Sandhu Dominic, Ritchie Grant A D, Robbins Peter A
Dept of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, Oxford, UK.
Dept of Physiology, Anatomy and Genetics, University of Oxford, Oxford, UK.
ERJ Open Res. 2021 Apr 19;7(2). doi: 10.1183/23120541.00858-2020. eCollection 2021 Apr.
Multiple-breath washout techniques are increasingly used to assess lung function. The principal statistic obtained is the lung clearance index (LCI), but values obtained for LCI using the nitrogen (N)-washout technique are higher than those obtained using an exogenous tracer gas such as sulfur hexafluoride. This study explored whether the pure oxygen (O) used for the N washout could underlie these higher values.
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. Carbon dioxide and O 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 O for the N washout or with air for the tracer gas washout.
At normal ventilation/perfusion ('/') ratios, the rate of washout of N and exogenous tracer gas was almost indistinguishable. At low '/', the N washout lagged the tracer gas washout. At very low '/', N became trapped in the acinus. Under low '/' conditions, breathing pure O introduced a marked asymmetry between the inspiratory and expiratory gas flow rates that was not present when breathing air.
The use of pure O to washout N increases O uptake in low '/' 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 N.
多呼吸洗脱技术越来越多地用于评估肺功能。获得的主要统计指标是肺清除指数（LCI），但使用氮气（N）洗脱技术获得的LCI值高于使用诸如六氟化硫等外源性示踪气体获得的值。本研究探讨了用于N洗脱的纯氧（O）是否可能是这些较高值的原因。
构建了一个均匀、相互通气的腺泡模型。灌注保持恒定，通过在呼吸周期中改变扫气量来调节通气。供应腺泡的血液具有标准的混合静脉成分。血液与腺泡气体之间的二氧化碳和O交换达到平衡。模型以空气或空气加示踪气体作为吸入气进行初始化。用纯O进行N洗脱或用空气进行示踪气体洗脱。
在正常通气/灌注（'/') 比率下，N和外源性示踪气体的洗脱速率几乎无法区分。在低'/' 时，N洗脱落后于示踪气体洗脱。在非常低的'/' 时，N被困在腺泡中。在低'/' 条件下，呼吸纯O会导致吸气和呼气气流速率之间出现明显的不对称，而呼吸空气时则不存在这种情况。
使用纯O洗脱N会增加低'/' 单位中的O摄取。这会产生一股进入腺泡的背景气流，在呼气时与流出腺泡的气流相反，从而延迟N的洗脱。
