Effect of Panting Frequency on the Plethysmographic Determination of Thoracic Gas Volume in Chronic Obstructive Pulmonary Disease1–3

Abstract

We studied the dependence of the Plethysmographic measurement of thoracic gas volume on the frequency of panting during occlusion in 3 normal subjects and in 10 patients with chronic obstructive pulmonary disease (COPD). Total lung capacity was derived using changes in both mouth (ΔPm) and esophageal (ΔPes) pressure to reflect changes in alveolar pressure (TLCm and TLCes, respectively). In the normal subjects, both TLCm and TLCes decreased slightly with increasing frequency of panting; the difference between TLCm and TLCes (TLCm—TLCes) was small and did not vary with frequency. In the patients, TLCes also decreased slightly, but, in contrast to the normal subjects, TLCm increased with increasing frequency. At low frequencies (less than 1 Hz), TLCm—TLCes averaged 0.14 ± 0.09 1 (mean ± SE) and was not significantly different from 0. However, TLCm — TLCes increased with frequency, averaging 1.49 ± 0.40 L at frequencies between 2.5 and 3 Hz. At all frequencies tested, cheek support reduced but did not abolish the error between TLCm and TLCes. At low frequencies, both ΔPm and ΔPes were in phase with changes in volume. However, at higher frequencies, changes in volume led changes in Pm, the phase angle between them increasing to almost 20° in one patient, whereas changes in Pes remained much more closely in phase with volume. Our results suggest that in patients with COPD, the Plethysmographic determination of absolute lung volume is valid only when the frequency of panting is low (less than 1 Hz); at higher panting frequencies, transmission of alveolar pressure to the mouth may be incomplete and result in artifactual increases in TLC.