I 1 06 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



does not significantly alter the effect of right atrial 

 pressure on venous return. 



Figure 4 illustrates one of the typical methods used 

 to study venous return. This procedure substitutes an 

 external perfusion circuit for the right heart. The 

 right atrium is cannulated, and blood is pumped from 

 this cannula through a system that includes a) a 

 collapsible tube that can be raised or lowered to adjust 

 the right atrial pressure, b) a heater used to maintain 

 a normal blood temperature, c) a pump that always 

 provides sufficient propulsive force to keep flowing 

 through the circuit all the blood that is allowed to 

 pass the collapsible tube, and d) a recording flow- 

 meter, usually of the rotameter type. After passing 

 through this external circuit, the blood is reinfused 

 into the distal stump of the pulmonary artery. Thus, 

 all the blood entering the right atrium must pass 

 through this external circuit. This system allows con- 

 tinuous recording of the blood flow through the heart, 

 which can be termed either "venous return" or 

 "cardiac output." Indeed, even all the coronary blood 

 flow, which empties into the right atrium, courses 

 through this circuit. Furthermore, the pressure inside 

 the collapsible tube automatically adjusts exactly to 

 the atmospheric pressure; therefore, the right atrial 

 pressure can be elevated or lowered and maintained 

 at a very exact value, regardless of how much blood 

 flows into the right atrium, by simply raising or 

 lowering the collapsible segment. 



Still another important feature of this system is that 

 the right atrial pressure, except during periods of 

 measurements, can be maintained at a negative value, 

 thus allowing blood to flow from the extrathoracic 

 veins into the right atrium with equally as much ease 



Flowmeter 



t ' Right atrium 



Collapsible tube 



fig. 4. External perfusion system in which a pump controls 

 the activity of the heart. Using this external system it is 

 possible to raise and lower the right atrial pressure at will 

 while studying the effect of right atrial pressure on venous 

 return. 



as in the closed-chest animal. Therefore, in using this 

 preparation, the circulatory system does not deterio- 

 rate, as is usually the case when the veins are cannu- 

 lated (178), but, instead, will continue in an active 

 state for many hours. 



EFFECT OF RIGHT ATRIAL PRESSURE ON VENOUS RE- 

 TURN THE NORMAL VENOUS RETURN CURVE. Figure 5 



illustrates the normal effect on venous return of in- 

 creasing the right atrial pressure (96). Note that this 

 curve, like the cardiac output curves discussed above, 

 does not tell one exactly what the venous return will 

 be, but tells instead what the venous return would be 

 if the right atrial pressure were known. Furthermore, 

 this venous return curve depicts the circulatory 

 conditions at a given instant, but it can become 

 considerably altered, as will be discussed below, from 

 one instant to another. For this reason, all the different 

 points along the curve must be measured under 

 identical circulatory conditions. This has been diffi- 

 cult to do, because any time the venous return falls 

 to subnormal values, circulatory reflexes immediately 

 ensue, attempting to return the venous return back 

 toward a normal value. In so doing, the venous 

 return curve becomes greatly altered, which also 

 will be discussed below. To prevent this, two different 

 methods have been used to measure the venous 

 return curve without altering systemic conditions by 

 circulatory reflexes or by other circulatory changes 

 during the course of measurement. First, venous 

 return curves have been measured in animals sub- 

 jected to total spinal anesthesia, which abrogates all 

 circulatory reflexes. However, to determine the 

 normal venous return curve, a drip of epinephrine or 

 norepinephrine is administered to the animal to 

 return the vasomotor tone back to normal prior to 

 making the measurements necessary to establish the 

 curve. 



Plateau 



c 1 200- 



z 

 a. 



3 



O 



z 



UJ 



> 



800 



400- 



Mean systemic 



pressure = 



7 mm Hg 



44 



RIGHT ATRIAL PRESSURE (mm Hg) 



fig. 5. The normal venous return curve. 



-4 

 ATRIAL 



+ 8 



