VENOUS RETURN 



U1 



Z> 



o 



z 



UJ 



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2400 



2000 



1600 



1200 



800 



400 H 







MEAN ART PRESS =184 



TOTAL SPINAL ANESTHESIA 

 EPINEPHRINE 

 ELEVEN DOGS 

 MEAN WEIGHT = 14 kg 



0.0035 mg /kg /minute 

 MEAN ART PRESS. = 120 



0.0015 mg /kg /minute 

 MEAN ART PRESS. = 62 



0.0005 mg /kg /minute 

 MEAN ART PRESS = 4 



NO EPINEPHRINE 



fig. II. Effect of different rates of epinephrine 

 injection on the venous return curves. Note that 

 the principal effect is to increase the mean systemic 

 pressure. [From Guyton el al. (95 I 



■16 -12 -8 -4 + 4 +8 +12 



RIGHT ATRIAL PRESSURE (mm Hg) 



epinephrine, as depicted in the experiments of figure 

 1 1 , there had to be equal dilatation somewhere else. 

 Indeed, measurements have shown that, as the 

 arterioles constrict under these conditions, there is a 

 tendency for the veins to dilate even though the walls 

 of the veins do tighten to a very great extent (55). 

 This elevates the mean systemic pressure but does not 

 increase the resistance to blood flow from the systemic 

 vessels toward the heart. In essence, then, we can say 

 that an increase in vasomotor tone affects venous 

 return principally by increasing the mean systemic 

 pressure, and, usually, an increase in vasomotor 

 tone does not increase the average resistance that 

 opposes the return of blood to the heart. 



SUMMARY OF FACTORS THAT AFFECT THE VENOUS 



return curves. Basically, there are only two different 

 patterns of changes in venous return curves, those 

 that result from a) changes in resistance in the systemic 

 circulation, and h) changes in the mean systemic 

 pressure. Figure 6 depicts the pattern of venous 

 return curves that results from alteration of vascular 

 resistance, while figure 9 illustrates the curves that 

 result from alteration of mean systemic pressure. 



Any factor that alters resistance, whether this be a 

 localized or generalized alteration, will correspond- 

 ingly alter the venous return curve. However, altera- 

 tion of the venous resistance affects venous return far 

 more drastically than alteration of the arterial re- 

 sistance. 



The factors that affect the mean systemic pressure 

 can be divided into two main groups: /) those that 

 affect the blood volume, and 2) those that affect the 

 ability of the circulatory system to hold blood. The 

 two most important factors of all that affect mean 



systemic pressure are blood volume itself and changes 

 in vasomotor tone. 



The interrelationships of all these different factors 

 on the return of blood to the heart can be expressed 

 mathematically by the following formula (98) : 



VR- 



P 



Pro 



R v C v+ (R v +R o )C a 



c v + c a 



In this formula VR is venous return, P ms is mean 

 systemic pressure, P ra is right atrial pressure, C,. is 

 capacitance of the veins, C„ is capacitance of the 

 arterial tree, R v is the average resistance to blood flow 

 from the veins to the heart, and R„ is the resistance 

 from the arterial tree to the venous tree. This formula 

 shows that venous return is approximately pro- 

 portional to the mean systemic pressure minus right 

 atrial pressure, which has been called the ''pressure 

 gradient for venous return," while, on the other hand, 

 venous return is inversely proportional to the re- 

 sistances in the systemic circulation. The capacitances 

 in the formula are constants for any given animal, 

 and they determine the relative importance of arterial 

 resistance versus venous resistance. In the normal 

 animal a given change in venous outflow resistance 

 affects venous return approximately eight times as 

 much as the same change in arterial resistance (87). 



Equating the Venous Return and Cardiac Output Curves 



If one understands the different factors that affect 

 venous return and cardiac output curves, he can 

 readily determine the approximate effects of any 

 given circulatory change on each of these two types 



