PHYSIOLOGIC CONSEQUENCES OF CONGENITAL HEART DISEASE 



463 



In studies with dog atria, Little et al. (164) meas- 

 ured the volume-elasticity properties of the right and 

 left atria. The amount of fluid needed to completely fill 

 the atria without distending them was measured, and 

 then iBcasured amounts of additional fluid were added 

 and the atrial pressure recorded. The right atrial 

 system was found to have an average initial filling 

 \olumc twice that of the left atrial system. The 

 volume-elasticity curves plotted for the right and left 

 atrial systems showed that for equal increments in 

 volume the right atrial system was more distensible 

 than the left. On the basis of these studies they con- 

 cluded that the pressure gradient between the left and 

 right atria is related to the different elastic properties 

 (distensibility) of the atria. Cournand and associates 

 (68) also attributed the pressure gradient to the 

 smaller capacity and distensibility of the left atrium 

 and pulmonary veins. Hickam (131), however, sug- 

 gested that when the two atria communicate, that 

 ventricle which normally operates under lower pres- 

 sure more readily accepts blood and has a larger 

 output. Since the filling pressure in the right ventricle 

 is lower than in the left, it accepts the greater amount 

 of blood, and the flow between the atria is left to right. 

 Hull (136) stated that the larger tricuspid valve and 

 the ease with which the right ventricle fills lower the 

 pressure in the right atrium. Undoubtedly the dis- 

 tensibility characteristics of both atria and ventricles 

 play a role in determining the pressure gradient be- 

 tween the atria. In patients with pulmonary hyper- 

 tension the right ventricle becomes hypertrophied and 

 mav fail. In these cases the right ventricle becomes 

 less distensiijle and the right atrial pressure may rise 

 with failure. The right-to-left shunt may then become 

 predominant. 



The study of patients with atrial septal defect by 

 means of indicator-dilution techniques has provided a 

 great deal of information about the direction and mag- 

 nitude of blood flow from the various pulmonary and 

 systemic veins. Swan and co-workers (238) first demon- 

 strated that blood from the right lung shunts preferen- 

 tially across an atrial septal defect. This was done by 

 injecting indicator into the right and left pulmonary 

 arteries, with sampling of the resultant dye-blood mix- 

 ture at a systemic artery. When indicator is injected 

 into the right pulmonary artery the resulting dilution 

 curve recorded from a systemic artery showed a 

 smaller peak concentration and greater distortion of 

 the disappearance slope than did the curve obtained 

 following injection into the left pulmonary artery, as 

 shown in figure 34. This anomalous drainage of rela- 

 tively greater magnitude from the right lung appears 



Injection into rioht 

 pulmonary artery 



DYE INJECTION INTO; (8 



0! SAT. 7o 

 -100 

 LEFT PULMONARY 



ARTERY 



RIGHT PULMONARY 

 ARTERY 



Injection into left 

 pulmonary artery 



8.9 mg/L 



PULMONARY TRUNK 



5.6 mg f\_ 



8.0 mg /L 



T-1824 

 0.18 mg /kg 



FIG. 34. Diagram of path taken by indicator dye after its 

 injection into both right and left pulmonary arteries and re- 

 sultant dilution curves recorded in systemic arterial system in 

 a case of atrial septal defect, a: Diagrammatic representation of 

 central circulation. Thick solid lines within diagram represent 

 circulatory route taken by indicator after its injection. Relative 

 thickness of these lines represents fraction of indicator passing 

 to different locations from left atrium. Small insert above each 

 diagram represents general contour of the systemic arterial 

 dilution curve associated with each site of injection, with 

 instant of injection indicated by arrow. Note proximity of right 

 pulmonary veins to location of septal defect, evident from patho- 

 logic anatomic studies of this condition, b: Systemic arterial 

 dilution curves recorded in a 16-year-old girl with atrial septal 

 defect. T-1824 was injected at sites indicated to left of figure. 

 Note smaller initial deflection and greater distortion of disap- 

 pearance slope recorded following injection of dye into right 

 pulmonary artery than into left, indicating that more of dye- 

 blood mi,\ture is shunted left to right from right pulmonary 

 veins than from left. Distortion of curve recorded following 

 injection into main pulmonary artery (bottom panel) is inter- 

 mediate between those recorded after injection into right and 

 left pulmonary arteries. 



to be a consistent feature in the usual case of atrial 

 septal defect and is most probably a consequence of 

 the juxtaposition of the atrial septal defect to the ori- 

 fices of the right pulmonary veins in the left atrium. 

 Since the pulmonary veins from the left lung enter the 

 left atrium farther from the atrial septum, less of the 

 blood from these sites crosses the defect. Similar con- 



