Studies of Lung Volume and Intrapulmonary Mixing. Nitrogen Clearance Curves: Apparent Respiratory Dead Space and its Significance

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Abstract
An open circuit method is presented for the study of pulmonary clearance curves by the sampling of collected expired air during normal breathing of oxygen. The method has the advantages that alveolar air samples are not required and that interference with normal breathing is reduced to a minimum. The equations appropriate to a "single chamber" model of pulmonary ventilation are derived and sources of error discussed. The clearance curves are used to calculate an "effective respiratory dead space," the value of which should include (a) the "true" or anatomic dead space and (b) a measure of the retarding effect on lung clearance brought about by slow intrapulmonary mixing or unbalanced distribution of inspired gas in the various portions of the lungs. Measurement of the effective respiratory dead space requires constancy of functional residual volume and tidal volume during the exptl. period, and data are presented illustrating the misleading results that may be obtained when this requirement is not met. Of a total of 47 expts. on 20 normal persons, 13 on 8 persons, were considered acceptable. It is concluded that the behavior of the normal lung is compatible with the single chamber model. The acceptable data can be expressed by the following equation relating the effective respiratory dead space to the tidal volume; Vd = 0.30 Vt[long dash]4.9; correlation coeff. +0.609. The pulmonary clearance curves were also used to calculate the alveolar dilution/breath (alveolar diln. ratio) and the pulmonary half-clearance time. The alveolar diln. ratio shows little variation even when breathing is erratic, but theoretical reasons are given for supposing that this ratio is of little value in assessing the efficiency of pulmonary ventilation, unless supported by the additional data used in calculating the effective dead space. Consequently the use of the alveolar diln. ratio, which can sometimes be measured for clearance processes that fail to fulfill certain of the requirements mentioned previously for calculation of effective dead space, has little to recommend it. The pulmonary half-clearance time is subject to variations which likewise require accessory data for their interpretation. The essential information provided by the half-clearance time can often be obtained more easily by measuring the lung volume and its subdivisions. It remains to be shown, by measurements in suitably chosen cases involving respiratory impairment, whether the effective respiratory dead space is indeed sensitive enough to abnormalities of ventilation to be valuable as a clinical test.