Barometric measurement of tidal volume: effects of pattern and nasal temperature

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Abstract
The accuracy of the Drorbaugh-Fenn formula for calculating tidal volume (VT) from the phasic pressure change measured when an animal breathes in a closed chamber is questionable. The formula may underestimate VT by up to 30%, and the error may increase as the ratio of inspiratory duration (TI) to total breath duration (Ttot) increases, and as the expired temperature at the nares (TN) increases. To test their theory, VT was measured in anesthetized rats by the barometric technique and by conventional pneumotachography simultaneously. TN was varied from ambient to body temperature by passing a variable current through the pneumotachograph heater; TI/Ttot was varied by changing FICO2 [inspired CO2 fraction] and by selecting different rats. The predictions were confirmed. A factor is derived for retrospectively correcting VT estimated by the Drorbaugh-Fenn formula. It requires knowledge of TN and TI/Ttot and reduces the error between experiments to under 20% and within each experiment to about 5%. To facilitate it use, TN was measured in rat, rabbit, cat, man and infant pigtail monkey [Macaca nemestrina].

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