348 EDEMA 



either osmotic pressure changes or changes in the affinity of the tissue 

 colloids for water, play the most important part; whereas in inflamma- 

 tory edema there can be no question that alteration in the capillary 

 walls is the most essential factor. But the mechanical factor of blood 

 pressure cannot be disregarded, although by itself seldom sufficient to 

 cause edema; associated with other factors it is undoubtedly an im- 

 portant agency, for there are few edemas that arc not associated with 

 increased blood pressure. Hydremia and hydremic plethora may al- 

 most be disregarded, except in so far as they may cause altered metab- 

 olism in the tissues, injury to vessel-walls, over-saturation of the blood 

 colloids, and decreased osmotic pressure within the vessels. Ljanph- 

 atic obstruction is possibly a factor of some secondarj^ 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 usually 

 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 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 bj^ the palpating 

 finger. A study of edema with this instrument in the hands of 

 Schwartz^^ has revealed many interesting facts, but as yet the appa- 

 ratus is too complicated for general clinical use. 



"Cardiac" Edema. — Passive congestion introduces nearly all 

 these aforementioned factors, for in addition to the increased blood 

 pressure there is also an opportunity for changes in the capillary wall, 

 either from stretching 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. When the stasis is nearly complete, or if it is comp'ete 

 for a time and then relieved, the endothelium may })e injured through 

 lack of nourishment. As the edematous fluid in chronic passive con- 

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

 probably less dependent upon capillary permeability than upon other 

 factors, except in the ease of acute stasis, when the fiuid partakes of 

 the character of the exudates. Presumably the accunuilation 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 



" Arch. Int. Med., 1910 (17), 390 and 4r-,9. 



