L 15-Cardiac Development & Congenital Anomalies PDF

15 Cardiac Development& Congenital Anomalies ILOs By the end of this lecture, students will be able to 1. Interpret the development of the heart tube in relevance to its divisions and derivatives. 2. Appraise the development and fate of sinus venosus relevant to its derivatives. 3. Differentiate bet...

15 Cardiac Development& Congenital Anomalies ILOs By the end of this lecture, students will be able to 1. Interpret the development of the heart tube in relevance to its divisions and derivatives. 2. Appraise the development and fate of sinus venosus relevant to its derivatives. 3. Differentiate between the developmental origin of the smooth and rough parts of both atria relevant to their function. 4. Interpret the different congenital anomalies affecting the heart septa in relation to their developmental process. 5. Correlate the clinical syndromes affecting the heart with its developmental origin. 6. Interpret the congenital defects of the heart in relevance to the changes in fetal circulation that occur after birth. CLINICAL PROBLEM A full-term male infant, on the first day after birth, presented with generalized cyanosis. A chest radiograph revealed slightly enlarged heart with a narrow base and increased pulmonary vascular markings. A probable diagnosis of complete transposition of the great blood vessels was made. Give the embryological basis of this anomaly, and explain how the infant was able to survive after birth with such a severe anomaly? Formation of the heart tube    The heart is mesodermal in origin. It develops from two primitive heart tubes, which are formed from mesenchyme in the cardiogenic area of the embryo. These tubes fuse with each other in a craniocaudal direction to form a single primitive heart tube. However, the caudal ends of two heart tubes fail to fuse with each other. This tube forms the endocardium of the heart. The splanchnic mesoderm surrounding the primitive heart tube forms myocardium and epicardium. Page 1 of 9 Formation of cardiac loop  During the 4th to 7th week, the heart undergoes looping followed by septation → a typical four chambered structure. The single endocardial tube elongates, forms dilations & constrictions, forming:  Sinus venosus,  Atrium,  Ventricle,  Bulbus cordis,  Truncus arteriosus. During the fifth week, the truncus region is divided by a spiral septum into an anterolateral portion (the outflow tract of the right ventricle) the pulmonary artery, and a posteromedial portion (the outflow tract of the left ventricle) the aorta. During fetal life, the left pulmonary artery is connected to the dorsal aorta by ductus arteriosus, which is forming a prenatal shunt. Page 2 of 9 Formation and looping of the heart tube Septum Formation in the Common Atrium Begins at the end of the 4th wk. The septum primum, a sickle-shaped crest descending from the roof of the atrium, begins to divide the atrium in two but leaves a lumen, the ostium primum.Later, the ostium secundum is formed by cell death that creates an opening in the septum primum. Finally, a septum secundum forms, but an interatrial opening, the oval foramen, persists. Only at birth, when pressure in the left atrium increases, do the two septa press against each other and close the communication between the two. Page 3 of 9 Septum formation in the ventricles The interventricular septum consists of a thick muscular part and a thin membranous portion. CLINICAL CORRELATES Page 4 of 9 1-Transposition of the great vessels: Occurs when the spiral septum fails to follow its normal spiral course and runs straight down. As a consequence, the aorta originates from the right ventricle, and the pulmonary artery originates from the left ventricle. Sometimes associated with a defect in the membranous part of the interventricular septum; It usually accompanied by an open ductus arteriosus. Transposition of the great vessels 2- Ventricular septal defects (VSDs): In the membranous or muscular portion of the septum- Mostly (80%) muscular region. 3- Tetralogy of Fallot: Abnormality resulting from anterior displacement of the spiral septum. It consists of: (a) Pulmonary stenosis; (b) A large defect of the interventricular septum; (c) Overriding of the aorta; and (d) Hypertrophy of the right ventricular wall (because of higher pressure on the right side). Page 5 of 9 Tetralogy of Fallot 4- Patent ductus arteriosus (PDA) Under normal conditions the ductus arteriosus is functionally closed through contraction of its muscular wall shortly after birth to form the ligamentum arteriosum. A patent ductus arteriosus, either may be an isolated abnormality or may accompany other heart defects especially in premature infants. 5- Coarctation of the aorta: The constriction may be above or below the entrance of the ductus arteriosus: preductal or postductal. Page 6 of 9 Coarctation of the aorta 6- Dextrocardia: Ventricular inversion, a condition in which the looping of the heart tube is reversed from its normal pattern, producing a heart that has its apex inferior and to the right (rather than left) side. Dextrocardia: D, Diaphragm; DLA, dilated left atrium; L, liver; LL, left lung; V, ventricle. CIRCULATORY CHANGES AT BIRTH Changes in the vascular system at birth are caused by cessation of placental blood flow and the beginning of respiration. Since the ductus arteriosus closes by muscular contraction of Page 7 of 9 its wall, the amount of blood flowing through the lung vessels increases rapidly. This, in turn, raises pressure in the left atrium. Simultaneously, pressure in the right atrium decreases as a result of interruption of placental blood flow. The septum primum is then apposed to the septum secundum, and functionally the oval foramen closes. To summarize, the following changes occur in the vascular system after birth: 1- Closure of the umbilical arteries: Distal parts of the umbilical arteries form the medial umbilical ligaments, and the proximal portions remain open as the superior vesical arteries. 2- Closure of the umbilical vein and ductus venosus: After obliteration, the umbilical vein forms the ligamentum teres of the liver. The ductus venosus, which courses from the ligamentum teres to the inferior vena cava, is also obliterated and forms the ligamentum venosum. 3- Closure of the ductus arteriosus forms the ligamentum arteriosum. 4- Closure of the oval foramen. SOLVING THE CLINICAL PROBLEM The transposition of the great blood vessels is caused by abnormal development of the spiral septum. The infant was able to survive after birth because this anomaly is often associated with patent ductus arteriosus, ASD, and/or VSD that allows mixing of blood between two circulations. N.B. The complete transposition of the great blood vessels is incompatible with life if there is no PDA or atrial and ventricular septal defects. Page 8 of 9 Fetal Circulation birth Circulation after Page 9 of 9