Plethysmography Overestimates Lung Volume in Patients With COPD
Last Updated: 2010-01-06 19:01:59 -0400 (Reuters Health)
"Preferential use of plethysmography in patients with obstruction may have incorrectly supported a longstanding belief about the degree of lung hyperinflation" in severe chronic obstructive pulmonary disease (COPD), said lead author Dr. Carl O'Donnell, of
Recent limited evidence has hinted at this problem, but plethysmography is still the standard method for measuring total lung volume. In Dr. O'Donnell's study, he and his colleagues used three different methods to measure total lung capacity in 132 patients: plethysmography, helium dilution and thoracic CT scans. All of the participants were scheduled for clinically indicated chest CT scans.
The mean total lung capacity was 6.18 liters by plethysmography, 5.55 liters by helium dilution and 5.31 liters by CT scan. Plethysmography gave significantly higher lung capacities than either helium dilution (0.63 liters greater, p = 0.001) or CT (0.87 liters greater, p = 0.001). There were no significant differences between the helium dilution and CT scan methods.
The differences were even greater in the 79 subjects with COPD. In this group, total lung capacity was 0.93 liters greater with plethysmography versus helium dilution (p < 0.001) and 1.07 liters greater than by CT scan (p < 0.001). Again, there was no difference between helium dilution and CT scan results.
Further analyses showed that most of the larger differences (>1 liter) between plethysmography and the other methods occurred when forced expiratory volume in one second was less than 30% of predicted. In other words, the more severe the airway obstruction, the more total lung capacity was overestimated by plethysmography.
Conventional wisdom has held that the helium dilution method underestimates lung capacity in some settings, but Dr. O'Donnell told Reuters Health that this study suggests that "lung volume can be measured accurately by the helium dilution method, even in severe airflow obstruction."
Chest 2009.
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