PHYSIOLOGIC CONSEQUENCES OF CONGENITAL HEART DISEASE 



465 



monary-artery pressure increases slowly or rapidly 

 during early adult life, usually with persistence of an 

 increased level of pulmonary blood flow. Develop- 

 ment of hypertension in the pulmonary circuit, 

 whether associated with increased pulmonary blood 

 flow or not, is unlike the response of normal pulmo- 

 nary vessels that are distended with but a small change 

 in pressure, when the flow through them is increased. 

 An increase in pulmonary hypertension in atrial 

 septal defect with an increased pulmonary blood flow 

 signifies an increase in pulmonary vascular resistance 

 from values below the range of normal to values that 

 equal or slightly exceed normal. Study of a relatively 

 few patients seen over a period of 6 to 8 years suggests 

 that once the level of pulmonary-arterv pressure is 

 significantly elevated, it may remain virtually the 

 same, whereas the progression of organic change in 

 the pulmonary ves.sels is manifested only by a steady 

 decline in pulmonary blood flow. Why pulmonary 

 hypertension develops in certain patients and not in 

 others is uncertain. Histologic studies of the small pul- 

 monary vessels of such patients have shown that once 

 pulmonary hypertension is established a distinct 

 muscular media forms in the arterioles, and the media 

 of the muscular pulmonary arteries hypertrophies. 

 The progression of histologic changes seems identical 

 to that of the changes in pulmonary hypertension 

 associated with ventricular septal defect or patent 

 ductus arteriosus. 



It has been suggested that pulmonary embolism 

 may play a role in this phenomenon. Dexter (79), how- 

 ever, pointed out that pulmonary embolism is rare in 

 the first two decades of life and begins to become ap- 

 parent in the third decade. He was unable to detect 

 any evidence that pulmonary embolism had occurred 

 in patients with pulmonary vascular disease, and con- 

 cluded that if this was a factor it must be rare. 



Although, when normal, the vessels responsible for 

 pulmonary resistance have relatively little smooth 



muscle as compared with similar vessels in the systemic 

 circulation, there is evidence from the eflfects of hy- 

 poxia (83, 86, 96) and more recently from the effects 

 of 5-hydroxytryptamine (203) that they are capable of 

 constriction. That tone is present in the smooth 

 muscle of the pulmonary vessels in some patients with 

 atrial septal defects can be demonstrated by two 

 different methods. First, a change from breathing air 

 to breathing 99 per cent oxygen is accompanied in 

 many of these patients by a fall in pulmonary vascular 

 resistance, often by more than a third of the initial 

 values (237). The method by which this occurs is 

 unknown. 



Second, acetylcholine chloride, a substance which 

 when injected intra-arterially into systemic vessels 

 causes local vasodilatation (85), has recently been 

 used in the study of the pulmonary circulation. Its 

 rapid destruction in the circulating blood offers the 

 possibility that an injection can be made into the pul- 

 monary artery in sufficient concentration to affect the 

 pulmonary vessels without altering the hemodynamics 

 on the left side of the heart or in the systemic circula- 

 tion. Wood and collaborators (278) have used a single 

 injection of this substance and have shown that tone is 

 present in the resistance vessels of the lungs in some 

 patients with mitral stenosis. Harris (128) found that 

 rapid injections into the pulmonary artery of patients 

 with pulmonary hypertension, some of whom had 

 congenital heart disease, resulted in a transient fall of 

 pressure in the pulmonary artery in slightly more than 

 one third of the patients. Shepherd and associates 

 (221) have administered acetylcholine cliloride by 

 continuous infusion into the pulmonary artery in pa- 

 tients with congenital heart disease. Figure 36 shows 

 the effects of such an infusion in a patient with pul- 

 monary hypertension and an atrial septal defect. 

 During the infusion there occurred a marked decrease 

 in systolic pressure of the right ventricle and an in- 

 crease in o.xygen saturation of the blood in the pul- 



BREATHING AIR 



Earpiece Oximeter - lo^ soi %) 



R. V, Blood (0, s.i. X) f^^'- 



Respiration ^T: '^ '^ '^, 



RADIAL ARTERY 

 I mm Hg I 



PULMONARY ARTERY 



I mm Hg I 



J ./ iJ J , v^ 1]^ \J :/ iJ 

 llji'l'llli^l|llllllll|IJ'lli|illllll||llllll||li» 



rr. n 



'J v/ y y, J , 



HSART RATE 

 (Bioti/minuttl 



FIG. 36. Effects of continuous in- 

 fusion of acetylcholine into pulmonary 

 artery of 33-year-old woman with atrial 

 septal defect and pulmonary hyperten- 

 sion. Note prompt fall in pulmonary- 

 artery pressure associated with rise in 

 oxygen saturation of right ventricular 

 blood on administration of acetyl- 

 choline. Vertical time lines are at in- 

 tervals of 10 sec and were 3 cm apart 

 before photographic reduction of record. 



