Institute of Mathematics and Computer Science, University of Southern Denmark, Odense, Denmark.
PLoS One. 2013 Sep 9;8(9):e73335. doi: 10.1371/journal.pone.0073335. eCollection 2013.
A large body of evidence has now accumulated describing the advantages of multiple breath washout tests over conventional spirometry in cystic fibrosis (CF). Although the majority of studies have used exogenous sulphur hexafluoride (SF6) as the tracer gas this has also led to an increased interest in nitrogen washout tests, despite the differences between these methods. The impact of body nitrogen excreted across the alveoli has previously been ignored.
A two-compartment lung model was developed that included ventilation heterogeneity and dead space (DS) effects, but also incorporated experimental data on nitrogen excretion. The model was used to assess the impact of nitrogen excretion on washout progress and accuracy of functional residual capacity (FRC) and lung clearance index (LCI) measurements.
Excreted nitrogen had a small effect on accuracy of FRC (1.8%) in the healthy adult model. The error in LCI calculated with true FRC was greater (6.3%), and excreted nitrogen contributed 21% of the total nitrogen concentration at the end of the washout. Increasing DS and ventilation heterogeneity both caused further increase in measurement error. LCI was increased by 6-13% in a CF child model, and excreted nitrogen increased the end of washout nitrogen concentration by 24-49%.
Excreted nitrogen appears to have complex but clinically significant effects on washout progress, particularly in the presence of abnormal gas mixing. This may explain much of the previously described differences in washout outcomes between SF6 and nitrogen.
大量证据表明，多呼吸冲洗测试在囊性纤维化（CF）中比传统肺活量计具有优势。尽管大多数研究都使用外源性六氟化硫（SF6）作为示踪气体，但这也导致了对氮冲洗测试的兴趣增加，尽管这些方法之间存在差异。肺泡中排出的身体氮的影响以前被忽视了。
开发了一个两室肺模型，该模型包括通气异质性和死腔（DS）效应，但也纳入了关于氮排泄的实验数据。该模型用于评估氮排泄对冲洗进展和功能残气量（FRC）和肺清除指数（LCI）测量准确性的影响。
在健康成人模型中，排出的氮对 FRC 的准确性仅有微小影响（1.8%）。用真实 FRC 计算的 LCI 误差更大（6.3%），并且在冲洗结束时，排出的氮占总氮浓度的 21%。DS 和通气异质性的增加都会导致测量误差的进一步增加。在 CF 儿童模型中，LCI 增加了 6-13%，而排出的氮使冲洗结束时的氮浓度增加了 24-49%。
排出的氮似乎对冲洗过程有复杂但具有临床意义的影响，特别是在存在异常气体混合的情况下。这可能解释了之前描述的 SF6 和氮之间冲洗结果差异的大部分原因。
