THE CONTROL OF THE RESPIRATION 347 



out correctly is the respiratory quotient (ratio of CO 2 excreted to O 2 absorbed). For 

 reasons which are set forth above, the quotients should be approximately equal in the 

 air collected in the large and in the small spirometers; if they are not so, the condi- 

 tions of the method have not been correctly carried out. 



Since the dead space and the average composition of the alveolar air under these 

 conditions may be considered constant, the percentage composition of the deep expira- 

 tion will differ from that of the mixed sample of several normal expirations in propor- 

 tion as the dead space exerts a greater diluting effect in the small than in the large 

 expiration. This being the case, the data obtained can be combined algebraically to 

 give either the capacity of the air passages or the percentage composition of the 

 alveolar air. 



Let A = amount of air in large expiration (small spirometer), 



Ai = amount of air in small or normal expiration (tidal air), 

 B the percentage of CO, or O 2 in the expired air of large expiration, 

 Bi = the percentage of CO 2 or O 2 in the expired air of small expiration, 

 x = the capacity of the dead space, 



y = the average percentage of CO, or O., in the alveolar air; then, 

 A x B = (A - x)y and Ai x Bi =r (Ai - x)y. 

 Solving this for x, y remaining constant under the same physiologic conditions, we 



A x Ai x (B-Bi) J 

 have: x = : , the dead space. Or solving for y, we have: 



y AxB ~ AlxBl , the mean percentage of CO 2 in the alveolar air. In case the 

 A-Ai 



dead space for O, is desired, B and Bi must be made to equal the O, absorbed. 



Clinical Method. The use of the kymograph and pneumograph, and 

 other complicating factors, make the method as just described quite im- 

 practicable for clinical procedure, but the use of the same apparatus 

 with the following modification will yield satisfactory results for most 

 clinical purposes. The patient is made to respire through the valves for 

 a short time, after which the observer collects a single expiration in a 

 small spirometer by turning the stopcock from Position 1 to 2. A sam- 

 ple of this is taken for analysis, and the spirometer is again emptied 

 and a series of successive samples of deeper expirations taken. This is 

 done by directing the patient, after he has started to breathe normally 

 into the spirometer, to breathe more deeply. The amount of air col- 

 lected in each expiration is controlled by the observer by closing the 

 stopcock when the desired volume is obtained. By this means one can 

 collect several expirations differing from one another by increasing 

 amounts but all occupying the same time. The samples of the various 

 expirations are collected in a series of numbered sampling syringes, and 

 the gaseous composition of each is determined. When the percentage 

 of C0 2 or 2 in each expiration is plotted on cross section paper on the 

 ordinates, with the volume of the expirations in c.c. on the abscissae, a 

 hyperbolic curve should be obtained. Any marked deviation from such a 

 curve indicates that some error has been made in taking a sample, and 



