Plethysmography overestimates lung volume in patients with airflow obstruction
MedWire News: Plethysmography overestimates lung volume compared with the helium dilution method and computed tomography (CT) scans in patients with airflow obstruction, research shows.
Writing in the journal Chest, Carl O’Donnell (Beth Israel Deaconess Medical Center, Boston, Massachusetts, USA) and colleagues explain: “Total lung capacity (TLC) is commonly measured in the pulmonary function testing laboratory either by gas dilution (usually with helium), or whole-body plethysmography. These two techniques yield equivalent results among those with normal lung mechanical function.”
However, they add that “conventional wisdom suggests that in the setting of heterogeneous airflow obstruction, such as occurs in chronic obstructive pulmonary disease or asthma, plethysmography is more accurate.”
To investigate further, the researchers studied 132 patients from three hospitals. Of these, 59.8% had airflow obstruction, with a FEV1/forced vital capacity ratio of less than 0.65.
Each participant’s TLC was measured using plethysmography, the helium dilution method, and CT scans.
The researchers found that average TLC values among all the participants was 6.18 l using plethysmography, 5.55 l using the helium dilution method, and 5.31 l using CT scans.
TLC values using plethysmography were significantly greater than those measured using the helium dilution method or CT scans. There were no significant differences in TLC values between the latter two methods.
Further analysis showed that these differences were evident in patients with airflow obstruction, but not among those without airflow obstruction.
O’Donnell and team conclude: “In the setting of airflow obstruction, plethysmography systematically overestimates lung volume relative to helium or thoracic imaging despite adherence to current recommendations for proper measurement technique.”
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By Mark Cowen