PHYSIOLOGIC CONSEQiUENCES OF CONGENITAL HEART DISEASE 



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AREA OF VENTRICULAR SEPTAL DEFECT ( cm ^ ) 



FIG. 26. Relationship between 

 size (area) of defect and "re- 

 sistance" to blood flow across 

 ventricular septal defects. Inset 

 shows relationship between area 

 of ellipse in square centimeters 

 and magnitude of major axis, 

 which is 125% of minor axis. In 

 scries of patients studied, maxi- 

 mal dimension of ventricular 

 septal defect averaged 125% of 

 minimal dimension. Broken line 

 indicates rectangular hyperbola, 

 K. = X-Y, in which K equals 

 average product of ordinate (Y) 

 %alues for pressure/flow indexes, 

 and abscissa (X) values for area 

 of defects. See text for discussion. 

 [From Savard rl al. (208).] 



left and the pulmonary artery on the right side of the 

 incomplete septum. The resistances to blood flow 

 out through the aorta and the pulmonary artery are 

 determined in turn by the degree of systemic and 

 pulmonary vascular resistances, respectively. If 

 pulmonary resistance is low in relation to systemic 

 vascular resistance, a large left-to-right shunt will be 

 present, and vice versa. Since systeinic vascular 

 resistance has been demonstrated to remain essen- 

 tially normal in patients with ventricular septal 

 defects, the alterations in pulmonary vascular resist- 

 ance that are frequently associated with this disease 

 are of paramount importance in determining the 

 hemodynamic effects associated with the defect. 



Savard and co-workers (208) have studied the 

 hemodynamic alterations in ventricular septal defect 

 in relation to the size of the defect measured during 

 open cardiotomy for repair of these defects. They 

 correlated the cross-sectional areas of the ventricular 

 septal defects and the calculated resistance to flow 

 across these defects (fig. 26). Their data indicate 

 that for defects with an area of less than i cm^ the 

 resistance to blood flow across the defect increases 

 rapidly with decrease in size of the defect, whereas 

 the resistance to blood flow across the defects with an 

 area of more than i cm' falls to practically zero. This 

 confirmed the statement of Wood et al. (279) that 



no obstruction to flow would be expected in defects 

 larger than i cm' in size. 



The relationship of the estimated areas of ventricu- 

 lar septal defects to the ratio of the systolic pressures 

 recorded simultaneously in the pulmonary and sys- 

 temic arterial circulations is shown in figure 27. This 

 systolic-pressure ratio has been expressed as the sys- 

 tolic pressure in the right ventricle divided by the 

 systolic pressure recorded simultaneously from the 

 radial or femoral arteries. Since in normal individuals 

 the systolic pressure in these arteries exceeds the 

 systolic pressure in the thoracic aorta by 14 per cent, 

 it would be expected that this ratio would approach 

 an average value of 0.88 for patients in whom sys- 

 tolic pressures in the right and left ventricles were 

 equal. This average ratio of 0.88 is shown in figure 

 27 as a horizontal broken line. These data indicate 

 that as the area of a ventricular septal defect increases 

 toward i cm- per m- there is a progressive increase 

 in the systolic-pressure ratio (that is, a decrease in 

 pressure gradient) across the defect, and as the area 

 of the defect approaches and exceeds i cm- per m^ 

 this ratio approaches 0.88, or, in other words, the 

 pressures in the right and left \entricles approach 

 unity with an area of more than i cm- per m'-. 



It must be kept in mind, however, that the systolic 

 pressure in the right ventricle is determined both by 



