PERIPHERAL VENOUS SYSTEM 



I0 93 



and greater distensibility at intermediate pressures. 

 A majority of authors, however, have stressed the 

 increment in volume between two arbitrary pressures, 

 and interpreted a decrease in the volume change be- 

 tween these two pressures as an indication of venous 

 constriction. 



To argue that a decreased volume increment is an 

 indication of venoconstriction might appear to repre- 

 sent a conflict with our previous discussion of dis- 

 tensibility patterns. The great distensibility of the 

 constricted vein over the intermediate range of the 

 sigmoid curve would seem to demand a great total 

 distensibility in the constricted vein. As can be ap- 

 preciated by reference back to figure 5, however, a 

 comparison of the volume increments in constricted 

 and dilated veins between any two arbitrary pressures 

 will yield different relationships at different pressure 

 levels. In the experiment shown in figure 7, for ex- 

 ample, the injection happens to have been stopped 

 at the point that the pressure-volume relationships 

 in the constricted and dilated veins were virtually 

 identical; a comparison of the total pressure-volume 

 increment would offer no suggestion of the significant 

 differences in distensibility above and below this 

 particular point. 



The author is not aware of any data, obtained with 

 sufficient precision to identify the pattern of the dis- 

 tensibility curve, in which apparent discrepancies 

 in the interpretation of decreased venous distensi- 

 bilities cannot be resolved by reference to the pressure 

 level. To illustrate this point, the data in figure 1 1 

 were taken from a report of a reasonably well-stand- 

 ardized application of the plethysmographic method 

 and described by the authors as a "'typical" response. 



fig. 1 1 . Volume increments in the human forearm recorded 

 by congesting the veins to successively higher pressure. Open 

 circles indicate control values; solid circles are the volumes 

 determined while infusing noradrenaline at the rate of 0.4 

 jug/min. [Redrawn from Glover el al. (34).] 



The authors interpreted these data as demonstrating 

 that noradrenaline acts to decrease venous disten- 

 sibility. One cannot argue with such an interpretation 

 as a correct description of the data as far as they go. 

 Nonetheless, this interpretation ignores the different 

 form of the curves. Although the control data ex- 

 hibit a relationship convex to the volume axis, the 

 data recorded during noradrenaline infusion are 

 clearly concave to the volume axis. These data are 

 therefore completely compatible with the pattern 

 interpretation used by Capps and generalized in 

 figure 5. An extrapolation of the curves in figure 1 1 

 to higher pressures would clearly lead to a relative 

 increase in the total distensibility of the constricted 

 vessels and a relative decrease in the total disten- 

 sibility of the dilated vessels. Unfortunately, since 

 the plethysmographic method cannot be used effec- 

 tively when venous congesting pressures approach 

 arterial pressures, this technique does not appear 

 suitable for bringing out the full sigmoid distensibility 

 pattern in these veins. It should be recognized that 

 veins in the extremities are subjected to much greater 

 hydrostatic loads than are visceral veins, and there- 

 fore it would not be at all surprising to discover that 

 much higher pressures were required to achieve the 

 full sigmoid pattern in arm veins. 



It is of interest to note that a similar argument has 

 arisen in reference to isolated vein preparations. 

 Leonard & Sarnoff (59) state unequivocally that a 

 constrictor drug always reduces venous distensibility. 

 Inspection of the Leonard and Sarnoff report, 

 however, reveals a definite alteration of the pattern 

 of their distensibility data which is quite compatible 

 with the suggestion that sufficient stretch will even- 

 tually reveal a very significant distensibility of the 

 constricted vein. 



Nevertheless, there is another factor in these disten- 

 sibility characteristics which must be considered. 

 The original interpretation of the sigmoid curve 

 visualized that at high pressures the constricted 

 vessel was pulled out to the same dimension as the 

 dilated vessel, and thus demands that the total 

 distensibility must be greater in the constricted 

 vessel. As has been discussed earlier, the validity of 

 this interpretation is open to some question. It is 

 conceivable that a sigmoid distensibility pattern may 

 be compatible with some reduction in total disten- 

 sibility. If, in addition, dilated veins were in a state 

 of partial collapse at the point that the initial volume 

 was measured, this would augment the possibility of 

 observing a greater total distensibility- in the dilated 

 vein. 



