Syncope in apparently normal patients has been attributed to an inhibitory reflex originating in cardiac sensory receptors. The reflex may be elicited by upright tilt with or without isoproterenol infusion. In this study, an upright 90 ° tilt protocol was evaluated in 20 young patiente aged 7 to 22 years with syncope but with normal cardiac and neurologic evaluations. The electrocardiogram and blood pressure were noninvasively recorded at 1 min intervals while the patient was supine (5 to 10 min) and during tilt (15 min) in the baseline state. The protocol was repeated during isoproterenol infusion at increasing doses until symptoms of syncope or near syncope were provoked or the maximal isoproterenol dose was achieved (0.07 to 0.1 μg/kg per min). Mean heart rate, mean blood pressure and RR interval variability, expressed as the standard deviation and the mean of the absolute difference between consecutive RR intervals, were assessed. Symptoms were elicited during tilt in 16 of the 20 patients (in 4 at baseline and in 12 with isoproterenol infusion); no symptoms were induced in 4 patients. Three patterns of heart rate and blood pressure response to tilt were observed during symptoms: 1) cardioinhibitory, in 3 patients with a maximal RR interval of 6.5 to 11s during baseline tilt; 2) vasodepressor, in 2 patients with a mean blood pressure ≤38 mm Hg and an RR interval ≤440 ms during tilt plus isoproterenol; and 3) mixed, in 11 patients with a mean blood pressure ≤54 mm Hg and a maximal RR interval of 580 to 1,200 ms (in 1 at baseline, in 10 with isoproterenol infusion). During tilt plus isoproterenol, four patients who did not develop symptoms had decreased heart rate variability, whereas symptomatic patients had increased heart rate variability (p <0.01). The tilt protocol utilized in this study elicited symptoms in 80% of patients with unexplained syncope and permitted characterization of three physiologically distinct heart rate and blood pressure responses during symptoms. Upright tilt is a useful provocative tool for evaluating young patients with syncope.
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