CHANGES IN VASCULAR PATTERNS 



-01 



association with severe chronic liver disease, in preg- 

 nancy, in persons with deficiency of the vitamin B 

 complex, in the carcinoid syndrome, and also in 

 certain apparently healthy individuals. The subject 

 has been well reviewed by Bean (10). 



TRAUMATIC OR SURGICALLY INDUCED 

 ARTERIOVENOUS CONNECTIONS 



The establishment of a connection of sufficient 

 magnitude between an artery and vein may have 

 major or even catastrophic consequences. These 

 have been carefully worked out by experiment (76, 



77> 79, i53)- 



The immediate effects upon opening the fistula are 

 a fall in blood pressure, an increase in the heart rate 

 and venous filling, and consequently a greater cardiac 

 output. The regional veins become engorged. A 

 thrill becomes palpable and a murmur audible over 

 the fistula, and these can be abolished by exerting 

 pressure over the vein proximal to the fistula. With a 

 sufficiently large shunt, the total blood volume in- 

 creases in course of time, and the blood becomes more 

 dilute. Although initially the size of the heart and of 

 the artery on both sides of the fistula becomes re- 

 duced, there is gradually a dilatation of the arteries 

 and veins proximal to the fistula. The heart also 

 becomes enlarged, chiefly because of dilatation. In 

 late stages, the proximal artery may even become 

 aneurysmally dilated (144). 



Blood flow is toward the fistula, even from the 

 distal artery. A large flow depends on a fistula which 

 exceeds in size that of the proximal artery. Lewis (98) 

 stated that the blood supply to the distal parts of the 

 limb is at first diminished, but that with passage of 

 time blood flow tends to become restored and may 

 even exceed that to the normal limb. This results 

 from the development of an extensive collateral circu- 

 lation as will be discussed (fig. 5). Evidence for in- 

 crease in the flow through the fistula is that cardiac 

 dilatation and decompensation can occur late after 

 the arteriovenous fistula is established. For the flow to 

 increase progressively the distal artery must be dis- 

 tensible. Excessive scarring can interfere with this 

 distensibility (79). Schenk el al. (153) made quantita- 

 tive observations on the regional blood flow in all 

 limbs of experimental arteriovenous fistulas using a 

 square wave electromagnetic flowmeter. They found 

 that in the femoral fistulas in dogs, flow tended to 

 increase and at the end of approximately 1 year had 



* 



fig. 5. Traumatic arteriovenous fistula. The trauma oc- 

 curred accidentally during attempted biopsy of lymph nodes 

 from the anterior scalene region in a 72-year-old man. Several 

 weeks later the patient noted pain and a pulsatile swelling in 

 the region of the wound over which a systolic bruit was audible 

 The specimen is a vinylite cast of the arteriovenous fistula 

 showing numerous tortuous arterial channels related to the 

 fistula. 



not yet reached stability. In contrast with the femoral 

 fistulas, flow through the proximal artery of carotid- 

 jugular fistulas tended to diminish with passage of 

 time, but in both types of fistulas there was a marked 

 increase in flow through both the arterial and venous 

 distal limbs. 



COLLATERAL CIRCULATION 



Collateral circulation may be defined as blood flow 

 that pursues a channel or system of vessels which is 

 alternative to or develops in substitution for a major 

 vascular pathway. To understand collateral circula- 

 tion would require not only a complete knowledge of 

 the mechanisms of angiogenesis, and therefore of all 

 growth, but also of the anatomical and functional 

 responses of blood vessels in general. At the present 

 writing only limited answers can be supplied to such 

 questions as: What starts the growth of collateral 

 vessels and what controls the rate of their increase; 

 what stops them from expanding indefinitely; when 

 newly formed, what guides them to their proper 

 place; what determines the structure of their walls. 



Some two hundred years ago the great John Hunter 

 was amazed to find that not only did the growth of 

 the stag's antler proceed uninterrupted when its 



