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An indication of obstruction to the upper airways (trachea and larynx) may be obtained by calculating the ratio of the forced expired volume in one second to the peak expiratory flow rate (FEV(1)/PEFR). This index was found to be usually less than 10 in normal subjects (mean 7.3), and in patients with asthma (mean 6.9), chronic bronchitis (mean 7.7), or interstitial lung disease (mean 6.3). A study of simulated upper airways obstruction showed that this index rises as the obstruction becomes more severe. All of 18 patients with proved upper airways obstruction had FEV(1)/PEFR indices greater than 10 (mean 14.0). This test can be carried out with forced expiratory manoeuvres only, and it does not require the use of complicated equipment. An FEV(1)/PEFR ratio greater than 10, when upper airways obstruction is suspected, indicates that significant obstruction may be present. High values suggest that the obstruction may be severe, and that further investigations are indicated.

Obstructing lesions of the trachea and larynx which cause a predominantly inspiratory obstruction can be satisfactorily diagnosed by measuring both F.I.V.(1) and F.E.V.(1). Chronic airways obstruction involving intrathoracic airways produces a much lower F.E.V.(1)/F.I.V.(1) percentage than normal, whereas obstruction to larynx and trachea causes a raised F.E.V.(1)/F.I.V.(1) percentage. If flow-volume measurements are not available the F.E.V.(1)/F.I.V.(1) percentage should provide a simple and useful method for diagnosis of upper airways obstruction.In one of the patients reported a predominantly inspiratory obstruction caused CO(2) retention. In patients with airways obstruction the correlation between Pco(2) and F.I.V.(1) was found to be the same as between Pco(2) and F.E.V.(1). This suggests that respiratory failure can be caused by either inspiratory or expiratory airways obstruction and that neither is of greater importance in producing CO(2) retention.