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A pulmonary shunt often occurs when the alveoli fill with fluid, causing parts of the lung to be unventilated although they are still perfused. [2] Intrapulmonary shunting is the main cause of hypoxemia (inadequate blood oxygen) in pulmonary edema and conditions such as pneumonia in which the lungs become consolidated. [2]
The most common cause of right-to-left shunt is the Tetralogy of Fallot, a congenital cardiac anomaly characterized by four co-existing heart defects. Pulmonary stenosis (narrowing of the pulmonary valve and outflow tract, obstructing blood flow from the right ventricle to the pulmonary artery)
Secondly, the pulmonary shunt is caused by zero or low V/Q ratio due to insufficient ventilation and excess perfusion. Improper ventilation lowers blood oxygenation and oxygen supply to body tissues. Although 100% oxygen is inspired, a pulmonary shunt prevents oxygen from being delivered to the alveoli and blood capillaries.
The pathophysiology of acute respiratory distress syndrome involves fluid accumulation in the lungs not explained by heart failure (noncardiogenic pulmonary edema). It is typically provoked by an acute injury to the lungs that results in flooding of the lungs' microscopic air sacs responsible for the exchange of gases such as oxygen and carbon dioxide with capillaries in the lungs. [1]
A cardiac shunt is when blood follows a pattern that deviates from the systemic circulation, i.e., from the body to the right atrium, down to the right ventricle, to the lungs, from the lungs to the left atrium, down to the left ventricle and then out of the heart back to the systemic circulation.
A left-to-right shunt can cause heart failure, with symptoms such as tachypnea, poor eating, and diaphoresis. Dyspnea and indications of laborious breathing can be caused by low lung compliance and increased airway resistance. Infants may have failure to thrive as well as recurrent pneumonia. [2]
A pulmonary-to-systemic shunt is a cardiac shunt which allows, or is designed to cause, blood to flow from the pulmonary circulation to the systemic circulation. [1] [2] This occurs when: there is a passage between two or more of the great vessels; and, pulmonic pressure is higher than systemic pressure and/or the shunt has a one-way valvular ...
Physiological shunts, occur due to the effect of gravity. The highest concentration of blood in the pulmonary circulation occurs in the bases of the pulmonary tree compared to the highest pressure of gas in the apexes of the lungs. Alveoli may not be ventilated in shallow breathing. Shunting may also occur in disease states: