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RESEARCH PRODUCT

Problems in Precise Measurement of Tidal Volumes

subject

description

Respiratory gas flows and volumes can conveniently be measured by pneumo-tachography.3 However, the pneumotachographic signal depends on gas temperature, humidity, and composition, and therefore the achievable precision of measurement is limited to about 5–10%2,8 — which is not sufficient for determining the human ventilation distribution with satisfactory accuracy.6,7,5 As a — potentially more accurate — alternative, a plethysmographic method may be used in which the tested subject breathes out of and into bags that are suspended in a large tank of constant volume (Figure 1). Volume V and pressure P in the system (consisting of tank, tubings, valves, and the subject) are related via Boyle-Mariotte’s law P.V= constant,and therefore, in- and expiratory tidal volumes may be registered in the form of pressure changes in the tank. Due to the availability of high performance pressure transducers, volumes may, on principle, be determined in this way at a precision of 1C−5 of the maximum volume range or better. As a drawback of the method, Boyle-Mariotte’s law requires temperature (and total amount of gas in the system) to be constant during the maneuver which — in a strict sense — does not hold true for a number of reasons. Most obviously, the in- and expiratory volume changes induce adiabatic compression and decompression of the gas in the tank. In addition, the time constant for thermal equilibration typically is more than an order of magnitude larger than the duration of the respiratory cycle which defines the temporal resolution required for the physiological analysis. Hence, during the experiments commonly performed in respiratory physiology (for calibration purposes as well as for physiological measurements), the system is not in thermal equilibrium. The present study deals with the effects of this deviation.