462 HANDBOOK OF PHYSIOLOGY >^ CIRCULATION I 



0-3 3-5 5-10 10-15 15-20 



f?IGHT VENTRICUL/JR SVSTOLIC PRESSURE MINUS 

 PULMONARV ARTERY SYSTOLIC PRESSURE— (mm Hg) 



FIG. 33. Frequency distribution of magnitude of systolic 

 pressure gradient across pulmonary valve in 59 cases of inter- 

 atrial communication. Measurements were made only from 

 records of pressures obtained as catheter tip was withdrawn 

 across pulmonary valve and in absence of cardiac irregularities. 



magnitude of the pulmonary and systemic blood 

 flows in these patients. In patients with uncomplicated 

 atrial septal defect the systemic blood flow remained 

 within the range of normal despite the magnitude of 

 pulmonary flow. 



Because of the normal difference in oxygen satura- 

 tion between blood from the superior vena cava and 

 blood from the inferior vena cava, use of the satura- 

 tion of superior vena caval blood as representative 

 of mixed venous lilood for the calculation of systemic 

 blood flow will result in a systematically lower value 

 for systemic blood flow. This may be the explanation 

 for the report of Dexter ( 78) that the left ventricular 

 output is usually reduced in patients with atrial septal 

 defect. 



Many patients with uncomplicated atrial septal 

 defect have a lower systolic blood pressure in the pul- 

 monary artery than in the right ventricle. Barratt- 

 Boyes & Wood (19) have demonstrated that in the 

 normal subject the gradient between the pulmonary 

 artery and the right ventricle varies from o to 5 mm 

 of mercury. The gradient associated with atrial septal 

 defect, however, may be as high as 20 mm of mercury 

 in patients in whom a normal pulmonary valve is 

 demonstrated at operation (fig. 33). 



The relative diameter of the pulmonary valve as 

 compared to the size of the right ventricular cavity 

 and the degree of dilatation of the pulmonary artery 

 could affect the gradient. In patients with atrial 

 septal defect the dilated right ventricle and pulmonary 

 artery separated by a normal valve could produce a 

 relative pulmonary stenosis. Weidman and associates 



(258) demonstrated a positive correlation iDctween the 

 pulmonary blood flow and the gradient. As the pul- 

 monary blood flow increased, the gradient across the 

 valve increased. Consequently, it is believed that the 

 high pulmonary blood flow with the resulting dilata- 

 tion of the right ventricle and pulmonary artery pro- 

 duces a "relative" stenosis at the pulmonary valve. 



FACTORS DETERMINING DIRECTION OF SHUNTS. The rea- 

 sons for the strongly predominant left-to-right shunt of 

 blood in patients with atrial septal defect have been 

 extensively studied. Dexter (78) found that in the 

 presence of a small atrial septal defect, left atrial pres- 

 sure was higher than that in the right atrium by 

 usually not more than 3 mm of mercury, but when 

 the opening was more than 2 cm- in cross-sectional 

 area there was no pressure difference discernible. Al- 

 though measurement of both right and left atrial pres- 

 sures is difficult, a number of such studies have been 

 carried out and, in general, are in agreement with 

 those of Dexter. Cournand et al. (69) studied three 

 subjects with atrial septal defects in whom left and 

 right atrial pressures were recorded. In these subjects 

 the amplitude of the pressure variations and the mean 

 pressure in the left atrium were greater than those in 

 the right. Little and co-workers (164) confirmed this 

 in dogs with surgically created atrial septal defects in 

 which the right and left atrial pressures were simul- 

 taneously recorded. They found in the dog that a 

 left-to-right pressure gradient persisted during the 

 entire cardiac cycle or for all but a short period before 

 or during atrial SNStole. Shaffer and collaborators 

 (220) plotted left atrial pressure against that in the 

 right in seven cases of atrial septal defect and showed 

 that fluctuations in the pressure gradient between the 

 atria can occur during the cardiac cycle. Braunwald 

 and colleagues (38) also found variations in the 

 gradient between the right and left atria during por- 

 tions of the cardiac cycle, particularly early in atrial 

 systole and immediately after atrial systole. During 

 these periods there may be a reversal in gradient with 

 momentary shunting of blood from right to left. This 

 venoarterial shunting is usually of insufficient quan- 

 tity to cause demonstrable decreases in the oxygen 

 saturation of peripheral arterial blood. 



It may be assumed then that the pressure in the 

 left atrium generally exceeds that in the right atrium. 

 However, w hen there is a large atrial septal defect, the 

 two atria can be considered to form a common pres- 

 sure chamber for practical purposes and the actual 

 pressure gradient between the right and left sides must 

 be minimal. 



