Malignant Vasovagal Syncope: a Case of Prolonged Asystole Induced by the "Tilt" Test and Aggravated by Therapy with a Beta Blocker (A Clinical Case and Diagnostic, Physiopathologic and Therapeutic Review) [Síncope Vasovagal Maligna: Um Caso de Assistolia Prolongada Induzida Pelo Teste de "Tilt" e Agravada Pela Terapêutica Com Bloqueador Beta (Caso Clínico e Revisão Diagnóstica, Fisiopatológica e Terapêutica).]