The ratio of the alveolar ventilations of SF6 and He in patients with lung emphysema and in healthy subjects

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Abstract

This study assesses the possible impact of changes in the morphometry of acinar airways and air spaces on the efficacy of intrapulmonary gas mixing for sulphur hexafluoride (SF6) relative to that for helium (He). To that end the alveolar ventilations of He and SF6 were determined in patients with macroscopic lung emphysema and in healthy subjects. He–SF6 washout tests were performed in 17 patients (15 emphysema, 2 chronic bronchitis) and 21 healthy subjects. Using a three-compartment model, the data obtained were used to estimate the overall, effective, alveolar ventilations of SF6 and He, and their ratio V̇aASF6/V̇aHe. Mean V̇aASF6/V̇aHe (±S.D.) for patients (0.80±0.06) was significantly smaller (P<0.001) than the value for the group of age-matched healthy subjects (0.90±0.05) which was non-significantly smaller than the result for the group of young, healthy subjects (0.93±0.03). In our patients, we also determined a score for emphysema using high resolution computed tomography, and this score correlated inversely with V̇aASF6/V̇aHe (r=−0.56, P=0.018). We have interpreted our observations to mean that in patients with lung emphysema, the efficacy of intrapulmonary gas mixing for SF6 as compared with that for He reflected by V̇aASF6/V̇aHe is diminished due to increased diffusive path-lengths within the enlarged air spaces of their lungs which impair diffusive gas mixing for SF6 more than for He.

Introduction

Washout curves of test gases that are poorly soluble in blood and lung tissue have been used for a long time to assess the properties of pulmonary ventilation and gas mixing (Hickam and Frayser, 1958, Briscoe and Cournand, 1959). von Nieding et al. (1977) evaluated the differences between the washout curves of helium (He) and sulphur hexafluoride (SF6) obtained from patients suffering from obstructive lung disease. They observed that in early washout the (normalized) concentration of He in end-tidal gas was larger than that of SF6. However, during washout this relationship reversed causing a crossover of the He–SF6 washout curves, where the crossover occurred much later in washout in patients with lung emphysema as compared with patients with asthma or chronic bronchitis. The authors attributed their observations to diffusion limited gas mixing in the lung where this limitation could be reinforced within the enlarged acinar airways and air spaces found in patients with lung emphysema. In other words, the efficacy of intrapulmonary gas mixing of SF6 relative to that of He would depend on the morphometry of the acini. In the present study, we have investigated this assumption from its consequence for the ratio of the alveolar ventilations (V̇a) of SF6 and He which in subjects with lung emphysema should be more affected by incomplete diffusive gas mixing than in healthy subjects. For that purpose multiple breaths He–SF6 washout tests were performed in patients with macroscopic emphysema and in healthy subjects. The washout behaviour of the test gases was described with a mathematical lung model consisting of a series dead space and two alveolar compartments ventilated in parallel. For each test gas the estimated effective alveolar ventilations of the two compartments were added to obtain V̇a, and the results for V̇aASF6/V̇aHe were used to evaluate our hypothesis that V̇aASF6/V̇aHe in patients with lung emphysema may be smaller than V̇aASF6/V̇aHe in healthy subjects.

Section snippets

Materials and methods

The study was performed on 15 patients with lung emphysema, 2 with chronic bronchitis (only used for the correlation analysis), 10 age-matched healthy subjects and 11 young, healthy subjects (Table 1). The emphysema patients were selected from the outpatient clinic showing macroscopic emphysema on high resolution computed tomography (HRCT). The HRCT of the two patients with chronic bronchitis showed only minor abnormalities. All of the patients had a history of heavy smoking. None of the

Results

Lung function data and characteristics of all subjects and the HRCT-scores of the patients are listed in Table 1. The HRCT-scores for the emphysema patients ranged from 38 to 120. This range corresponds to almost no emphysema and severe disease. The lung volumes and DlCO of the healthy subjects were within normal limits (Quanjer et al., 1993). Concerning age the patients did not differ significantly from the older, healthy subjects, whereas FEV1 and DlCO did (both P<0.001). FEV1 and DlCO of the

Discussion

The washout from residual gas of test gases that are poorly soluble in blood and lung tissue have been used for a long time to study the properties of intrapulmonary gas mixing and to assess unequal ventilation in patients with obstructive lung disease (Hickam and Frayser, 1958, Briscoe and Cournand, 1959). So far, the multiple breaths washout of those test gases was represented by a plot of the partial pressure of the test gas in end-tidal gas or in mixed expiratory gas (Pē) versus the breath

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