Air-Moistener

Since with any type of breathing appliance the purified air passing along the tube is too dry to be breathed comfortably by the subject, a small moistening device, consisting of a wire-gauze frame covered with linen and thoroughly drenched with water, is inserted in the connection between the mouthpiece or other breathing appliance used and the circulating air.2

Oxygen Supply

If the whole apparatus is filled with air at the beginning of a period, the percentage of oxygen in the confined air being breathed will fall considerably and might easily reach the point at which oxygen-want would be felt. Consequently, since the spirometer is of generous size, and it has been established by Higgins3 that the respiration of oxygen-rich atmospheres is without effect upon the gaseous metabolism, a liberal supply of pure oxygen from a cylinder is introduced into the system prior to connecting the mouthpiece with the subject and after an equivalent volume of air has been expelled through the 3-way valve. During the experiment the volume of air in the spirometer is decreased by approximately 250 c.c. per minute, or 2.5 liters in a 10-minute period. A rough calculation will indicate the probable amount of pure oxygen to introduce. In the course of an experiment there is an exchange of oxygen for nitrogen in the lungs and blood, so that the air in the system contains a little more nitrogen than at first, but repeated tests have shown that the percentage of oxygen in the residual air at the end of a 10-minute or even 15-minute experiment is always very considerably above that of the oxygen in outdoor air. Consequently subjects can never suffer from oxygen-want.

1 For a full description of these nosepieces, also method of use, see Carpenter, Carnegie Inst. Wash. Pub. No. 216, 1915, pp. 22-23.

2AI1 workers in gaseous metabolism should refer to the monograph by Dr. T. M. Carpenter (Carnegie Inst. Wash. Pub. No. 216, 1915) for detailed description of respiratory apparatus of practically all the current types, with special reference on pp. 23, 36, and 37 to mouthpiece, nosepieces, and moistening device.

3 Benedict and Higgins, Am. Journ. Physiol., 1911, 28, p. 1.

Experimental Procedure

The use of the apparatus should be preceded each day by a test for tightness which is easily made by placing a 100-gram weight on top of the spirometer. If there is a leak at any point the spirometer will show it definitely in 3 minutes. After the weight is removed, the motor is started and approximately 2.5 liters of pure oxygen admitted from the cylinder through the pet-cock, P (fig. 3). As each millimeter on the scale corresponds to approximately 21 c.c. (the exact value is determined at the time of construction),1 this amount of oxygen is sufficient to raise the bell approximately 120 mm. The oxygen is best introduced when the spirometer bell is about one-third of the distance above its lowest level, as this allows for exaggerated expiration or inspiration without completely filling or emptying the spirometer.

Since the apparatus is primarily designed for measuring oxygen only, for the time being we need not consider the method of measuring the carbon dioxide. The connection of the subject with the air circuit is controlled by the 3-way valve (see V, fig. 2), which may be opened directly to the room air or, by turning it 90 degrees, connected with the ventilating air circuit. A small double-headed arrow, stamped on the top of the valve, indicates in which direction it should be turned. The opening to the room air usually points down and is not shown in figure 2. A short elbow is attached to this opening and over the end of the elbow a small piece of goose-down is fastened with a bit of wax. The movements of this feather indicate the respiration rate and the end of each expiration.

With the breathing appliance attached, the subject is first allowed to breathe room air through the side opening of the 3-way valve. The temperature of the spirometer bell and the position of the pointer on the scale are recorded, also the barometer,2 which should be read to the nearest millimeter. As soon as the respiration appears normal, the 3-way valve is quickly turned 90 degrees to connect the subject with the air current. This should be done at the end of a normal (not forced) expiration. At the moment of turning the valve, the exact time is noted, preferably with a stop-watch. Practically no further attention need be given to the subject or apparatus until the end of the experiment. It is preferable to have the subject keep awake during the experimental period and we frequently find it necessary to tap on the air-pipe to make sure that he has not fallen asleep. At the end of 10 or 12 minutes, the excursions of the bell are carefully watched and at the conclusion of a normal expiration the 3-way valve is again turned, disconnecting the subject from the ventilating air current; the time is then noted. The mouthpiece and nose clips are removed, and the subject lies quietly until another period is begun.

1 Seven bells made on the same form gave by actual calibration the following values per millimeter: 20.99. 20.97. 20.86, 20.86. 20.83. 20.97, and 20.97 c.c, with an average of 20.92 c.c.

2 A good aneroid barometer reading in millimeters is satisfactory, as an error of 7 or 8 mm. introduces an error of but 1 per cent in the final calculations.

It is a fundamental rule in this Laboratory that all subjects, prior to measurements of the gaseous metabolism, must be lying down, resting quietly for not less than 30 minutes. The length of time between periods need be determined only by the activity of the operator in recording his observations. After the end of the period 1 minute is allowed to elapse to insure a thorough mixing of the air and removal of carbon dioxide. When the pointer indicates that the level of the spirometer is constant, this is read and the temperature is recorded.