474 



H.^NDBOOK OF PHYSIOLOGY ^^ CIRCULATION I 



T.ABLE lo. Average and Range of Hemodynamic Values 

 in /J Patients With Atrial Septal Defect 

 and Pulmonary Stenosis 



T.-\BLE 1 I . Comparison of Differences in Riglit 

 Ventricular, Pulmonary and Systemic Arterial 

 Systolic Pressures in Patients With Pulmonary 

 Stenosis and Ventricular or Atrial Septal Defects 



* S.A. = systemic artery; R.V. = right ventricle; P. A. = 

 pulmonary artery. 



septa reduced left and increased right atrial pressures. 

 With marked stenosis, right atrial pressures exceeded 

 left atrial pressures at all points in the cardiac cycle. 

 They were, however, able to prevent right-heart fail- 

 ure by creation of an interatrial septal defect. 



Amorim and co-workers (9) have studied the efifect 

 of atrial septal defects on hemodynamic alterations 

 caused by acute partial obstruction of the pulmonary 

 artery in dogs. These studies were carried out in 

 closed-chest dogs in which a balloon catheter was ad- 

 vanced to the pulmonary artery and inflated to pro- 

 duce various degrees of occlusion. 



In these dogs with chronic atrial septal defects, 

 graded obstruction of the main pulmonary artery asso- 

 ciated with right ventricular systolic pressure levels of 

 more than 80 mm of mercury was produced without 

 evidence of right-heart failure. A considerable decrease 

 in the magnitude of the left-to-right shunts and an 

 increase in the right-to-left shunts occurred which in 

 some cases exceeded 50 per cent of the systemic blood 

 flow. These changes were associated with only minor 

 changes in the filling pressure of the right ventricle 

 and no systematic changes in the systemic arterial 

 pressure. There was no evidence for significant tri- 

 cuspid regurgitation under these circumstances. 



These findings contrasted with similar studies in 



dogs with intact cardiac septa in which obstruction of 

 the pulmonary artery, so as to cause an increase in 

 right ventricular systolic pressure to more than 60 mm 

 of mercury, was associated with marked elevation of 

 mean right atrial pressure and a striking decrease in 

 cardiac output and in systemic arterial pressure asso- 

 ciated with a significant degree of tricuspid regurgita- 

 tion. 



It appears that atrial septal defect provides a con- 

 siderable increase in resistance to the hemodynamic 

 effects associated with right ventricular hypertension 

 caused by acute partial obstruction of the pulmonary 

 artery. The defect in the atrial septum is in effect a 

 "safety valve" allowing blood to shunt in the right-to- 

 left direction, thus preventing overloading of the right 

 ventricle. 



It has also been pointed out that an increase in right 

 atrial pressure associated with a decrease in systemic 

 pressure and the consequent decrease in the pressure 

 gradient across the coronary artery -coronary sinus 

 system may decrease coronary flow. An atrial septal 

 defect allows the blood to shunt into the left atrium 

 and hence into the systemic circulation, thus main- 

 taining tiie systemic pressure as well as preventing a 

 marked increase in right atrial pressure. Thus the pres- 

 sure gradient through the coronary vessels would be 

 maintained in such animals in spite of a severe increase 

 in resistance to right ventricular outflow (g). 



Although it does appear that a right-to-left shunt 

 via an atrial septal defect is protective in an acute in- 

 crease in resistance to right ventricular outflow, it 

 should be recognized that the resulting systemic ar- 

 terial hypoxemia is in it.self deleterious. 



Atrial septal defect with tricuspid stenosis. Stenosis of the 

 tricuspid valve increases resistance to flow of blood 

 into the right ventricle so that an increase in right 

 atrial pressure occurs. The left-to-right shunt through 

 the atrial septal defect is decreased or rever.sed depend- 

 ing on the severity of the stenosis. Pulmonary blood 

 flow never attains the high levels encoimtered in un- 

 complicated atrial septal defects and, if the tricuspid 

 stenosis is severe, pulmonar\ l:)lood flow may be de- 

 creased to less than systemic flow. 



Patients who develop tricuspid stenosis as a result 

 of rheumatic endocarditis may also develop cyanosis 

 as a result of the occurrence of a right-to-left shunt via 

 a valve-competent foramen ovale, present in 25 per 

 cent of the normal population. 



ebstein's malformation. The hemodynamic altera- 

 tions occurring in Ebstein's malformation (87) may be 

 considered the result of obstruction to the flow of 



