Patent Ductus Arteriosus
Definition
Patent ductus arteriosus (PDA) is the most common extra cardiac shunt which represents a persistent patency of the vessel that normally connects the pulmonary arterial system and the aorta in the fetus. It accounts for 10-12% of all varieties of congenital heart defects.
A normal ductus arteriosus allows for a partial shunt between the pulmonary artery system and aortic system in the fetus allowing for partial bypass of the lungs. It represents a muscular artery with an intact, wavy, internal elastic lumina, a circulatory arranged media of smooth muscle cells, and a medial component. It is found 2-10mm beyond the left subclavian being 5-10mm in length with a wide aortic orifice and a narrow pulmonary orifice thereby allowing a restrictive flow. The dusctus arteriosus should close within 2-4 weeks after birth beginning at the pulmonary end to become the non-functional ligamentum arteriosum.
Pathology
Before the closure of the ductus arteriosus the right ventricle provides 85% of all aortic blood flow in the fetal circulation. Ductus arteriosus is mediated by the release of vasoactive substances (acetylcholine, bradykinin, endogenous catecholamines, etc.), and variations in pH.
PDA is classified according to vascular resistance as: small, moderate, or large. Resistance of the ductus is related to the cross sectional area and the length of the duct itself. With increased resistance there is decreased flow across the PDA. There is a small increase in LV work in this instance.
Left sided PDA refers to an opening on the PA side.
Right sided PDA refers to an opening on the aortic arch side.
Bilateral PDA refers to the opening of both sided of the ductus.
Hemodynamics
In a large PDA there is free communication between the aorta and the pulmonary artery. This increases the systolic pressure in the pulmonary artery and causes a volume overload on the left ventricle. The LV begins to dilate and hypertrophy to carry the load. The RV acquires a pressure overload.
The effects of the PDA depend upon 4 factors:
1. size of communication
2. pulmonary vascular resistance (PVR)
3. presence and degree of prematurity and respiratory distress syndrome (RDS)
4. functional capability of the volume loaded LV
Small PDA: normal pulmonary arterial pressure and PVR
Large restrictive PDA: increase in pulmonary blood flow, volume overload on the left side of the heart, normal PVR, increased LA and LV hypertrophy, little or no RV hypertrophy.
Large non-restrictive PDA: pulmonary hypertension, pressure overload on RV, direction of flow dependant on SVR and PVR
SVR > PVR: left to right shunt, left side volume overload
PVR > SVR: right to left shunt, right side volume overload
Clinical Manifestations
Symptoms are found in those with large shunts, heart failure, or other underlying problems. Some of the symptoms are: effot dyspnea, hoarsness due to pulmonary hypertension pushing on the laryngeal nerve, cyanosis, angina and syncope seen in restrictive PDA.
Surgical Interventions
Surgical ligation of PDA represents one of the few curative operations for congenital heart disease.
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