THE MECHANICS OF RESPIRATION 331 



In cardiac disease the air in the lungs at the end of a normal expiration 

 is usually increased. This is similar to the condition which attends exer- 

 cise, and is probably a physiological adaptation to give optimum aeration 

 to the blood, as explained above. 



It has become more and more evident, since Peabody and Wentworth's 

 researches, that a determination of the vital capacity is of great importance 

 in the diagnosis and prognosis of several diseases, including heart disease 

 and tuberculosis. It is also important in guaging the effects of treatment. 

 In order that the value actually found in a given patient may be compared 

 with the value which a healthy individual of the same body build would 

 give, it is necessary for clinical purposes that some reliable and yet simple 

 method be available from which the normal value may be computed. 

 Lundsgaard and Van Slyke 51 and Dreyer 52 have worked out several ratios 

 for this purpose, and West 50 has shown, by observations on 129 persons, 

 the most useful of these is one based on the body surface. The body sur- 

 face is determined from measurement of height and weight by the graphic 

 chart of DuBois, the use of which is explained on page 576. 



The vital capacity can be satisfactorily measured by using a simple 

 spirometer of about 8 liters capacity and three trials should be allowed, the 

 largest expiration being recorded. The average value for vital capacity 

 (in liters divided by the body surface (in square meters) is 2.61 1. per sq. 

 m. and for women 2.07 (e.g., vital capacity = 5,300 1. ; body surface 2.01, 

 therefore, ratio =2.63). The deviation from the values should not be be- 

 yond 15 per cent, the great proportion of normal individuals being within 

 10 per cent of the above averages. Athletes give decidedly higher values 

 and old people give lower ones. 



