VENOUS RETURN 



I 121 



curves of the figure, with the systemic venous return 

 curve being governed by a new mean systemic pres- 

 sure of only 4 mm Hg and the pulmonary venous 

 return curve being governed by a very high pulmo- 

 nary pressure of +28 mm Hg. Blood continues to 

 shift from the systemic circulation into the pulmonary 

 circulation until the output of the left ventricle rises 

 to equal the falling output of the right ventricle. These 

 conditions are reached at equilibrium point C for 

 the right ventricle and equilibrium point C" for the 

 left ventricle. Since the mean pulmonary pressure 

 has risen to 28 mm Hg, the pulmonarv circulation 

 has become engorged with blood, and the pulmonary 

 capillary pressure will probably be above the critical 

 value of about 25 mm Hg, above which pulmonary 

 edema begins to appear (94). 



This is only a partial analysis because within the 

 next 30 sec or so sympathetic reflexes will elevate at 

 least three of the curves, the right ventricular output 

 curve, the systemic venous return curve, and the left 

 ventricular output curve, thereby resulting in a fur- 

 ther elevated equilibrium level of cardiac output 

 but also further increase in atrial pressures. 



EFFECT OF ACUTE RIGHT HEART FAILURE. Figure 26 



illustrates the sequence of events when the right heart 

 fails acutely. Points A and A' represent normal con- 

 ditions, and point B represents the instantaneous 

 effect of the acute failure on right heart output, show- 

 ing that the right heart output is only about one-half 

 the output of the left ventricle at that point. Immedi- 



ately, blood begins to shift from the pulmonary circu- 

 lation into the systemic circulation (138, 139), and 

 this shift continues until the left ventricular output 

 falls to equal the rising right ventricular output. The 

 new equilibrium points are C for the right heart and 

 C" for the left heart, both of which now have the same 

 ventricular outputs and venous returns of 700 ml per 

 min. During the re-equilibration of blood between 

 the systemic and pulmonary circulation, the mean 

 systemic pressure has risen from 7 to 7.4 mm Hg, 

 while the mean pulmonarv pressure has fallen from 

 7.0 to 4 mm Hg. This minute increase in mean sys- 

 temic pressure explains the failure of systemic vascu- 

 lar pressures to rise greatly in acute right heart failure 

 (122, 181). After another moment or so, sympathetic 

 reflexes tend to elevate the different curves as ex- 

 plained above, and the cardiac output can return 

 part way toward normal. 



effect of blood volume change. Figure 27 analyzes 

 the effect of hemorrhage on the outputs of both ven- 

 tricles. Note that the primary effect of reduced blood 

 volume is to shift the scales for left and right atrial 

 pressures, moving the zero pressure points toward 

 each other. An increase in blood volume causes ex- 

 actly the opposite effect. Here again, since the 

 capacitance of the pulmonary circulation is only one- 

 seventh that of the systemic circulation, the left atrial 

 pressure scale is still one-seventh that of the right 

 atrial pressure scale. Thus, in figure 27, the mean 

 systemic pressure has fallen to 1.7 mm Hg and the 



■| 2800- 



fig. 26. Analysis of the effect of sudden right 

 ventricular weakness on cardiovascular dynamics 

 showing a decrease in right ventricular output to 

 point C and left ventricular output to point C". 



-4 -2 +2 + 4 +6 



RIGHT ATRIAL PRESSURE (mm Hg) 



+ 50 +40 +30 +20 +10 -10 

 LEFT ATRIAL PRESSURE (mm Hg) 



