ALKALI RESERVE 139 



because of various factors (psychical, etc.) other than acidosis 

 which may influence the carbon dioxide tension, nevertheless, it 

 is of considerable value and has been rather widely adopted for 

 clinical use. 



Principle. By rebreathing air under certain definite con- 

 ditions a sample is obtained whose carbon dioxide tension is vir- 

 tually that of venous blood. The method of analysis of this sample 

 depends on the fact that if a current of air containing carbon 

 dioxide is passed through a solution of sodium carbonate or bicar- 

 bonate until the solution is saturated, the final solution will contain 

 sodium bicarbonate and dissolved carbon dioxide. The reaction 

 of such a solution will depend on the relative amounts of the 

 alkaline bicarbonate and the acid carbon dioxide present. This, 

 in turn, will depend on the tension of carbon dioxide in the air 

 with which the mixture has been saturated and will be independent 

 of the volume of air blown through, provided saturation has once 

 been attained. High tensions of carbon dioxide change the 

 reaction of the solution toward the acid side. Low tensions have 

 the reverse effect ; hence the reaction of such a solution is a measure 

 of the tension of carbon dioxide in the air with which it has been 

 saturated. A suitable indicator is added to the solution and its 

 reaction (after the passage of the alveolar air) is determined by 

 comparison with a set of suitable standards. 



Procedure. Collection of the alveolar air. The method of 

 collection is essentially that of Plesch, as modified by Higgins. 

 A rubber bag of approximately 1500 c.c. capacity (A basket-ball 

 bladder or a hot-water bag answers very well. If the latter is 

 used, the neck may be closed by a rubber stopper carrying a short 

 glass tube 3/8 inch in internal diameter) is connected by means 

 of a short rubber tube to a glass mouth-piece. (An ordinary 

 piece of glass tubing with rounded edges, If inches long and 3/8 

 inch in internal diameter.) About 600 c.c. of air are blown into 

 the bag with an atomizer bulb, and the rubber tube clamped off 

 by a pinchcock. The subject should be at rest and breathing 

 naturally (especially to be guarded against is a deep, voluntary 

 inspiration just before the collection begins, as this causes too low 

 a determination). At the end of a normal expiration, the subject 

 takes the tube in his mouth; the pinchcock is released and the 

 subject's nose closed by the observer. The subject breathes back 

 and forth from the bag four times in twenty seconds, emptying 



