The Effect of Change in Body Position on Lung Volume and Intrapulmonary Gas Mixing in Normal Subjects 1

Abstract

In many normal people a sizable part of the lung is ventilated at a much slower rate than the remainder. Means have been devised for measuring the volume and ventilation rate of these "slow spaces" (1-4). It is also established that a change in body position from sitting to recumbent will alter the size of the various subdivisions of the lung volume (5, 6). In the course of some observations on intrapulmonary gas mixing, it was found that changes in body position caused significant changes in size and ventilation rate of the "slow spaces." It is the purpose of this paper to present the results of a study of the effects of various body positions on lung volumes and on the size and ventilation rate of the poorly ventilated regions of the lung. METHODSThe functional residual capacity. (FRC) and the vollume (Vs) and minute ventilation rate (Vs) of the most poorly ventilated lung space were measured by an opencircuit helium method (4). The subject breathes a mixture of 50 per cent helium and 50 per cent oxygen for fifteen minutes to achieve a nearly uniform concentration of helium throughout the lungs. At the end of a normal expiration he is switched to tank oxygen, and the expired gas is thereafter collected for subsequent measurement of volume and helium concentration. In addition, the helium concentration of the expired gas is continuously followed by a sensitive katharometer between he- homogeneously ventilated lung space. The straight line toward which the curve tends during the last few minutes of the washout is drawn in "by eye." From the slope of this line, the point at which it intersects the ordinate at zero time, and from the minute ventilation rate of the subject, it is possible to estimate the ventilation rate and volume of the slow space.In the present study the expressions k. and f are used to characterize the slow space. k. is the "turnover rate"of the slow space, or ratio of its minute ventilation to its volume (V./V.). For the lung as a whole, k is usually between 2 and 3 if the subject is in a basal or nearbasal state. For the whole lung, k represents the ratio between total minute ventilation and the functional residual capacity. The present subjects were not basal and the ratio between minute ventilation and FRC was about 4, for seated subjects. The relative size of the slow space is expressed as the ratio, f, between the volume of the slow space and the functional residual capacity. The present method is not well suited to the measurement of rapidly ventilated lung spaces because of the lag in response of the katharometer to change in helium concentration. A 95 per cent response occurs in 20 seconds. In addition, it requires 5 to 8 seconds for expired gas to pass from the subject to the sampling point. Although an allowance can be made for these delays, inexactness in the allowance may introduce serious errors into the estimates of ventilation rate and volume of lung spaces which have a very rapid turnover rate. The analytical system can measure accurately k values of at least 3.0, as demonstra...

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