Effect of Variation of Tidal Volume on Size of Physiological Dead Space in Dogs

¹ From the Department of Cardiorespiratory Diseases, Walter Reed Army Institute of Research, Walter Reed Army Medical Center, Washington, D. C.

The size of the lung dead space, measured by substituting arterial for alveolar Pco₂ in the Bohr equation, has been found to vary directly with the size of the tidal volume in anesthetized dogs. Simultaneous measurement of the dead space by this technique and by Pappenheimer's technique revealed that ‘alveolar’ CO₂ tension measured by the latter was consistently lower than the arterial CO₂ tension. Thus, one of the two assumptions necessary for measurement of a constant, isosaturation dead space did not prove correct. This finding, plus the linear relationship between Pe_COCO2 and i/V_T, indicates that the dead space equals a constant plus an amount which varies directly in magnitude with the size of the tidal volume.

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