Investigations concerning vital capacity

Abstract

Since having been established nearly a century ago 1 as a quantitative measure of respiration, measurement of the vital capacity has found universal clinical application because of the ease with which it is done and its utility in clinical orientation, especially with regard to heart disease. Vital capacity measures the extensibility of the lungs, and is inversely proportional to the degree of engorgement of the lungs. Measurements of vital capacity enable us to estimate quantitatively the degree of pulmonary congestion, and thus, together with the other hemodynamic data, affords a clearer view of circulatory conditions. Its measurement is therefore considered indispensable in cases of heart disease. The present study treats of some new aspects of spirometric research which have, in my experience, not only theoretical, but practical value. According to the classical definition, the vital capacity is the volume of air, in cubic centimeters, which can be expelled by maximum expiration after maximum inspiration. For the actual study, the minimum duration of the expiration that represents the vital capacity was ascertained. In this way, two values were obtained: the vital capacity and the minimum duration of the forced expiration. Dividing the vital capacity by the number of seconds thus ascertained, we arrive at the value for the velocity of spirometric respiration. By this term we understand that quantity of air which corresponds to the average velocity of the quickest expiration. In order to characterize more exactly the respiratory function in relation to spirometric determination of the maximum volume, I looked for, and found, by means of a manometer, another respiratory factor: the expiratory pressure. Following the definition of vital capacity, I ascertained the maximum pressure which the subject will generate when exhaling into a manometer after having inhaled as deeply as possible. The altitude of the mercury column represents the maximum expiratory pressure. Thus the maximum volume and pressure during maximum inspiration and expiration were ascertained.