1092 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



then distends more slowly toward its equilibrium 

 condition. As a consequence, volume increase due 

 to the delayed distension of the veins merges indis- 

 tinguishably with the volume increase due to capillary 

 leakage. One might attempt to control this problem 

 by using rigorously standardized intervals of pressure 

 exposure before reading the volume. Since the magni- 

 tude of the effect increases as higher pressures are 

 reached, however (cf fig. 6), virtually all reports of 

 the use of the plethysmography technique have de- 

 scribed vague and highly subjective criteria for select- 

 ing the end point at which to read the volume. There 

 results a random scatter of the determined points 

 which often obscures the actual form of the distensi- 

 bility curve. Therefore most recent authors have 

 abandoned any attempt to interpret the form of the 

 curve, and base their interpretations solely on the 

 total volume increment between two arbitrary pres- 

 sure levels. We will return to consider the significance 

 of this type of interpretation after first considering 

 other techniques which have focused on the form of 

 the distensibility curve. 



The author has developed a method based upon 

 the change in distensibility pattern of intestinal veins 

 in the dog. After surgical isolation of the blood flow 

 through an intestinal loop, the circulation is momen- 

 tarily interrupted while blood is injected in a retro- 

 grade direction into the venous bed. By dividing 

 the volume change required to raise the venous pres- 

 sure from 10 to 20 cm saline into the volume change 

 required to raise the pressure from 20 to 30 cm saline, 

 it is possible to calculate a "venomotor index" which 

 expresses numerically the degree of sigmoid curvature. 

 This method has been standardized by use of a motor- 

 driven syringe and accurate timing of the injections 

 so that highly reproducible readings can be obtained, 

 permitting following changes in venomotor tone in 

 an animal over a period of several hours (5). Also, 

 if injection rates are adjusted so as to yield equivalent 

 rates of pressure rise in preparations of different sizes, 

 reasonably quantitative comparisons can be made 

 between different animals. 



Unfortunately, this method is not without its limi- 

 tations. There remain some unanswered questions 

 as to the exact nature of such a retrograde injection. 

 The method was originally designed on the assump- 

 tion that there were no valves in this bed. Since India 

 ink injections invaded the minute vessels of the viscus, 

 and yet there was no suggestion of any rise in pressure 

 on the arterial side of the loop, there was an empiri- 

 cal basis for accepting with some confidence that 

 the distensibility measurements were representative 



of the venous bed. Recognition that these veins are 

 in fact well supplied with valves of rather poor com- 

 petence (fig. 1) raises the question as to whether the 

 valves might be making some contribution to the 

 pattern of the distensibility curves. To admit this 

 possibility, however, would not alter the actual inter- 

 pretation of the data. The retrograde pressure re- 

 quired to force blood past a valve must be a function 

 of the muscular tone in the wall of the vessel. A dilated 

 vein should develop valvular incompetence quite 

 readily, while a constricted vein should be able to 

 withstand somewhat higher back pressures before 

 developing incompetence. Such valve action might 

 act to exaggerate the degree of sigmoid curvature 

 in the distensibility of a constricted venous bed, but 

 this would reinforce rather than detract from the 

 acceptance of the degree of sigmoid pattern as an 

 index to the degree of venoconstriction. 



A more serious difficulty relates to the moderately 

 extensive surgical preparation required in the tech- 

 nique for setting up the loop, which taxes the com- 

 pensatory ability of the animal. Any further major 

 manipulation, such as an open-chest procedure, often 

 leads to deterioration of its circulatory status. This 

 method is also restricted to the analysis of pressure- 

 volume increments; a satisfactory quantitation of the 

 total venous volume has not yet proved feasible. This 

 negates some of the potential value of obtaining a 

 quantitative index, since it cannot be related in mean- 

 ingful terms to total circulatory function. Although 

 this is a defect which this technique shares in common 

 with most other methods for assessing the functional 

 activity of the peripheral venous bed, it is important 

 not to lose sight of the ultimate goal of being able to 

 evaluate quantitatively the contribution of the pe- 

 ripheral venous bed to over-all circulatory dynamics. 



A further extension of this general method has 

 recently been introduced by Bartelstone (7), w f ho 

 has studied the pattern of pressure development be- 

 hind a sudden obstruction of the vena cava. His re- 

 sults also afford evidence of the sigmoid distensibility 

 pattern which develops with venous constriction, 

 although in this method the quantitative interpreta- 

 tion is complicated by simultaneous changes in flow. 



Distensibility by Volume Increment 



As implied above, the plethysmographically re- 

 corded distensibility of the venous bed has been sub- 

 jected to two types of interpretation. Capps stressed 

 the pattern of the data, with the constricted veins 

 showing relatively less distensibility at low pressures 



