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HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



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AREA OF VENTRICULAR SEPTAL DEFECT (cm^/m^ of Surface Area) 



FIG. 27. Relationship between .systolic pressures in pulmonary and systemic circulations and size 

 (area) of ventricular septal defect. Horizontal broken line indicates average relationship between 

 systolic pressure in left ventricle and that in radial artery in normal persons. It would be anticipated 

 that in patients with equal systolic pressures in left and right ventricles, values for right ventricular/ 

 radial-artery systolic pressures would cluster along this line. Note that for patients with defects with 

 an area of more than i cm-/m-, values for systolic pressure do scatter along this line, indicating that 

 in this situation systolic pressures in right and left ventricles were closely similar or equal. Note also 

 that ratios obtained when patients were breathing air (open circles) nearly always exceeded those ob- 

 tained when breathing oxygen. This finding is a consequence of reduction of pulmonary vascular 

 resistance associated with oxygen breathing and indicates that in the presence of lowered pulmonary 

 vascular resistance a larger ventricular septal defect is required to elevate right ventricular systolic 

 pressure toward level being maintained by left ventricle. 



the quantity of blood flowing into it via the detect 

 and by the pulmonary vascular resistance against 

 which the ventricle is emptying. Thus, if the pul- 

 monary vascular resistance is normal (that is, low), 

 a very high blood flow through the defect will be 

 required to equalize right and left ventricular systolic 

 pressures and still maintain a left ventricular systolic 

 pressure and consequently systemic arterial pressure 

 compatible with life. Since the capacity of the left 

 ventricle and pulmonary vascular bed to maintain 

 a very high flow is limited, a normally low pulmonary 

 vascular resistance is incompatible with prolonged 

 survival in the presence of a large ventricular septal 

 defect. Thus all surviving patients with large ventricu- 

 lar septal defects by necessity have an increa.sed 

 resistance to outflow from the right ventricle. 



It would be expected that in the presence of defects 

 in the intermediate size range of approximately 0.5 

 to 2.0 cm- per m- the pressure gradient between the 



left and right ventricles would \ary from zero to a 

 relatively large value, depending on pulmonary 

 vascular resistance. If pulmonarv vascular resistance 

 is high in .such cases, relatively little blood flow across 

 the defect is required to equalize pressures between 

 the ventricles so that right and left ventricular systolic 

 pressures would be essentially equal. If, however, 

 pulmonary vascular resistance were very low, the 

 systolic runoff into the pulmonary artery would be 

 very rapid, and hence a very high flow across the 

 defect would be required to equalize right and left 

 ventricular pressures during systole. In this .situation, 

 significant differences in the systolic pressure levels 

 in the two sides of the heart would be expected in 

 the presence of moderate-sized defects. 



That the magnitude of the pulmonary vascular 

 resistance may be important in determining the rela- 

 tive levels of right and left ventricular systolic pres- 

 sures has been demonstrated by infusion of acetyl- 



