What does it mean if the ECG showed signs of ischemia in the postural valve?

Posterior wall? You probably meant to say posterior wall, instead of posterior valve. But it probably means there is a blockage in the coronary artery that is feeding the posterior wall of your heart. This can be confirmed with a coronary angiogram and potentially fixed with coronary angioplasty and stent placement or else coronary bypass surgery.
Lack of blood flow. First to clarify: there's no such thing as the "postural valve." ekgs can show signs of ischemia in certain regions of the heart muscle, which means there could be a lack of blood flow to those areas, often caused by clogged arteries.