Comparison between two breathing protocols in adults using Multiple Breath Nitrogen Washout (MBNW) to measure ventilation heterogeneity

Abstract

Ventilation Heterogeneity (VH) refers to the uneven distribution of ventilation within the lungs. Multiple breath washout tests measure VH by breathing in a tracer gas and tracking its concentration in each exhaled breath as the gas is progressively “washed out” of the lungs. In Multiple Breath Nitrogen washout (MBNW), the participant breathes 100% oxygen (O2), and resident nitrogen (N2) is washed out of the lungs. From the washout, multiple indices can be derived: functional residual capacity (FRC) as a measure of resting lung volume, lung clearance index (LCI) as a global measure of VH, and regional measures of VH in the primarily conductive (Scond) and acinar (Sacin) regions of the lung. MBNW is a highly valuable research tool and gaining traction as a clinical test. A consensus statement on device requirements and analysis procedures has been published to assist in establishing standardised protocols for testing. Two main breathing protocols are commonly used in MBNW testing, controlled breathing – where restrictions are placed on tidal volume (to 1L) and respiratory rate – and free breathing, where no such restrictions are placed. However, these two protocols have not been directly compared in adults, whether in health or disease, nor has there been characterisation of the between-test repeatability of the two protocols. This thesis aimed to compare MBNW indices between the two breathing protocols, and to explore if other anthropometric factors, such as height, weight, age, tidal volume or respiratory rate might affect these differences, in health and asthma. It also characterised between-test repeatability for the two protocols. In healthy adults, the FRC and LCI indices were found to be largely comparable between protocols, with a statistically but not clinically significant difference evident in LCI. However, regional measures of VH were less comparable; there was poor correlation between the protocols in Scond, and Sacin was higher, i.e. acinar ventilation heterogeneity was worse, when measured by the free breathing protocol. Importantly, differences in Sacin between breathing protocols were greater in patients who either breathed faster (increased respiratory rate) or shallower (reduced tidal volume) during free breathing than what the 1-L controlled breathing criteria imposes. Between-session variability was greater with the free breathing protocol. In asthma, the findings were mostly similar, in that FRC and LCI were largely comparable. While Scond was not different, between-protocol difference was observed in Sacin, demonstrating a proportional bias, i.e. the difference was dependent on the magnitude of Sacin. Interestingly, unlike in health, no dependence was seen with respiratory rate or tidal volume, which may reflect the relative contributions of the breathing pattern versus disease to between-protocol differences. In summary, this thesis demonstrates that the two breathing protocols commonly used in MBNW testing are not interchangeable, particularly for Sacin, not only in health but also in disease. Further work is warranted to better understand if correction for these differences is possible, to investigate more robust methods of calculating Scond and Sacin and other possible sources contributing to differences between these two established protocols. The presented findings will help inform ongoing efforts to achieve standardisation of test protocols, to facilitate multiple breath washout as a clinically useful test to measure ventilation heterogeneity.