The bias flow nitrogen washout technique for measuring the functional residual capacity in infants

Abstract

The functional residual capacity (FRC) is the most commonly measured static lung volume in infants. It is important for interpreting volume-dependent pulmonary mechanics,e.g.airway resistance, and defining normal lung growth. The bias flow nitrogen washout technique is widely used for measuring FRC because the dead space and circuit resistance are low, making it suitable for small or sick infants. Moreover, data acquisition and calculation are easily programmed for a personal computer.The aim of this paper is to provide recommendations pertaining to equipment requirements, study procedures and reporting of data for functional residual capacity measurements. While measuring the functional residual capacity is regarded as physiologically and clinically important, the accuracy of the measurement is undoubtedly equally important. Hence, the paper also emphasizes factors influencing the accuracy of functional residual capacity measurements independent of equipment requirements. These recommendations represent the “State of the Art” in 2000.

Lung Function Tests in Neonates and Infants with Chronic Lung Disease of Infancy: Functional Residual Capacity

“…17 Apart from using the ''resident'' gas N 2 as an indicator for washout, other inert gases such as He and SF 6 have also been widely tested. Respiratory mass spectrometers can detect virtually any gas and remain the ''gold standard,'' but they are relatively expensive, bulky, and not currently in widespread use.…”

“…5,31 Conversely, in the presence of airway obstruction and uneven ventilation, FRC may be seriously underestimated when using the gas dilution or washout techniques if insufficient time is allowed for complete equilibration of gas concentrations. 17,31,32 Assessments of FRC by gas dilution or washout are similarly noninvasive and, in contrast to plethysmography, can be applied in sick and ventilated infants. Guidelines for the bias-flow-N 2 -washout technique were published, 17 although equipment for this technique is not currently being marketed.…”

“…17,31,32 Assessments of FRC by gas dilution or washout are similarly noninvasive and, in contrast to plethysmography, can be applied in sick and ventilated infants. Guidelines for the bias-flow-N 2 -washout technique were published, 17 although equipment for this technique is not currently being marketed. N 2 washout with pure O 2 may alter both breathing patterns and FRC, 33 and is impractical for ventilated infants receiving high fractional inspired oxygen concentration (FiO 2 ).…”

“…These difficulties emphasize the advantages of adhering to published guidelines developed following intensive international collaboration. 5,17,[41][42][43] Considerable care is required with both gas dilution and plethysmographic techniques to estimate and subtract the appropriate apparatus dead space when calculating FRC. 29,41 In addition to the errors arising from miscalculated dead space, one of the major difficulties imposed by circuitry and software used for gas dilution/washout is the lack of quality-control measures either to ascertain stability of a breathing pattern or to identify the phase of respiration at which an infant was switched into the system, such that an unknown proportion of tidal volume will be included in the estimated FRC.…”

“…43 Episodes of prolonged hypopnea or apnea with resultant ''loss of FRC'' or intermittent sighs, as frequently observed during ''irregular breathing patterns'' in preterm infants and neonates, will influence FRC and make comparisons within and between individuals difficult. 44 Recent guidelines 5,17 therefore recommend that stability of the end-expiratory level prior to airway occlusion (FRC pleth ) or commencement of washout be assessed and documented. Means of quantifying this in relation to the magnitudeof the child's tidal volumewere incorporated into most of the currently available commercial systems for measuring FRC in infants, 29 and such data should be reported as an integral part of quality control in future publications.…”