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HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



servations in 120 cases of ventricular septal defect, 

 considered that the anatomic relationship of the defect 

 to the orifice of the pulmonary artery is just as impor- 

 tant a factor as size. Imperial and colleagues (141) 

 found in their series that defects of the muscular 

 septum had different effects from those in the mem- 

 branous septum. They thought that the explanation 

 for this was that the muscular defect may contract 

 during ventricular systole, and on this assumption 

 the effective size rather than the location per se is the 

 hemodynamic determinant. In the more direct 

 studies of Savard and co-workers (208), the data 

 support the interpretation that the effective size of 

 the defect is the primary determinant of its hemo- 

 dynamic effects irrespective of its position in the 

 septum. 



EFFECT ON PULMON.\RV VASCULAR RESISTANCE. It Seems 



logical to conclude that the changes in pulmonary 

 vascular resistance in patients with ventricular septal 

 defects are related to the size of the defect. If the 

 defect is small, the pulmonary vasculature is pro- 

 tected, since the ventricular septum still constitutes 

 an effective (high resistance) although incomplete 

 barrier to blood flow from the left ventricle into the 

 pulmonary circulation. In patients with small ventric- 

 ular septal defects no increase in vascular resistance 

 ordinarily occurs in the pulmonary circulation irre- 

 spective of the age of such patients (fig. 30). However, 

 if the defect is large so that the ventricular septum 

 provides an ineffective (low resistance) barrier to 

 blood flow from the left ventricle into the pulmonary 

 circulation, the pulmonary vasculature is subjected 

 to very high blood flows and, in addition, to pressures 

 equal to or approaching systemic arterial pressures 

 generated by the forceful contractions of the left 

 ventricle. An increase in pulmonary vascular resist- 

 ance occurs in this situation and usually attains or 

 approaches the level of systemic vascular resistance 

 by the age of 20 years or before, as shown in figure 30. 



Adams and collaborators (7) made serial observa- 

 tions of 20 patients with ventricular septal defect and 

 also concluded that pulmonary resistance tends to 

 increase progressively with time after varying intervals 

 of "latency." 



Keith and co-workers (147) noted that the associa- 

 tion of severe pulmonary hypertension with a patent 

 ductus arteriosus is uncommon in childhood. Since 

 the ductus is usually considerably smaller than the 

 aorta, the shunt (left-to-right flow) that occurs 

 through it is, as a rule, equivalent to that which 

 occurs via a relativelv small- or moderate-sized ven- 



10 20 30 40 50 



FIG. 30. Relationship of ratio of pulinonary/systemic vascular 

 resistance to age in 53 patients with ventricular septal defect. 

 Patients in whom systemic exceeded pulmonary systolic pres- 

 sure by more than 20 mm of mercury are presumed to have 

 small ventricular septal defects. In these patients (open circles) 

 this ratio remains low, that is, vascular resistance is normal or 

 only slightly elevated, and there is no apparent tendency for 

 pulmonary vascular resistance to increase with age. Patients 

 with closely similar systolic pressures in pulmonary and systemic 

 arteries are presumed to have large ventricular septal defects. 

 In these patients (solid circles) there is a significant tendency for 

 ratio of pulmonary /systemic vascular resistance to increase 

 widi age. Note that pulmonary vascular resistance was greatly 

 elevated to values closely similar to or in excess of those for 

 systemic vascular resistance in all patients in this group who 

 were more than 20 years of age. 



tricular septal defect. These authors found that 1 1 per 

 cent of their series of children with patent ductus 

 arteriosus had severe pulmonary hypertension, 

 whereas approximately one third of their patients 

 with ventricular septal defect had .severe pulmonary 

 hypertension. This difference in incidence of pulmo- 

 nary hypertension in the two conditions is believed 

 related to the fact that the resistance to blood flow 

 through ventricular septal defects (which can be large) 

 is usually less than that through a patent ductus 

 arteriosus, the diameter of which is usually consider- 

 ably less than the diameter of the aorta. Another 

 factor is that the ductus is of variable length and the 

 greater the length the greater will be the resistance 

 to blood flow through the ductus. 



The exact cause or causes for the increase in pul- 

 monary vascular resistance that occurs in these 

 patients are not known. It appears that the increase 

 in systolic pressure in the right ventricle and pulmo- 

 nary arteries that is associated with a large ventricular 

 septal defect is an important inciting cause for the 

 development of increased pulmonary vascular resist- 

 ance. Blount et al. (34) have suggested that the force 

 of ejection into the pulmonary \ascular tree may 



