Leaks during multiple-breath washout: characterisation and influence on outcomes

Abstract

Nitrogen multiple-breath washout (N2MBW) is increasingly used in patients with cystic fibrosis. The current European Respiratory Society/American Thoracic Society consensus statement for MBW recommends the rejection of measurements with leaks. However, it is unclear whether this is necessary for all types of leaks. Here, our aim was to 1) model and 2) apply air leaks, and 3) to assess their influence on the primary MBW outcomes of lung clearance index and functional residual capacity.We investigated the influence of air leaks at various locations (pre-, intra- and post-capillary), sizes, durations and stages of the washout. Modelled leaks were applied to existing N2MBW data from 10 children by modifying breath tables. In addition, leaks were applied to the equipment during N2MBW measurements performed by one healthy adolescent.All modelled and applied leaks resulted in statistically significant but heterogeneous effects on lung clearance index and functional residual capacity. In all types of continuous inspiratory leaks exceeding a certain size, the end of the washout was not reached. For practical application, we illustrated six different “red flags”, i.e. signs that enable easy identification of leaks during measurements.Air leaks during measurement significantly influence N2MBW outcomes. The influence of leaks on MBW outcomes is dependent on the location, relation to breath cycle, duration, stage of washout and size of the leak. We identified a range of signs to help distinguish leaks from physiological noise.

Effect of intermittent inspiratory leaks on measurement of lung clearance index using nitrogen and sulfur hexafluoride

“…We thank C.A. Whitfield and colleagues for their interest in our study [ 1 ], and agree that the effects of leaks on multiple-breath washout (MBW) outcomes are dependent on the type of tracer gas. Efforts should be made to develop an online feedback to inform operators of the tightness or leakiness of the system [ 2 ].…”

“…We, however, do not assume that modelled leaks in our study would be easily visible during testing. Figure 3a [ 1 ], for example, displays traces during a temporary leak across two tidal breaths that cannot be easily identified. The current mathematical lung model described by C.A.…”

Impact of leaks in sulfur hexafluoride and nitrogen multiple-breath washout systems

“…A recent study by L enherr et al . [ 1 ] helps quantify impacts of leaks on multiple-breath washout (MBW) systems for functional residual capacity (FRC) and lung clearance index (LCI) measurements. In my view, technical leaks (post-capillary and intra-capillary leaks) are avoidable by careful testing of the equipment before connecting to the patient.…”

“…However, pre-capillary leaks (interface between the device and patient) will occur, even in carefully controlled tests. The study [ 1 ], based primarily on computer modelling, provides detailed data showing that LCI measurements are relatively insensitive to expiratory leaks, but quantitative data on errors due to inspiratory leaks are not reported. It is, however, important to note that inspiratory leaks have a much higher impact on nitrogen (N 2­ ) MBW LCI and FRC measurements than expiratory leaks.…”

Effect of intermittent inspiratory leaks on measurement of lung clearance index using nitrogen and sulfur hexafluoride

“…Leaks during inert gas washout are a major potential source of measurement error in multiple-breath washout (MBW) testing. Visible leaks require exclusion of the whole test repeat [ 1 ], whilst undetected leaks may lead to significant errors in estimation of washout indices such as lung clearance index (LCI) and functional residual capacity (FRC) [ 2 ]. A recent study attempted to assess the impact on FRC and LCI of leaks of varying magnitude and type using clinically generated data modified to replicate the impact of leaks [ 2 ].…”