INCIDENCE AND HEMODYNAMIC CHARACTERISTICS OF NEAR-FAINTING IN HEALTHY6-YEAR-OLD TO 16-YEAR-OLD SUBJECTS

Objectives. We studied the incidence and hemodynamic characteristics o f near-fainting under orthostatic stress in healthy children and teena gers. Background. Orthostatic stress testing is increasingly used to i dentify young subjects with unexplained syncope. However, the associat ed incidence of syncope and hemodynamic responses in normal young subj ects are not well known. Methods. Eighty-four healthy subjects 6 to 16 years old performed forced breathing, stand-up and 70 degrees tilt-up tests. An intravenous line to sample blood for biochemical assessment of sympathetic function was introduced between the stand-up and tilt- up tests. Finger arterial pressure was measured continuously. Left ven tricular stroke volume was computed from the pressure pulsations.Resul ts. Sixteen of the 84 subjects were excluded because of technical prob lems. The incidence of a near-fainting response in the remaining 68 su bjects was 10% (7 of 68) for the stand-up test and 40% (29 of 68) for the tilt up test. Baseline parasympathetic and sympathetic activity of nonfainting and near fainting subjects was not different. Near-fainti ng was characterized by attenuated systemic vasoconstriction and exagg erated tachycardia that occurred as early as 1 min after return to the upright position. On tilt-up, plasma adrenaline levels increased by a factor of 2, with slightly higher increments in the near fainting sub jects. Conclusions. Inadequate vasoconstriction is the common underlyi ng mechanism of near-fainting in young subjects. The remarkably high i ncidence of near-fainting during the tilt-up test after intravascular instrumentation raises serious doubts about the utility of this proced ure in evaluating syncope of unknown origin in young subjects.