Test For Rate Of Ventilation

The spirometer, when filled, holds about 7 liters. The rate of ventilation may be approximately determined by opening the system at the coupling, S (fig. 3), and pinching the tube, W, thus closing the intake tube. Air from the spirometer is then discharged by the blower at the open coupling, and the time in seconds is noted for the spirometer to fall completely (350 mm.). If 15 seconds is required for the spirometer to fall, the ventilation rate is approximately 28 liters per minute.

Absorbing System

The calcium-chloride and soda-lime bottles are of simple form and, for convenience, are placed upon a shelf attached to the base of the apparatus. The soda lime is made by the formula of Haldane.1 It has a distinct yellowish tint which changes to a chalky whiteness as the carbon dioxide is absorbed by the reagent. The efficiency of the reagent and the time for renewal may be judged by the progress of this change in color. When a determination of the oxygen consumption only is desired, as is usually the case, the calcium-chloride bottles may be dispensed with. A greater efficiency of soda lime is thus obtained, for it has been found that the moister the soda lime is, the greater is the absorption of carbon dioxide. Indeed, the expired air may be passed directly through the soda lime if care is taken that the excessive absorption of water does not make the absorbent so pasty as to interfere with the free passage of air. As this is a very important point, the ventilation rate should be frequently tested.

If the reagent becomes exhausted the physiological effect is a somewhat labored respiration due to the unabsorbed carbon dioxide acting as a stimulus to the respiratory center. This is not a serious defect and usually has no quantitative significance in the measurement of the oxygen consumed in the 10 or 12 minute period. At the end of the period the motor must be run a few seconds longer to insure a complete absorption of the carbon dioxide in the air by the soda lime.

1 Haldane, Journ. Physiol., 1892, 13, p. 422. The prepared soda lime may be purchased from Stanley Jordan & Co., 93 Water Street, New York City.

While the labored respiration is evident to the practised eye in a more rapid rate and a larger amplitude of the excursions of the bell, a pneumograph about the chest with attachment for a kymograph record supplies a good picture of the mechanics of respiration and instantly records dyspnoea. The excursions of the bell may be directly written on a kymograph by attaching a light pointer to the counterpoise, though the continuous upward trend of the curve as the oxygen is absorbed will permit of but short records on the kymograph drum of standard height.

Spirometer

The spirometer used is a modification of the form of spirometer employed in the universal respiration apparatus, the two chief differences being (1) the recessed part which contains the mechanical blower and (2) the unusual length of the bell. The first provides space for the blower without intricate connections and absolutely precludes leaks. It is a feature of the apparatus that has proved especially satisfactory in practice. The bell is made of sufficient size to allow not only for a full excursion if a deep breath is taken, but also for a considerable contraction in total volume of the air in the ventilating circuit as the oxygen is consumed.

To minimize the labor of breathing, the bell is delicately counterpoised by a weight (see X, figure 3) on the end of a silk cord running over a light aluminum pulley, Z, at the top. A pointer attached to the counterpoise shows, on a millimeter scale, the fluctuations in the height of the bell. To indicate accurately the somewhat large temperature changes a light-weight thermometer,1 T, is inserted permanently in the top of the bell. To avoid getting water on the blower and consequent electrical damage, with danger of setting fire to the insulation in the oxygen-rich atmosphere, the water level in the spirometer should be considerably lowered when the apparatus is to be moved on an uneven floor.

Adjustment To Subject

Almost universal adjustment of the mouthpiece is possible by means of the two hand wheels, A and B. (See fig. 2.) The former controls the movement of the 3-way valve and mouthpiece around the extension arm, L, as an axis, and also their location on the extension arm, while B permits raising or lowering the arm L on an upright standard, S. The extension arm can also be swung about S within the scope of the two rubber tubes, W and G. With no further adjustment the arm L may be placed in position for a subject either lying in bed or sitting in a chair. When used for experiments with the subject in the standing position, the base is mounted on a wooden box of the desired height.

1 At present a small Fahrenheit thermometer is used, as a centigrade instrument of like weight and temperature range is not available.

Breathing Appliances

For the sake of simplicity in this description of the portable respiration apparatus, and since it was regularly used in all our experiments, the mouthpiece only is referred to and shown in the diagram, but we have reason to believe that the original form of inflated nosepiece used in the Nutrition Laboratory may be of even greater practical value, inasmuch as it has less influence upon the type of respiration.1

At first sight the use of a mask in a closed-circuit apparatus of this type would seem impracticable, as the slightest leak between the mask and the face would have a pronounced effect upon the measurement of the oxygen. Actual experience with the mask in the Nutrition Laboratory has, however, given good results. This form of breathing appliance is much more comfortable for subjects, permitting as it does free nose and mouth breathing, without the objectionable features of the mouthpiece.