Then blood pass to right ventricle this will increase pulmonary artery pressure  Due to the incorrect blood flow, symptoms such as fatigue (from decreased systemic blood flow), palpitations (from blood flowing from left atria to right atria), weight gain, edema, right upper chest pain (all caused from the left to right atria blood flow), and paroxysmal nocturnal dyspnea (shortness of breath during sleep), orthopnea (difficulty in breathing while lying down), and hemoptysis or coughing up blood (all caused by small ASD that cause blood flow from left to right atria). Atrial Septal Defect Ostium secundum
During the more common form of Lutembachers syndrome, ASD Ostium secundum, a hole will form in the flap of tissue (septum primum) that should close between the two atria after birth. With the onset of a hole created in the tissue flap that closes the larger hole between the left and right atrium, blood can again flow from the left atrium to the right. Ostium secundum causes many of the same symptoms seen in ASD primum. With either type of ASD, blood will flow from the right atrium skipping the right ventricle (or very little flowing into the ventricle) and instead flow to the left atrium introducing the possibility of blood lacking oxygen to go the rest of the body. Sometimes, the direction of blood flow is largely determined by the left and right ventricle ability to squeeze (contract) and relax (compliance).Apart from congential or birth defects causing ASD, ASD is thought to be also acquired.
Mechanism
There is no exact mechanism for Lutembachers syndrome but instead a combination of disorders as the result of Atrial septal defect (ASD) and/or Mitral valve stenosis.It is thought ASD is caused by the failure to close the hole (foramen ovale) between the right and left atrium normally found within the heart during fetal development; the creation of a hole between the atrium may also be acquired. There are two types of ASD: Ostium secundum and ASD Primum. Atrial Septal Defect Primum
The failure of the hole between the right and left atrium to close shortly after birth is the cause behind ASD primum. During fetal development blood will pass from the umbilical cord and flow into the left atrium through a hole between the two atria. Once a baby is born and the lungs begin to fill with air, the blood flow of the heart changes; a tissue flap (septum primum) normally closes the hole (foramen ovale) between the two atria and becomes part of the atrial wall. During ASD primum, after birth the hole is not completely closed allowing blood to flow into the right atria from the left atria.