This would be analogous to causing syncope with vigorous carotid-sinus massage in normal persons. After the exclusion of serious cardiac disease, arrhythmias, and central nervous system and metabolic diseases, syncope associated with head-up tilt testing may be of neurocardiogenic origin. The hallmarks of neurocardiogenic syncope are severe hypotension associated with paradoxical bradycardia after a period of sympathetic excitation. If upright-tilt testing without the infusion of isoproterenol reproduces these hemodynamic changes in less than 15 minutes and causes presyncope or syncope, the diagnosis of neurocardiogenic syncope can be made and treatment with metoprolol or disopyramide is appropriate. Implantation of a pacemaker, however, would not be indicated.
Several questions await further study.
It is possible that a vasodepressor response occurred in those patients as a result of sympathetic inhibition but that the parasympathetic response was blunted. For example, patients with diabetes may have a blunted parasympathetic response as one of the earliest manifestations of autonomic dysfunction. Thus, despite the absence of bradycardia in some patients with positive upright-tilt tests, the choice of medical therapy is reasonable, particularly if the presence of vasodilatation can be confirmed. These are reasonable questions that require further investigation. Overall, I believe that upright-tilt testing should be used more regularly to support the diagnosis of neurocardiogenic syncope and to assess the effectiveness of a contemplated treatment.