Correction of Single-Breath Helium Lung Volumes in Patients With Airflow Obstruction

Study objective: To determine whether alveolar volume (V_A) measured during the single-breath diffusing capacity for carbon monoxide (Dco) can be used as a substitute measure for the multiple-breath total lung capacity (TLC) in subjects with and without airways obstruction.

Design: Retrospective review of pulmonary function test (PFT) results.

Setting: Pulmonary function laboratories at the Johns Hopkins Hospital (JHH) and the Johns Hopkins Asthma and Allergy Center (JHAAC).

Participants: Patients referred for spirometry, helium lung volumes, and Dco during a single visit between November 1993 and November 1996.

Results: JHAAC patients (n=2,477) were used to assess the relationship between V_A and TLC. In patients with an FEV₁/FVC ≥0.70, there was good agreement between V_A and TLC (V_A/TLC=0.97 to 0.99). However, in patients with an FEV₁/FVC <0.70, V_A systematically underestimated TLC (V_A/TLC=0.67 to 0.94). The degree of underestimation was related to the severity of airflow obstruction. To predict TLC using V_A, a regression equation was used to “correct” V_A for the severity of obstruction. This equation was used to predict the multiple-breath TLC for JHH patients (n=2,892). Patients with an FEV₁/FVC ≥0.70 showed a high degree of correlation between V_A and TLC (Pearson's correlation coefficient [r]=0.96 to 0.99; p<0.05). After adjusting for the severity of airflow obstruction, patients with an FEV₁/FVC in the range of 0.40 to 0.70 also had a strong correlation between the corrected V_A and the multiple-breath TLC (r=0.83 to 0.94; p<0.05).

Conclusions: V_A accurately predicts TLC in patients with mild or no airflow obstruction. For patients with moderate to severe obstruction, correcting V_A for the severity of obstruction improves the accuracy of this relatively simple and rapid technique for measuring TLC.

(CHEST 1998; 114:907–918)

Abbreviations: cV_A=alveolar volume corrected for the severity of airflow obstruction; Dco = single-breath diffusing capacity for carbon monoxide; FRC=functional residual capacity; JHAAC=Johns Hopkins Asthma and Allergy Center; JHH=Johns Hopkins Hospital; PFT=pulmonary function test; r=Pearson's correlation coefficient; RV=residual volume; SVC=slow vital capacity; TLC=total lung capacity; V_A=alveolar volume