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HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



form, mitral and tricuspid valves as such are not 

 present. There is only a common atrioventricular 

 orifice and valve, so that venous and arterial blood 

 intermix as they traverse the atrioventricular canal. 

 There is also a defect in the atrial septum immediately 

 above the common valve. In the partial form there 

 is a defect in the lower part of the atrial septum but 

 the tricuspid valve is properly formed. The mitral 

 valve shows a cleft in its anterior leaflet. In a rare 

 case of the partial form the atrial septum is normally 

 formed but there is a cleft in tlie mitral valve. Those 

 patients with a defect in the atrial septum suffer 

 predominantly from the arteriovenous shunt, whereas 

 in the rarer type of partial form with an intact atrial 

 septum the functional disturbance is that of mitral 

 regurgitation resulting from the deformity of the 

 mitral valve. With the complete form survival be- 

 yond infancy is uncommon, whereas with the partial 

 form some patients survive to adult life. 



EBSTEIN's M.-'iLFORM.ATION OF THE TRICUSPID VALVE. 



Ebstein's malformation of the tricuspid valve (87) 

 is rare; it occurs in less than i per cent of congenital 

 cardiovascular defects and is characterized by attach- 

 ment of the septal and posterior leaflets of the tricuspid 

 valve to the right ventricular wall at its apex. The 

 anterior leaflet is normally attached to the annulus 

 fibrosus. The abnormal valvular attachment may 

 result in tricuspid regurgitation or cause the tricuspid 

 orifice to be reduced in size. One feature of the ab- 

 normal valvular attachment is that the greater por- 

 tion of the right ventricle forms a large, common 

 receiving chamber with the right atrium. The only 

 portion of the right ventricle that functions as such 

 is the anatomic outflow portion of this chamber. 

 An atrial septal defect is commonly associated with 

 Ebstein's malformation via which a relatively large 

 right-to-left shunt may occur. Patients with Ebstein's 

 malformation usually survive to adulthood, the 

 average survival period being about 25 years. 



ENDOCARDi.J^L SCLEROSIS. Endocardial sclerosis was 

 present in 5 per cent of a series of 357 cases of major 

 cardiac malformations reported from the Mayo 

 Clinic (106). It is characterized by elastic and col- 

 lagenous thickening of the mural endocardium. 

 Usually the left ventricle is involved and the left 

 atrium may be involved as well. The valves on the 

 affected side of the heart may be stenotic, but as a 

 rule they are normal. The endocardial thickening 

 of the left ventricle probably prevents normal excur- 

 sion of the veniricle during diastole and .so causes a 



progressive impediment to pulmonary venous drain- 

 age. The effects on the pulmonary circulation and 

 right ventricle are similar to those that would be 

 caused by mitral stenosis. Survival beyond infancy 

 is uncommon. 



Conduction Defects 



Abnormalities of rhythm of the heart are not 

 uncommon. Usually, however, the arrhythmias are 

 secondary to some underlying cardiac condition. 



SINUS ARRHYTHMIA. Sinus arrhythmia is present to 

 some degree in most children more than 4 or 5 years 

 of age and in most adults. This is a rhythmic variation 

 of heart rate occurring synchronously with respira- 

 tion. The heart rate increases toward the end of 

 inspiration and slows toward the end of expiration. 

 It is apparently due to alteration in the vagal tone 

 transferred from the central respiratory mechanism. 



HEART BLOCK. Depression of the conducting mecha- 

 nism of the atrioventricular node results in a delay 

 of the wave of excitation passing from atria to ven- 

 tricles. This is referred to as atrioventricular block. 

 A 2-to-i block or complete heart block is less fre- 

 quently encountered than minor degrees of heart 

 block (147). Ninety-five per cent of the higher grades 

 of heart block occurring in childhood appear to be 

 congenital in origin. These may be associated with 

 congenital heart disease, particularly corrected trans- 

 position of the great vessels which may coexist with 

 ventricular septal defect, single ventricle, aortic 

 atresia, and the like. Many of the patients show 

 evidence of left ventricular hypertrophy. This is to 

 be expected since the slow ventricular rate produces 

 a lengthening of the heart muscle in the prolonged 

 diastole, and this leads to hypertrophy. In children, 

 isolated heart block causes very few symptoms. Most 

 of them lead normal, active lives although a few 

 tire easily. It apparently is rare for Stokes-Adams 

 attacks to occur in congenital heart block. 



WOLFF-PARKINSON-WHITE SYNDROME. Ill I93O, Wolff, 



Parkinson, and White (265) first described an electro- 

 cardiographic entity consisting of a short P-R interval 

 and a wide QRS, as is seen in bundle branch block. 

 This anomaly is considered to be due to either an 

 accessory pathway around the atrioventricular node, 

 such as the bundle of Kent, or an accelerated spread 

 of the impulse through the atrioventricular node and 

 bundle. One complication is the frequent occurrence 



