Comparison of functional residual capacity (FRC) from two multiple breath washout (MBW) systems and body plethysmography

Abstract

Background: FRC is the volume of air left in lungs after a passive expiration, being influenced by lung compliance, also gas trapping and airway obstruction. Gold standard FRC is obtained using body plethysmography (FRC PLETH ) which captures all of the gas in the thorax. FRC from gas dilution methods (MBW) may be limited to communicating lung units so may be lower than FRC PLETH particularly in disease states. Debate within the field on the ideal MBW method is pertinent so accurate measures of FRC is vital. Methods: MBW trials with N 2 (FRC N2 ) (Exhalyzer D, Ecomedics AG) and SF 6 (FRC SF6 ) (Innocor TM Innovision) were performed on the same day, FRC PLETH was taken from the closest test from 11 Healthy Controls (HC) and 24 CF. Results: FRC was similar in HC with all measuring systems (FRC SF6 2.87 ± 0.48, FRC N2 3.09 ± 0.6, FRC PLETH 2.93 ± 0.49(NS)). However, in CF patients (FEV1% 76.1% ±17.18) FRC N2 was higher than FRC SF6 (p= N2 was similar to FRC PLETH p= >0.05; FRC SF6 was lower than FRC PLETH p= N2 2.41 ± 0.96, FRC SF6 1.89 ± 0.67, FRC PLETH 2.54 ± 0.85 (p= Conclusions: In HC FRC measurements are similar between devices, previous data suggest FRC PLETH should be higher than FRC from MBW but that was not the case here. In CF FRC PLETH is higher than FRC SF6 but not for FRC N2 . Thus, the FRC N2 may be giving a more accurate reflection of FRC than FRC SF6 ; we suggest using N 2 as a washout gas may enable contribution from poorly ventilated areas of lung not obtainable for the larger molecule SF 6 . More investigation is required, particularly where debate amongst MBW techniques is rife.