Thermal artifacts in plethysmographic airway resistance measurements. Peslin, R., C. Duvivier, M. Vassiliou, and C. Gallina. Unit[acute]e 14 de Physiopathologie Respiratoire, Institut National de la Sant[acute]e et de la Recherche M[acute]edicale. Universit[acute]e H. Poincar[acute]e Nancy I, 54500 Vandoeuvre-les -Nancy, France
APStracts 2:0320A, 1995.
Measurements of airway resistance (Raw) by body plethysmography, when breathing unconditioned air, are implicitly based upon the assumption that the warming and humidification of air in the airways is instantaneous. Simulation with a simple model suggests that Raw may be frequency dependent and substantially underestimated at most breathing frequencies if the time constant of gas conditioning (q) is between 0.01 and 0.3 s. We measured the frequency dependence of the real (Re) and imaginary parts of the relationship between the plethysmographic signal and airway flow from 0.5 to 3 Hz in six healthy subjects in several situations. When breathing unconditioned air through a heated pneumotachograph, Re increased by 47 +/- 70 % between 2 and 3 Hz; the data were consistent with a q of 0.087 +/- 0.023 s. Additional deadspaces moderately increased q (0.105 +/- 0.031 and 0.120 +/- 27 s with 50 cm long PVC and copper tubes, respectively). When breathing saturated air conditioned at 34, 36, 38 and 40 C, Re exhibited a much smaller positive frequency dependence, most of which was probably due to a flow dependence of Raw. We conclude that, unless the inspired gas is conditioned, plethysmographic airway resistance is likely to be substantially underestimated, particularly when measured during spontaneous breathing. 

Received 23 June 1995; accepted in final form 12 July 1995.
APS Manuscript Number A82-5.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 30 July 1995.