PERIPHERAL VENOUS SYSTEM 



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should not be overlooked, at present it has not been 

 developed to the point that it can be applied to the 

 accurate measurement of venous tone. 



Venous Distensibility Patterns 



As has been pointed out in reference to figure 5, 

 constriction of a vein alters its distensibility diagram. 

 The maximally dilated vein exhibits a smooth curve 

 convex toward the volume axis. As progressively 

 more constriction occurs, the distensibility curve is 

 transformed into a sigmoid form showing initially a 

 relatively rapid rise in pressure with initial volume 

 increments, a very much slower pressure rise as inter- 

 mediate volumes are added, and then a final steep 

 rise in pressure as still further volume is introduced. 

 If the rate of venous distension is carefully controlled 

 so as to prevent stress relaxation effects from dis- 

 torting the slopes of these curves, evidence of venocon- 

 striction should be obtainable from studies of the 

 shape of the distensibility curve. 



The first application of this principle was presented 

 by Capps (16) using a plethysmographic method on 

 the human forearm. The use of the plethysmographic 

 technique for venous distensibility measurements 

 was a logical outgrowth of measurements of blood 

 flow by the Hewlett & Van Zwaluwenburg method 

 (50). In the latter method, while the distal portion 

 of the arm is enclosed in a plethysmograph to record 

 arm volume, a pressure cuff around a proximal por- 

 tion is suddenly inflated to a pressure slightly less 

 than the arterial diastolic pressure. This suddenly 

 blocks venous outflow from the arm without any 

 immediate interference with arterial inflow, and 

 hence the arm will increase in volume at a rate equal 

 to the rate of blood flow into the arm. After a short 

 interval, this blood flow will be reduced by the pro- 

 gressive congestion of the distal vascular bed. The 

 point is reached eventually where arm volume be- 

 comes relatively stable, and this must mean that 

 venous pressure has increased to equal cuff pressure 

 so that venous blood will be forced past the occluding 

 cuff at a rate equal to the reduced inflow rate. As 

 has been pointed out earlier, the most significant 

 factor in the change in the volume of the arm under 

 these conditions is the congestion of the venous bed 

 (17, 39). Therefore, using stepwise increments in 

 pressure in the occluding cuff yields a series of incre- 

 ments in arm volume which, as a reasonably good 

 first approximation, represents the increase in venous 

 volume at the corresponding occluding pressures. 

 With this method, Capps obtained clear evidence of 



a sigmoid distensibility curve in veins constricted by 

 cold and other venoconstrictor stimuli, while dilated 

 veins exhibited a typical convex distensibility pattern. 



Many subsequent authors have reported on veno- 

 motor reactions using the plethysmographic method. 

 A number of modifications in technique have been 

 introduced to minimize the artifact associated with 

 inflation of the pressure cuff, and to permit accurate 

 pressure reference levels. Errors in the pressure-volume 

 determinations due to unequal distribution of pres- 

 sure in the transitional zone at the margin of the 

 plethysmograph may be corrected by use of a double 

 plethysmograph. Both compartments are exposed to 

 equal pressures, but only the distal segment is used 

 for volume recording (93). Burch (12) has used 

 another method to correct for the occlusion artifact 

 in developing the plethysmographic technique for 

 use on the digit. By measuring the volume change 

 produced by the venous occlusion cuff during an 

 interval when all blood flow had been arrested by 

 arterial compression, he obtains an uncomplicated 

 record of the artifact alone, which can be subtracted 

 from the blood flow curves. When these corrected 

 curves are compared with unoccluded digital pulse 

 curves, he feels that he can analyze arterial inflow 

 and venous outflow dynamics with sufficient accuracy 

 to quantitate the phasic changes that occur during 

 each pulse wave. 



Other plethysmographic devices, such as the mer- 

 cury-in-rubber resistance strain gauge (89) or the 

 impedance plethysmograph (75), may be adapted 

 for venous studies, although the simplicity of their 

 application should not encourage neglect of estab- 

 lishing their quantitative reliability. 



There is one inherent difficulty in the plethysmo- 

 graphic method for distensibility determinations, 

 however, which in the opinion of the author has not 

 been adequately resolved. The sudden increment in 

 venous outflow pressure will have its most direct 

 effect in elevating venous pressure; to a lesser extent 

 it will elevate capillary pressure, and to a slight ex- 

 tent it will elevate the distal portions of the arterial 

 pressure gradient. This justifies the assumption that 

 most of the immediate volume change will occur on 

 the venous side, an assumption which has been rea- 

 sonably well substantiated. On the other hand, the 

 elevated capillary pressure promotes capillary transu- 

 dation, so that the recorded volume never reaches a 

 true plateau, but shows a slow increase persisting 

 after the initial major increase. Owing to the time- 

 dependent characteristics of vascular elasticity, more- 

 over, venous distension occurs rapidly at first and 



