348 EDEMA 



atic obstruction is possibly a factor of some secondary importance if 

 we consider that distended vessels and tense tissues may occlude the 

 lymph capillaries. 



SPECIAL CAUSES OF EDEMA 



We may now consider which of the above factors are at work in 

 bringing about edema under the conditions in which it is usuallj^ 

 observed clinically. Before taking up the detailed consideration of 

 edematous conditions, however, it may be well to call attention to the 

 fact that our knowledge of edema, and especially its clinical recog- 

 nition and study, has been handicapped by the lack of a suitable ob- 

 jective method of detecting and measuring edema. We are in the 

 same position in respect to edema that we were to blood pressure 

 when the only clinical measure was the clinician's forefinger. An 

 attempt to remedy this defect has been made by Schade,^"'' whose 

 "elastometer" reveals and measures degrees of edema not discernible 

 by the palpating finger. A study of edema with this instrument in the 

 hands of Schwartz ^°^ has revealed many interesting facts, but as yet 

 the apparatus is too complicated for general clinical use. 



"Cardiac" Edema. — Passive congestion introduces nearly all these 

 factors, for in addition to the increased blood pressure there is also 

 an opportunity for changes in the capillary wall, either from stretch- 

 ing and thinning of the cells and cement substances, or from "loss 

 of tone" in the endothelium surrounding the stomata (Meltzer), 

 or from toxic injury by accumulated products of tissue metabolism. 

 AVhen the stasis is nearly complete, or if it is complete for a time and 

 then relieved, the endothelium may be injured through lack of 

 nourishment. As the edematous fluid in chronic passive congestion 

 is usually of a watery type, poor in proteins, the edema is prob- 

 ably less dependent upon capillary permeability than upon other 

 factors, except in the case of acute stasis, when the fluid partakes of 

 the character of the exudates. Presumably the accumulation of 

 crystalloids within the tissues also plays a part in this form of edema, 

 as the osmotic pressure is raised in tissues having deficient oxygen 

 supply. But Fischer holds that the reduction in oxidation acts chiefly 

 by increased production of acids, which greatly increase the affinity 

 of the tissue colloids for water and at the same time alter the colloidal 

 state of the capillary endothelium so that the capillaries become more 

 permeable. Finally, there is probably more or less obstruction to 

 lymphatic outflow because of the increased pressure on the lymphatic 

 channels, and perhaps, also, in the case of cardiac incompetence, ob- 

 struction to the discharge of lymph from the thoracic duet into the 

 sul)clavian vein against the higli intravenous ])i-essnre. 



Renal Edema. — AVe must recognize under tliis heading two dififer- 



soaZeit. cxp. Path. u. Thor., 1!)]2 (11), .309. 

 30b Arch. Int. Med., 1910 (17), 396 and 459. 



