THE SIGNAL IN TOTAL-BODY PLETHYSMOGRAPHY - ERRORS DUE TO ADIABATIC-ISOTHERMAL DIFFERENCE

Total-body plethysmography is a technique often employed in comparativ e physiology studies because it avoids excessive handling of the anima ls. The pressure signal obtained is generated by an increase in intern al energy of the gas phase of the system. Currently, this increase in internal energy is ascribed to heating (and water vapour saturation) o f the inspired gas. The standard equation for computing tidal-volume i mplies that only temperature and saturation differences can be respons ible for generating the ventilation signal. In this study, we were abl e to demonstrate that the difference between the external process of t he thoracic expansion, which is adiabatic, and the internal process of it, which is isothermic, is an important factor of internal energy ch ange in the total-body plethysmography method. In other words, organic tissues transfer heat to the entering gas but also to the present gas , in a way that keeps internal expansion an isothermic process. This e xtra amount of energy was never taken into account before. Therefore, experiments using such a technique to measure tidal-volume should be d one using isothermic chambers. Moreover, due to uncertainties of the c omplementary measurements (ambient and lung temperatures, ambient wate r vapour saturation) needed to compute tidal-volume using total-body p lethysmography, a minimal temperature difference about 15 degrees C be tween body and ambient should exist to keep uncertainties in tidal-vol ume values below 5%. However, this limit is not absolute, because it v aries as a function of humidity and degree of uncertainty of the compl ementary measurements. (C) 1998 Elsevier Science B.V. All rights reser ved.