Background Lung hyperinflation associates with adverse outcomes in smokers without airflow obstruction (AFO), and identifies subgroup of COPD patients who may benefit from lung volume reduction procedures. Undertaking lung volume measurements routinely in all such patients is unrealistic. Two equations to calculate%predicted residual volume (RV%calc) have recently been described. We retrospectively reviewed a lung function dataset to validate and compare these, and to determine potential of RV%calc to stratify patients who do not require plethysmography.

Methods Retrospective analysis of spirometry and plethysmography data from 716 consecutive patients who attended for lung function testing at one of our hospital sites in 2017. RV%calc was derived from the Elbehairy equation¹: [RV% predicted=3.58(FVC%) −164(FEV1/FVC) −81(SQRT-FVC%) −0.83(age) −10.7(gender) +732 (male=1, female=0)]and Evankovich equation²: [RV% predicted = FVC(%pred)*2.96 –(FEV1/FVC)*177 –FVC(sqrt)*71 –0.83*age –10.2*gender +704 (male=1, female=0)].

Results AFO (FEV1/FVC <0.7) was present in 271 (of 716). RV%measured was >150% in 76 patients, and >175% in 47 patients.

Bland-Altman plots indicated good agreement between RV%measured and RV%calc from both equations (median difference -10%, 95% agreement -77% to 58%). Agreement was better at lower values of RV%.

Both equations showed good performance predicting plethysmography confirmed hyperinflation at RVmeasured >150% and >175% thresholds in the overall cohort (AUROCs 0.91 for Elbehairy equation and 0.94 for Evankovich equation) and in the sub-cohort of patients with AFO (AUROCs 0.89 and 0.93). Table 1 shows sensitivity/specificity for prediction of measured hyperinflation for selected RV%calc cutoff values, derived from the ROC curves.

Conclusions Both equations for estimating residual volume%predicted from spirometry data showed good performance vs RV%measured. Including RV%calc in spirometry reports seems appropriate. Prospective validation of an approach stratifying patients who do not require plethysmography - based on RV%calculated <95% (hyperinflation excluded) or >195% (definite hyperinflation) – is merited. These cutoffs would have potentially allowed plethysmography to be omitted in 22% (154/716) of patients in this cohort.

References

1. Elbehairy, A., Whittaker, H., Quint, J, et al. ( 2018). Identifying Patient Suitability for Lung Volume Reduction - Estimation of Gas Trapping from Spirometry. Thorax, 73(4), pp.A30-A31.

2. Evankovich, J., Nouraie, S., Karoleski, C. and Sciurba, F. ( 2017). A Model to Predict Residual Volume from Forced Spirometry Measurements. C47. COPD: Physiologic Assessment, p.A5682.