Measurement of Inertia of the Lungs at Increased Ambient Pressure

¹ From the Department of Physiology, Harvard School of Public Health, Boston, Massachusetts

Measurement of the pressure required to produce volume acceleration of the lungs and gas stream was made in five human subjects. In three, measurements were made at increased ambient pressures. Transpulmonary pressure was recorded as the pressure difference between mouth and esophagus. The accelerative component of the transpulmonary pressure change between instants of zero air flow at the extremes of the tidal volume was obtained by subtraction of the estimated elastic component from the total pressure change. In order to increase the accelerative pressures, measurements were made during rapid, shallow breathing. The ratio of the accelerative pressure change to the simultaneous change in volume acceleration, expressed the ‘inertance’ of the lungs and gas stream. At atmospheric pressure (ca. 15 psi) approximately 0.01 cm H₂O was required to produce a volume acceleration of 1 l/sec.² Inertance increased approximately in proportion to ambient pressure up to 4 atmospheres absolute pressure. It was concluded that the inertance, as measured, was predominantly in the gas stream. An estimate of the inertance of the gas stream, based on the anatomy of the respiratory tree, was found to be of the same order of magnitude as the observed values.

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