EDAIC MCQ (Multiple Choice Question) and answer. FREE UPLOADS BY WWW.TARGETEDAIC.COM (81)

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Question: True statements regarding PDA

• The high partial pressure of oxygen is the critical factor in the closure
• The isolated PDA arises from the anterior surface of the main pulmonary artery near its junction with the left pulmonary artery and joins the posterior descending aorta after the origin of the left subclavian artery.
•  Permanent anatomic closure of the ductus occurs typically within the first 3 weeks of life
• Administration of indomethacin will produce functional closure of the Ductus in 50-70 % of infants. 
• The surgical approach to the PDA is usually through a left postero-lateral thoracotomy.

Answers:

1. 1. True
   2. True
   3. True
   4. True
   5. True

Explanation:

• The Ductus Arteriosus (PDA) is a persistent foetal communication from the main pulmonary artery (PA) to the descending aorta. The incidence of this defect is 1 in 2500 live full-term births, accounting for approximately 10% of all congenital heart defects.
• The isolated PDA arises from the anterior surface of the main pulmonary artery near its junction with the left pulmonary artery and joins the posterior descending aorta after the origin of the left subclavian artery.
• The shunt between the aorta and the PA can be restrictive or non-restrictive. The magnitude and direction of flow are determined by the pressure in the aorta and resistance to flow in the two vascular beds, pulmonary and systemic. With non-restrictive PDA, the normal low pulmonary vascular resistance leads to an increased pulmonary blood flow and rise in pulmonary vascular pressure while systemic blood flow is frequently reduced
• At birth, several important changes occur in the heart and peripheral circulation. The circulation of the newborn infant is in transition from the foetal to the adult pattern. It can be shifted back to the foetal pattern by the presence of congenital heart disease, prematurity, anaesthesia, hypoxia and other conditions. Shunting through the PDA and across the foramen ovale is characteristic of foetal circulation. 
• After birth and the cessation of placental circulation, an increase in systemic vascular resistance occurs. Pulmonary vascular resistance decreases by 75 % and pulmonary blood flow increase 450 % with the beginning of neonatal respiration. From a prenatal pressure of 70/45, the pulmonary artery pressure decrease to 50/30 at 24 hours after birth and to 30/ 12 a few days later. 1 Vasoactive drug, including bradykinin, prostaglandins and endothelium-derived relaxing factor contribute to pulmonary vasodilation. Oxygen and ventilation contribute to increases in the release of these substances. 
• The PDA begins to close within 10-15 hours of birth as a result of the increased arterial oxygen pressure and vasoactive factors. The ductus is physiologically closed by the second day of life in normal infants, but we must remember that it can reopen in response to hypoxia. In neonates, acute moderate hypoxia increases cardiac output if metabolic acidosis is absent. However, in the presence of metabolic acidosis, cardiac output decreases. Acute severe hypoxia causes bradycardia. Other changes associated with hypoxia are decreased myocardial contractility, increased pulmonary vascular resistance and reopening of the ductus with shunting. Permanent anatomic closure of the ductus occurs typically within the first 3 weeks of life. The haemodynamics of PDA are similar to those of Ventricular Septal Defect in that the amount of left to right shunt is governed by the size of the defect and vascular resistance. When the ductus is large, the magnitude of the shunt is determined by the ratio of pulmonary vascular resistance to systemic vascular resistance.
• The surgical approach to the PDA is usually through a left postero-lateral thoracotomy. Intercostal nerve blocks at the end of the procedure are effective in controlling post-operative pain and can be done by the surgeon under direct vision before the chest is closed

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‘EDAIC MCQ (Multiple Choice Question) and answer’ covers a different topic each day with the intent that candidates can prepare one small topic each day, making steady progress in their exam preparation.