VENOUS RETURN 



I I 05 



2400-, 



2000- " 



1600 



1200 



-4 + 4 +8 + 12 



RIGHT ATRIAL PRESSURE (mmHg) 



+ 16 



fig. 3. Cardiac output curves, showing the effect of open- 

 ing the chest and of cardiac tamponade. 



the degree, of course, depending upon the degree of 

 cardiac tamponade. 



SUMMARY OF FACTORS THAT AFFECT THE CARDIAC 



output curves. From the above discussion of the 

 different factors that affect cardiac output curves, we 

 see that the curves fall into two simple patterns. First, 

 any factor that increases the effectiveness of the heart 

 as a pump or decreases the load against which it must 

 pump will elevate the curves. Conversely, any factor 

 that decreases the effectiveness of the heart as a pump 

 or increases its load will depress the curves. The 

 second pattern of cardiac output curves occurs when 

 pressure on the outside of the heart alters the effective 

 cardiac filling pressures. The greater the pressure 

 outside the heart, the further to the right is the cardiac 

 output curve shifted. 



Bv keeping these basic principles in mind one can 

 determine with relative accuracy what the cardiac 

 output curve of any given heart under any given 

 condition will be, but it must always be remembered 

 that at any instant the pumping effectiveness of the 

 heart is depicted by only a single cardiac output 

 curve, not by a family of curves. With this discussion 

 of cardiac output curves as a background, we can now 

 proceed to the somewhat more complicated venous 

 return curves which are the "complements" to the 

 cardiac output curves. 



Venous Return Curves 



VENOUS RETURN CURVES AS COMPLEMENTS TO CARDIAC 



output curves. Referring back to figures 2 and 3, one 



sees that each cardiac output curve represents an 

 infinite series of cardiac outputs over a range of right 

 atrial pressures. However, it is obvious that the cardiac 

 output at any given time can be only one value, not 

 an infinite number of values. The next question that 

 arises is, how does one determine where on the cardiac 

 output curve the circulatory system will be operating 

 at a given time? Obviously, if one can determine the 

 right atrial pressure, he can then determine the 

 cardiac output. However, the right atrial pressure, 

 like the cardiac output, is a variable. For this reason, 

 we must find some method to determine the right 

 atrial pressure at the same time that we determine the 

 cardiac output. A method for doing this is to analyze 

 the systemic circulation in terms of right atrial pres- 

 sure and flow, just as the heart-lung segment has been 

 analyzed for right atrial pressure and flow. This time 

 we use the term "venous return" for the flow, but we 

 still use right atrial pressure as the opposite coordinate. 

 If we remember that venous return equals cardiac 

 output except for transient instantaneous differences, 

 we see that we are using exactly the same coordinates 

 for analyzing the characteristics of the systemic circu- 

 lation as we have used for the heart-lung segment of 

 the circulation. Once this is done, the curves de- 

 picting the respective functions of the two segments of 

 the circulation can be plotted on the same coordinates, 

 and the point at which the two curves cross provides a 

 solution that gives the venous return, the cardiac 

 output, and the right atrial pressure, all simul- 

 taneously (81). 



METHOD FOR RECORDING VENOUS RETURN CURVES. To 



determine the effect of right atrial pressure on the 

 return of blood from the systemic circulation to the 

 heart, we must use one of three different procedures: 

 / ) isolate the systemic circulation and use a con- 

 trolled pump in place of the heart, 2) control the 

 action of the heart itself by placing a controlled pump 

 in place of one portion of the heart, or 3) make 

 measurements of flows and pressures in different parts 

 of the systemic circulation while artificially altering 

 the pumping ability of the heart. In our laboratory, 

 we have studied the effect of right atrial pressure on 

 venous return using all these three different methods, 

 in open-chest animals when using the first two 

 methods (87, 96, 98, 99, 102, 103, 105) and in closed- 

 chest animals when utilizing the third method (97). 

 The results have been identical within the limits of 

 experimental error. Furthermore, contrary to the 

 effects on the cardiac output curve, opening the chest 



