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Background: Respiratory admissions are over-represented in emergency medical admissions; we tested whether forced expiratory flow at 25 to 75% of vital capacity (FEF_25-75) reflecting small airways function, was predictive of 30-day mortality outcomes.

Methods: Between 2002 and 2017, there were 25,274 emergency admission episodes in 8071 patients with a primary respiratory diagnosis. We employed a logistic multiple variable regression model, to determine whether a pre-existing lung function measurement of (FEF_25–75) was prognostic for 30-day hospital mortality, having adjusted for other outcome predictors including Acute Illness Severity and Case Co-morbidity / Complexity.

Results: Respiratory admissions represented 23.7% of all admissions but 33.3% of readmissions. FEF_25–75 values linearly and inversely predicted 30-day hospital mortality outcomes - OR 0.88 (95% CI: 0.85, 0.91); consecutive deciles (falling values) of FEF_25–75 demonstrated progressively rising mortality rates. Respiratory admissions with a lower FEF_25–75 status were older 70.3 yr. (IQR: 60.9, 77.7) vs. 64.5 yr. (IQR: 50.3, 76.2), had a longer hospital length of stay – 6.2 days (IQR: 3.2, 10.9) vs. 5.8 days (IQR: 2.7, 7.3%) and a higher 30-day hospital episode mortality – 3.2% vs. 2.6%. The range of per patient mortality prediction was from decile 1 (lowest FEF_25–75 function) 17.0% (95% CI: 14.9%, 19.1%), decile 5 of 11.7% (95% CI: 10.8%, 12.5%), and decile 10 of 7.0% (95% CI: 5.8%, 8.1%). Comorbidity interacted with the lung function estimate – the threshold to influence outcome negatively was reduced in those with lower FEF_25–75 values.

Conclusion: Baseline FEF_25–75 linearly and inversely predicted 30-day hospital mortality outcomes. Outcomes in those with lower FEF_25–75 parameter showed Comorbidity dependence.

Key Words: Lung function, FEF_25–75, Mortality Outcomes, Comorbidity Score.