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.
The outcome of pregnancies complicated by fetal tachycardia is related to the underlying cause of the arrhythmia. PACs and PVCs are benign and the prognosis is excellent. In cases where these extrasystoles result in SVT and in utero medical therapy is successful, a favorable outcome is also expected.