A Simple Method for Measuring Functional Residual Capacity by N2 Washout in Small Animals and Newborn Infants

Abstract

An open circuit N2 washout technique is described for the determination of functional residual capacity in infants. Either 100% O2 or any oxygen/helium mixture can be used as the washing gas. The subject breathes the washing gas through a T-tube and the washed out nitrogen is mixed with this gas in a mixing chamber, placed into the exhalation part of the circuit. The N2 concentration of the mixed gas is analyzed continuously, and the concentration signal is electronically integrated over time. Calibration of the system is accomplished by injecting known amounts of nitrogen or room air into the circuit. The gas flow through the system must remain constant and is adjusted to approximate peak inspiratory flow of the infant. In vitro testing of the system showed that the technique gives reproducible values (coefficient of variance less than 1.0%) and that the integrated signal output has a close linear correlation with the amount of N2 washed out (r = 0.99). In vivo measurements in 10 cats confirmed the accuracy and reproducibility of the method when compared with N2 collection. The technical advantages of the system are simplicity of components, absence of valves, easy calibration, low dead space, and no need to collect or measure expired gases. For the infant this means no added resistance during washout and no risk of hypoxia, hyperoxia, or hypercapnea. In the presence of pulmonary disease and poor gas mixing the washout period can be prolonged as needed. There is no lower limit of weight or size for functional residual capacity measurements in small infants or animals.

Multiple-breath nitrogen washout techniques: including measurements with patients on ventilators

“…This technique has been shown to be very accurate and highly reproducible. Measurements of FRC by N 2 washout on spontaneously breathing subjects are highly reproducible, with a COV for repeated measurements of <7.1% [40]. 4.…”

“…Unlike the He dilution method, the N 2 washout method may also wash out blood and tissue N 2 . However, unlike the original descriptions for adults, where a 7 min period of O 2 breathing was calculated to wash out 220 mL of tissue nitrogen (up to 10% of the FRC) [24,25], it has been suggested in newborn infants that the volume of tissue and blood N 2 washed out in a test is not more than 1% of the total [40]. That is, no significant amount of N 2 is washed out during a test which ends within 2-3 min at most, and at most will cause a 5% error [39,53].…”

“…3. The technique developed by GERHARDT et al [40] is not immediately applicable to ventilated children, mainly because the gas flow during calibration does not equal the flow during the test. In order to overcome this difficulty, SIVAN et al [41] used the respiratory mass spectrometer already "in-line" for measuring the instantaneous N 2 concentration, to record the minute ventilation (V 'E) by the argon dilution technique [49].…”

“…In 1985, GERHARDT et al [40] provided a method to overcome the change in gas flow in nonventilated infants and children. They used an open washout system, to which a constant background O 2 flow was delivered.…”

“…9. As with the integration technique of GERHARDT and co-workers [40,53] for spontaneously breathing infants, the background flow must be extremely constant. Flow and volume are inversely related, so that an increase in flow of 10% will decrease the N 2 volume recorded by approximately the same amount, and thus the FRC measured.…”