Use of Air in Basal Metabolism

Abstract

In the past there has been no reference in the literature to the use of air instead of oxygen in the semi-portable type of basal metabo-limeters which are commonly used clinically. Yet, obviously, there may be certain advantages to using air instead of oxygen in a clinical metabolimeter. An article describing an air-using machine has just been published in a South American periodical by a doctor in Mexico City. 1 In certain parts of the world, oxygen is expensive and difficult to obtain.

It has been definitely shown 2 that the oxygen tension of the air breathed can fall decidly before any detrimental physiological changes are noted, and Schneider 3 and others have found that the oxygen intake begins to fall only when the oxygen concentration in the air breathed is 14.8% or lower.

Our first experiments using air were carried out on a standard basal metabolimeter∗ which uses a motor-driven fan for circulating the gas in the closed system. Air was merely substituted for oxygen, and the tests were limited to periods of 4 minutes, as the total capacity of the machine was only 12 liters. Oxygen control tests were made immediately preceding and following each test using air. Four different subjects were used. It was found that the results of the tests in which air was used agreed to 0.1% with those in which oxygen was used.

As a 4-minute test was not considered to be a period of sufficient length for a good determination, an auxiliary tank of air was connected in series with the metabolimeter. Using a 6-minute period, a preliminary group of 20 trials and 19 control determinations were made with this apparatus on 3 normal subjects. The average of the results of these 6-minute periods agreed within 1.5% with the results obtained with oxygen. (Table II.)

These results caused us to construct a machine with the spirometer incorporated into an auxiliary tank. The construction of this machine is shown in the diagram. The oxygen consumption is measured by the fall of the spirometer which is recorded on a kymograph drum in the usual way. The spirometer was calibrated by carefully measuring the distance traversed by the pen on the kymograph drum when air was added or subtracted in 500 cc. increments from the machine. The results on patients were also compared and found to agree with data obtained with a Benedict machine. Our machine was constructed from plans calculated to give it a total capacity of 40 liters, which was sufficient to provide the patient with fresh air for the entire 6-minute period of the test. Circulation of the air within the apparatus was maintained by means of a small 6-volt motor, driven by a storage battery. This motor did not heat the air to any measurable extent.

It was noticed that a rise of temperature of one degree centigrade produced an error of about 4% in this machine. In order to avoid this source of error, the air was heated before each test by a light-bulb inside the machine. The temperature during the test was then found to remain constant.

Checks on the oxygen level in the various containers were obtained by means of gas analyses, using a modified form of the Henderson-Orsat gas analysis machine. 4 In the preliminary series of tests, the oxygen concentration fell from 20.9%, to between 14 and 15%. (Table II.) In the tests with the 40 liter machine it fell to 16%, which is not below the oxygen concentration in ordinary expired air, 5 and indicates a wide margin of safety for this machine.