Cause Of Fluid Around The Heart?

My father is 45 years old and he is a heart patient. He is on high blood pressure medication  for the last 3 years , yesterday he was complaining of sharp, stabbing chest pain behind  the left side of his chest. That  sharp pain was travelling upto his neck. It was intensifying  when he  inhaled deeply. We took him to hospital immediately where he was diagnosed to have fluid around the heart. I want to know the cause of such fluid around his heart?