I dont think this problem what you are suffering from is because of dehydration
What you can do is get a 2d echocardiogram
which is better then ecg in picking up any cardiac problems.
The reason of chest tightness
can be cardiac or respiratory in origin but why only on standing is difficult to explain.
Such kind of chest tightness and breathlessness on standing is seen with liver
problems generally and its called platypnea.
You need to see a good physician who can evaluate things further