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In page Atrial septal defect:

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If the ASD is left uncorrected, the pulmonary hypertension progresses and the pressure in the right side of the heart becomes greater than the left side of the heart. This reversal of the pressure gradient across the ASD causes the shunt to reverse – a right-to-left shunt. This phenomenon is known as Eisenmenger's syndrome. Once right-to-left shunting occurs, a portion of the oxygen-poor blood gets shunted to the left side of the heart and ejected to the peripheral vascular system. This causes signs of cyanosis.[citation needed]