454 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



Without Pulmonory Hypertension With Pulmonory Hypertension 



(12 Cases) 



(24 Coses) 



10 20 30 



flGE-Yeors 



to 20 30 



/IGE-reors 



FIG. 23. Relationship of pulmonary blood flow to age in 36 

 patients with isolated ventricular septal defect. Note decrease 

 in blood flow with increase in age. 



no. 24. Relationship of pulmonary /systemic resistance ratio 

 to direction and magnitude of shunt in 24 patients with isolated 



-{20 -BO 



MAGNITUDE OF SHUNT 



-10 



ventricular septal defect and pulmonary hypertension. Note 

 that, as pulmonary resistance increases to levels in excess of 

 systemic vascular resistance, left-to-right shunt decreases 

 and is eventually exceeded by shunt in right-to-left direction. 



values. Savard et al. (208) compared left atrial pres- 

 sures measured directly during operation in 36 

 patients before and after closure of a ventricular 

 septal defect (fig. 25). The data demonstrate that the 

 left atrial pressure was significantly reduced in the 

 majority of cases after closure of the ventricular 

 septal defect. 



FACTORS DETERMINING DIRECTION AND MAGNITUDE OF 



SHUNT. The hemodynamic alterations associated with 

 a ventricular septal defect are determined chiefly by 

 two factors; /) the area of the defect and 2) the 

 reactions of the pulmonary vasculature, that is, 

 changes in resistance to pulmonary blood flow asso- 

 ciated with the defect. 



For purposes of simplification, ventricular septal 

 defects may i)e separated into two categories, large and 

 small, on the basis of the cross-sectional area of the 

 defect (208, 279). Of course, there may be many 

 defects the size of which is intermediate between 

 these two categories. 



A small ventricular septal defect is one the size of 

 which provides a significant resistance to blood flow 

 between the left and right ventricles. In spite of such 

 a defect the ventricular septum still constitutes an 

 effective, although incomplete, barrier to blood flow 

 between the left and right ventricles. It would be 

 expected, therefore, that the pressure in the right 

 ventricle would be significantly lower than that in 

 the left ventricle. Under such circumstances the 

 magnitude of a left-to-right shunt is determined 

 chiefly by the size of the defect, that is, the resistance 

 to blood flow across it. 



A large ventricular septal defect is defined as one 



4 8 12 16 20 



MEAN LEFT ATRIAL PRESSURE 

 POSTCARDIOTOMY 



24 28 



- mm Hg 



FIG. 25. Comparison of left atrial pressure recorded at opera- 

 tion before and after cardiotomy for closure of \entricular 

 septal defect in 36 patients. Diagonal broken line is line of 

 identity for preclosure and postclosure values. The fact that 

 left atrial pressure is nearly always reduced after closure of 

 ventricular septal defect suggests that increased left ventricular 

 work load associated with defect had resulted in increase in 

 left atrial pressure. 



the size of which is such that the resistance to blood 

 flow through the defect is very small, and therefore 

 the ventricular septum is an ineffectual barrier 

 between the left and right ventricles. Thus, in these 

 cases, the magnitude and direction of the ijlood flow 

 through the defect arc determined primarily by the 

 relative resistances to the outflow of blood from these 

 two common-pressure chambers \ia the aorta on the 



