AP Ch 34 Cardiovascular Development PDF

# Summary Review: Development of the Cardiovascular System ## Development of the Cardiovascular System 1. The heart arises from the mesenchyme and begins as an enlarged blood vessel with a large lumen and a muscular wall. By approximately the seventh week of gestation, all structures of the fetal...

# Summary Review: Development of the Cardiovascular System ## Development of the Cardiovascular System 1. The heart arises from the mesenchyme and begins as an enlarged blood vessel with a large lumen and a muscular wall. By approximately the seventh week of gestation, all structures of the fetal heart and vascular system are present. 2. The endocardial cushions are instrumental in closing the atrial septum, dividing the AV canals into the right and left AV orifices, and closing the septum. 3. In the fetus the pulmonary and systemic circulatory systems are connected by the foramen ovale, an opening between the atria; by the ductus arteriosus, a fetal vessel that joins the PA to the aorta; and by the ductus venosus, a fetal vessel that connects the inferior vena cava to the umbilical vein. 4. Fetal circulation is different from postnatal circulation because of the presence of fetal shunts and altered metabolic needs of the various organs. 5. Fetal blood flow depends on resistance for its distribution through the body. Resistance in the pulmonary circulation is higher than resistance in the systemic circulation, so myocardial thickness is about the same in the right heart and the left heart. 6. After birth, SVR increases and PVR decreases. 7. PVR drops suddenly at birth because the lungs expand and the pulmonary vessels dilate. It continues to decrease gradually during the first 8 weeks after birth. Decreased PVR causes the right myocardium to become thinner. 8. SVR increases markedly at birth because severance of the umbilical cord removes the low-resistance placenta from the systemic circulation. Increased SVR causes the left myocardium to become dominant and thicken over time. 9. Changes in resistance cause disappearance of the fetal connections between the pulmonary and systemic circulatory systems. The foramen ovale closes functionally at birth and anatomically several months later; the ductus arteriosus closes functionally 15 hours after birth and anatomically within the first several weeks; and the ductus venosus closes within 1 week after birth. 10. At birth a series of circulatory changes occur that affect blood flow, vascular resistance, and oxygen tension. The most important change is the shift of gas exchange from the placenta to the lungs. 11. After birth, significant postnatal changes occur, including thinning of the right ventricular myocardium as the PVR drops. As the SVR increases, the left ventricular myocardium becomes thicker and more dominant as it is in the adult heart. # Summary Review: Congenital Heart Defects - Most congenital cardiovascular defects have begun to develop by the fourth week of gestation, and most have many causes, both environmental and genetic. - Environmental risk factors associated with the incidence of CHD typically are maternal conditions. Among these are viral infections, diabetes, drug intake, alcohol intake, metabolic disorders, and advanced maternal age. - Genetic factors associated with CHD include, but are not limited to, trisomy 21 or Down syndrome, trisomy 13, trisomy 18, cri du chat syndrome, and Turner syndrome. It now appears, however, that most genetic mechanisms of causation are multifactorial. - Classification of CHDs is based on whether they (a) cause blood flow to the lungs to increase or decrease, (b) obstruct ventricular blood flow patterns, or (c) cause mixing of unoxygenated and oxygenated blood. - Symptoms of HF are usually the result of CHDs that increase blood volume and pressure in the pulmonary circulation, or myocardial failure. Clinical manifestations are almost the same as the manifestations of HF in adults, with the addition of FTT in children. - Cyanosis, a bluish discoloration of the skin, indicates that the tissues are not receiving fully adequate oxygenated blood. Cyanosis can be caused by defects that (a) reduce pulmonary blood flow; (b) overload the pulmonary circulation, causing pulmonary hypertension, pulmonary edema, and respiratory difficulty; and (c) cause large amounts of unoxygenated blood to shunt from the pulmonary to the systemic circulation. - Congenital heart defects that maintain or create direct communication between the pulmonary and systemic circulatory systems cause blood to shunt from one system to another, mixing oxygenated and unoxygenated blood and increasing blood volume and pressure on the receiving side of the shunt. - The direction of shunting through an abnormal communication depends on differences in pressure and resistance between the two systems. Flow is always from an area of high pressure to an area of low pressure. The resistance to flow determines the volume of the shunting. - Acyanotic CHDs that increase pulmonary blood flow consist of abnormal openings (PDA, ASD, VSD, AVC defect, or truncus arteriosus) that permit blood to shunt from left (systemic circulation) to right (pulmonary circulation). Cyanosis does not occur because the left-to-right shunt does not interfere with the flow of oxygenated blood through the systemic circulation. - If the abnormal communication between the left and right circuits is large, volume and pressure overload in the pulmonary circulation leads to HF. - In truncus arteriosus the main trunk fails to divide longitudinally into the aorta and PA. All blood from both ventricles enters the truncus, so that mixed blood is delivered by both circulatory systems, causing varying degrees of cyanosis and HF. - In CHDs that decrease pulmonary blood flow (TOF, tricuspid atresia), myocardial hypertrophy cannot compensate for restricted right ventricular outflow. Flow to the lungs decreases, and cyanosis is caused by mixing of systemic and pulmonary venous return. - Obstruction of ventricular outflow commonly is caused by PS, AS, COA, or interrupted aortic arch. - Despite obstruction, ventricular output remains normal for a long time because of compensatory ventricular hypertrophy stimulated by increased afterload and, in postductal COA, development of collateral circulation around the coarctation. - Complex CHDs that depend on mixing of the pulmonary and systemic circulations for survival during the postnatal period include TGA, HLHS, and TAPVC. This mixing results in desaturated systemic blood flow and cyanosis. - In TGA the circulatory systems are not connected serially or through a shunt, so that oxygenated blood remains permanently in the pulmonary circulation and unoxygenated blood remains in the systemic circulation. Survival depends on patency of the ductus arteriosus; in the absence of patency, surgical intervention is mandatory. - TAPVC is caused by abnormal pulmonary vein development and the lack of direct pulmonary venous return to the LA. All blood from the pulmonary and systemic circulations enters the RA. Mixed blood enters the LA through an ASD; it then flows into the systemic circulation and causes cyanosis. - Tricuspid atresia [left] and HLHS [right] are types of single-ventricle defects that commonly require three staged palliative surgical procedures. - Treatment for all hemodynamically severe CHDs is surgical or interventional palliation of the anomaly and management of cyanosis and HF. # Summary Review: Acquired Cardiovascular Disorders - The most common acquired cardiovascular disorders of childhood are Kawasaki disease, rheumatic heart disease (see Chapter 33), obesity, and HTΝ. - Kawasaki disease is an acute systemic vasculitis that may result in the development of coronary artery aneurysms and thrombosis. - Essential or primary HTN in children is the same as that in adults, except that it is more likely to be diagnosed at a younger age and most are at an asymptomatic stage. Most cases of secondary HTN in young children are because of an underlying cause, such as renal disease or COA. - Obesity in childhood is epidemic in the United States and other countries, but more recent data shows that obesity prevalence is not increasing. - Obese children are at risk for acquiring numerous other serious and potentially life-threatening illnesses, such as asthma, sleep apnea, HTN, type 2 diabetes mellitus, dyslipidemia, and cardiovascular disease.