THE MECHANICS OF RESPIRATION 



313 



cardiac dyspnea, can not breathe as comfortably when lying as when 

 sitting. This condition is known as orthopnea. The advantage of the sit- 

 ting over the lying position for breathing can not be satisfactorily ex- 

 plained. The greater vital capacity in the upright position; the favor- 

 ing of the return of the venous blood from the cerebral vessels by 

 gravity; the increased caliber of the pulmonary vessels because of the 

 enlarged thoracic cavity (see page 318) ; and the increase in the reserve 

 air of the lungs are all factors to be considered. 



The Vital Capacity. At one time it was thought that the vital capacity 

 of the lungs was related to their ventilatory capabilities, but for years 

 the determination of this value in patients has been considered unimpor- 

 tant. Recently Peabody and Wentworth 10 have called attention to the 

 fact that patients with heart disease become dyspneic more readily than 

 do healthy subjects, and that this tendency seems to depend largely 

 on their inability to increase the depth of the respiration in a normal 

 manner. They find that this inability to breathe deeply corresponds to 

 a diminished vital capacity of the lungs as measured in a spirometer, 

 by the volume of the greatest possible expiration after the deepest in- 

 spiration. They believe that any condition which limits the possibility 

 of increasing the minute volume of air breathed must be an important 

 factor in the production of dyspnea. 



In normal adults the following averages (Table I), were secured from 

 a large series of clinical cases. The subjects are grouped into two 

 classes, each group being subdivided according to height. 



TABLE I 

 THE VITAL CAPACITY OF THE LUNGS OF NORMAL MALES 



THE VITAL CAPACITY OF THE L,UNGS OF NORMAL FEMALES 



(Peabody and Wentworth.) 



It would appear that in normal people the vital capacity is at least 

 85 per cent, and almost always 90 per cent or more, of the standard 

 adopted for each group. In elderly persons a slight decrease from these 

 standards may be expected. 



