneous breathi ng'' and ''mechanical ventilation,'' each with volumes of FRC in the r ange of a premature and a newborn lung (20-80 ml), using both techniqu es at different baseline inspired oxygen concentrations (FiO(2)). Corr elations between known and measured volumes were high and identical fo r the two techniques (r = 0.996), and the mean error was not significa ntly different from zero (P = 0.111). Measurements of FRC in 6 infants gave a correlation coefficient of r = 0.989 between the two technique s; reproducibility, as measured by the coefficient of variation, was h igh, showing no significant differences between both techniques (P = 0 .792). However, values of individual infants were different (P = 0.011 ), and the slope of the regression line relating measurements by the 2 techniques was 1.04, with an intercept on the y-axis at 1.46. We conc lude that FRC can be measured with the modified N-2 washout technique, using heliox as a washout gas. Volumes can be measured with high prec ision and reproducibility, even in premature infants with low lung vol umes and/or high baseline FiO(2). A correction factor may be necessary to equate FRC measurements made by oxygen-N-2 vs. heliox-N-2 washouts . Hyperoxemia and hypoxemia can be avoided by admiring different flows of oxygen to a standard heliox mixture. (C) 1997 Wiley-Liss, Inc.