Abstract
Background Multiple-breath washout techniques are increasingly being used to assess lung function. The principal statistic obtained is the lung clearance index (LCI), but values obtained for LCI using the N2-washout technique are higher than those obtained using an exogenous tracer gas such as SF6. This study explored whether the pure O2 used for the N2 washout could underlie these higher values.
Methods A model of a homogenous, reciprocally-ventilated acinus was constructed. Perfusion was kept constant, and ventilation adjusted by varying the swept volume during the breathing cycle. The blood supplying the acinus had a standard mixed-venous composition. CO2 and O2 exchange between the blood and acinar gas proceeded to equilibrium. The model was initialised with either air or air plus tracer gas as the inspirate. Washouts were conducted with pure O2 for the N2 washout or with air for the tracer gas washout.
Results At normal ventilation-perfusion (V̇/Q̇) ratios, the rate of washout of N2 and exogenous tracer gas was almost indistinguishable. At low V̇/Q̇, the N2 washout lagged the tracer gas washout. At very low V̇/Q̇, N2 became trapped in the acinus. Under low V̇/Q̇ conditions, breathing pure O2 introduced a marked asymmetry between the inspiratory and expiratory gas flow rates that was not present when breathing air.
Discussion The use of pure O2 to washout N2 increases O2 uptake in low V̇/Q̇ units. This generates a background gas flow into the acinus that opposes flow out of the acinus during expiration, and so delays the washout of N2.
Footnotes
This manuscript has recently been accepted for publication in the ERJ Open Research. It is published here in its accepted form prior to copyediting and typesetting by our production team. After these production processes are complete and the authors have approved the resulting proofs, the article will move to the latest issue of the ERJOR online. Please open or download the PDF to view this article.
Conflict of interest: Dr. Sandhu reports personal fees from Clarendon scholarship (University of Oxford), personal fees from New College (University of Oxford), during the conduct of the study.
Conflict of interest: Dr. Ritchie reports grants from National Institute for Health Research (NIHR) Oxford Biomedical Research Centre (BRC), grants from Engineering and Physical Sciences Research Council (EPSRC), during the conduct of the study; In addition, Dr. Ritchie has a patent European Patent Application No. 09756339.9 pending, and a patent European Patent No. 3314213 issued.
Conflict of interest: Dr. Robbins reports grants from National Institute for Health Research (NIHR) Oxford Biomedical Research Centre (BRC), grants from Engineering and Physical Sciences Research Council (EPSRC), during the conduct of the study; In addition, Dr. Robbins has a patent European Patent Application No. 09756339.9 pending, and a patent European Patent No. 3314213 issued.
This is a PDF-only article. Please click on the PDF link above to read it.
- Received November 15, 2020.
- Accepted March 2, 2021.
- Copyright ©The authors 2021
This version is distributed under the terms of the Creative Commons Attribution Non-Commercial Licence 4.0. For commercial reproduction rights and permissions contact permissions{at}ersnet.org
