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.
Clinical Approach to Patients with Frequent PVCs. Premature ventricular complexes, or PVCs, are a common clinical problem. While patients may be asymptomatic, typically these PVCs cause sensations of skipping, heart pounding, and possibly chest pain, shortness of breath or dizziness. The 12-lead EKG is very useful in identifying the PVC morphology, as certain clinical syndromes exhibit very characteristic PVC morphologies (see below).