Physical effects of heliox versus oxygen on measurements of functional residual capacity by the nitrogen washout technique in small lung volumes: a model study.

Abstract

Measurements of functional residual capacity (FRC) by the nitrogen (N(2)) washout technique yield low N(2) signals in neonates and preterm infants, especially when they are on high fractions of inspired oxygen (FiO(2)). Thus, recorded values often lie in the low range of detectability. We hypothesized that using heliox instead of oxygen as a washout gas would affect the electric discharge conditions of N(2) molecules in a standard UV analyzer and thus yield higher N(2) signals. We performed three laboratory experiments using conditions similar to those in neonates with pulmonary disease, reproducing different initial FiO(2) values and very small lung volumes. Standard calibration procedures with physical models between 13.5-87 mL using a calibration syringe and purpose-built small calibration cylinders were carried out, and washout gas was either pure oxygen (as is general practice) or heliox at different ratios. We observed that the calibration line with heliox was shifted upwards and the slope was increased, depending on helium concentration and initial FiO(2). Since this effect was dose-dependent with respect to the proportion of helium in the washout gas, this strongly suggests a physical process elicited by the noble gas helium. We conclude that the heliox wash-out modification may help to increase the accuracy of FRC measurements and thus may enable studies of smaller patients or patients on high inspired FiO(2).