Comparison of the components of diffusing capacity utilizing the effective alveolar volume in patients with emphysema and chronic asthma

Quantification of single-breath underestimation of lung volume in emphysema

The extent to which a single breath measurement represents available gas dilutional as well as compressible thoracic volume in emphysema patients has not been quantified. We therefore measured single breath (TLCSB) and rebreathe helium dilution (TLCRB), and plethysmographic lung volume (TLCpleth), in fifty-five outpatients with clinical and radiographic emphysema, and in twenty-one normal controls. Among emphysema patients, TLCSB increasingly underestimated both TLCpleth and TLCRB as FEV1% predicted decreased (p for interaction=0.001 for both) by a mean of 1.7 l for TLCRB (p<0.001) and 2.2l for TLCpleth (p<0.001). In contrast, TLCRB underestimated TLCpleth by a mean of 0.5l (p<0.001) regardless of FEV1% (p for interaction=0.25). TLCSB, TLCRB, and TLCpleth showed strong agreement among normal subjects. We conclude that TLCSB underestimates available gas dilutional and compressible lung volume as physiologic emphysema severity increases. In contrast, TLCRB and TLCpleth show closer agreement which is unaffected by physiologic emphysema severity.

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 (HAAC).

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 > or = 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 > or = 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.