The adrenal medulla develops from neural tissue and secretes two hormones, epinephrine and norepinephrine. These two hormones are secreted in response to stimulation by sympathetic nerve, particularly during stressful situations. A lack of hormones from the adrenal medulla produces no significant effects.
While the sinoatrial node typically regulates the heartbeat during normal sinus rhythm, PACs occur when another region of the atria depolarizes before the sinoatrial node and thus triggers a premature heartbeat.
Cause. 1  Thrombosis of the superior vena cava caused by an indwelling central venous catheter which caused superior vena cava syndrome. 2  A CXR of a person with lung cancer which was causing superior vena cava syndrome. 3  A CT image showing compression of the right hilar structures by cancer.