Letter: Role of spirometry in fixed upper airway obstruction

To the Editor: We read with interest the article by Johnson et al¹ on fixed upper airway obstruction in the April issue and would like to highlight the following points.

There is a typo in Table 2 of the article: the FEV₁/FVC ratio is misstated as FVC/FEV₁. The Tiffeneau-Pinelli index, or FEV₁/FVC ratio, represents the proportion of forced vital capacity (FVC) a person can exhale in the first second of forced expiration (FEV₁). The presence of airflow obstruction is recognized by a decrease in the FEV₁/FCV ratio.

Spirometry provides both qualitative and quantitative criteria to diagnose fixed upper airway obstruction.² Qualitatively, the shape of the flow-volume curve with a significant truncation of both the inspiratory and expiratory limbs suggests a fixed upper airway obstruction of the glottis or upper trachea. However, these patterns are effort dependent and may vary with the patient and operator, at times providing variable results.² Thus, the characteristics of the flow-volume loop should be interpreted in conjunction with quantitative criteria.

A few quantitative criteria need to be highlighted. In upper airway obstruction, there is usually a low peak expiratory flow rate in comparison with FEV₁.³ This is because the FEV₁ integrates flows over both the effort-dependent and -independent portions of the maximum expiratory flow-volume curves and as such is “insensitive” to the narrowing of the central airways. Peak expiratory flow, which is completely effort dependent, is reduced to a greater extent.⁴

The Empey index, calculated as FEV₁ (L)/peak expiratory flow (L/minute), is a measure to detect upper airway obstruction by evaluating the disproportionate reduction of peak expiratory flow rate relative to FEV₁. A value over 10 indicates possible upper airway obstruction.³

A modification of this is the expiratory disproportion index ([FEV₁/peak expiratory flow rate] × 100); a value above 50 also identifies upper airway obstruction, with high specificity for central airway lesions (as mentioned by the authors).

Maximum voluntary ventilation (MVV) is also reduced disproportionately to the decrease in FEV₁. Thus, the MVV/FEV₁ ratio is another helpful indicator of upper airway obstruction, particularly when distinguishing it from chronic airflow limitation. A low ratio, typically defined as less than 25, suggests central airway narrowing.⁵

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