RESPIRATION AND ACIDOSIS 321 



and in diabetic coma may go as low as i or 2 per cent. A value of 

 2 per cent means that coma may supervene within 24 hours. A value 

 of 3 per cent or 4 per cent is less dangerous; in the worst event coma 

 will not come on for at least two or three days. For detailed data as 

 to alveolar carbon dioxide tension under different conditions see table 

 on page 317. 



3. Alkali Reserve. Indirect Method. Alveolar Carbon Dioxide Tension 

 Marriott's Method.^ While this method is open to criticism because of the liability 

 of error in the collection of the sample and, more fundamentally, 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 conditions a sample is 

 obtained whose carbon dioxide tension is virtually that of venous blood. The 

 method of analysis of this sample depends on the fact that if a current of air con- 

 taining carbon dioxide is passed through a solution of sodium carbonate or bi- 

 carbonate until the solution is saturated, the final solution will contain sodium 

 bicarbonate and dissolved carbon dioxide. The reaction of such a solution will de- 

 pend 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. 



Apparatus. The complete apparatus, including rubber bag for collection of 

 sample, standardized phosphate mixture sealed in tubes, the standard bicarbonate 

 solution, tubes, color comparison box and other accessories may be obtained from 

 Hynson, Westcott and Dunning, Baltimore, Maryland. 



Procedure. Collection of the alveolar air. The method of collection is essen- 

 tially that of Plesch, 2 as modified by Higgins. 3 A rubber bag of approximately 

 1500 c.c. capacity 4 is connected by means of a short rubber tube to a glass mouth- 

 piece. 5 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. 6 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 



1 Marriott: Jour. Am. Med. Ass'n, 66, 1594, 1916. 



2 Plesch: Ztschr. f. exper. Path. u. Therap., 380, vi, 1909. 



3 Higgins: Pub. 203, Carnegie Institution of Washington, 1915, p. 168.; Boothby, W. M., 

 and Peabody, F. W.: A comparsion of Methods of Obtaining Alveolar Air, Arch. Int. 

 Med., March, 1914, p. 497. 



4 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 % inch in 

 internal diameter. 



5 An ordinary piece of glass tubing with rounded edges, i^ inch long and % inch in 

 internal diameter. 



6 Especially to be guarded against is a deep, voluntary inspiration just before the collec- 

 tion begins, as this causes too low a determination. 



