Gas transfer excels in predicting mortality in COPD

Researchers say that measuring gas transfer is the most effective way to predict mortality in chronic obstructive pulmonary disease (COPD), after finding that it outperformed both spirometry and plethysmography for the purpose.

Nicholas Hopkinson (Imperial College London, UK) and colleagues studied 604 COPD patients with predominantly severe or very severe disease who were prospectively followed-up from their first full lung function testing.

Over a median follow-up of 83.1 months, 229 (37.9%) patients died, and the authors found significant differences between the baseline clinical characteristics of these patients and those of patients who survived. These included being older (63.0 vs 61.1 years), having a lower body mass index (23.4 vs 24.7 kg/m²), being more likely to be Global Initiative for Chronic Obstructive Lung Disease (GOLD) stage 4 (71.2 vs 41.1%), and being more likely to be on long-term oxygen therapy (27.1 vs 11.0%).

These patients also had reduced forced expiratory volume in 1 second (30.9 vs 40.6% predicted), lower carbon monoxide transfer factor (TLCO; 34.1 vs 44.7% predicted), lower arterial oxygen partial pressure (PaO₂; 9.2 vs 9.6 kPa), and higher functional residual capacity (184.4 vs 166.1% predicted), compared with survivors.

However, in multivariate analysis, only age, TLCO % predicted quartile and PaO₂ remained significant predictors for mortality, with TLCO showing the greatest power.

Hopkinson and team say that the predictive strength of TLCO may be due to the fact that it reflects both changes in functional lung volume and in gas transport across the alveoli. Additionally, TLCO is reduced when pulmonary venous pressure is increased as a result of processes such as left heart failure, pulmonary edema, and mitral stenosis, further enhancing its prognostic value.

Similarly, PaO_2, which also performed well in the study, could reflect the integration of several factors that are not captured by conventional lung function measurements but could impact on cardiovascular morbidity and mortality, the authors suggest.

“Spirometry, although easy to perform and low-cost, cannot offer as much information as gas transfer, which is also superior to plethysmographic lung volume measurements,” comments the team in the European Respiratory Journal.

“An argument can therefore be made for performing measures of gas transfer in routine practice for patients with COPD to provide them with the best prognostic information, and future classifications of COPD should include TLCO and not rely merely on the severity of airflow obstruction,” they conclude.

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