The next two slides list the organ systems and tissues involved in anaphylaxis and their clinical manifestations. Urticaria and/or angioedema occur in 90% of episodes, upper airway edema in 56%, dyspnea and/or bronchospasm in 47%, flushing in 51%, lightheadedness or syncope (hypotension) in 33%, and gastrointestinal manifestations in 30%. Tachycardia is the rule in anaphylaxis but bradycardia may occur in patients with atrial conduction defects or in patients who take sympatholytic medications, such as beta-blockers. Fisher also observed in a retrospective study of 186 patients who experienced anaphylaxis during surgery that bradycardia occurred in about 4% of patients in the absence of beta blocker use. A possible explanation may be the activation of the cardiac Bezold-Jarish reflex, which results from ischemic stimulation of vagal receptors in the inferoposterior wall of the left ventricle. Upper respiratory, eye, and genitourinary symptoms occur less frequently in anaphylaxis.