Journal of Applied Physiology, Vol 62, Issue 1 359-363, Copyright © 1987 by American Physiological Society
Measurement of thoracic gas volume by low-frequency ambient pressure changes
R. Peslin, C. Duvivier, B. Hannhart and C. Gallina
When the whole body is exposed to sinusoidal variations of ambient pressure
(delta Pam) at very low frequencies (f), the resulting compression and
expansion of alveolar gas is almost entirely achieved by gas flow through
the airways (Vaw). As a consequence thoracic gas volume (TGV) may be
computed from the imaginary part (Im) of the delta Pam/Vaw relationship:
TGV = PB/[2 pi f X Im(delta Pam/Vaw)], where PB is barometric minus
alveolar water vapor pressure. The method was tested in 35 normal subjects
and compared with body plethysmography. The subjects sat in a chamber
connected to a large-stroke-volume reciprocating pump that brought about
pressure swings of 40 cmH2O at 0.05 Hz. delta Pam and Vaw were digitally
processed by fast Fourier transform to extract the low-frequency component
from the much larger respiratory flow. Total lung capacities (TLC) obtained
by ambient pressure changes and by plethylsmography were highly correlated
(r = 0.959, p less than 0.001) and not significantly different (6.96 +/-
1.38 l vs. 6.99 +/- 1.38). TLC obtained by ambient pressure changes were
not influenced by lowering the frequency to 0.03 Hz, adding an external
resistance at the mouth, or increasing abdominal gas volume. We conclude
that the method is practical and in agreement with body plethysmography in
normal subjects.
